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Louisiana State UniversityLSU Digital Commons
LSU Historical Dissertations and Theses Graduate School
1999
Pine-Polyethylene (Wood -Polymer) Composites:Synthesis and Mechanical BehaviorCharacterization.Parviz S. RaziLouisiana State University and Agricultural & Mechanical College
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Recommended CitationRazi, Parviz S., "Pine-Polyethylene (Wood -Polymer) Composites: Synthesis and Mechanical Behavior Characterization." (1999). LSUHistorical Dissertations and Theses. 7007.https://digitalcommons.lsu.edu/gradschool_disstheses/7007
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PINE-POLYETHYLENE (YVOOD-POLYMER) COMPOSITES: SYNTHESIS AND MECHANICAL BEHAVIOR
CHARACTERIZATION
A Dissertation
Submitted to Graduate Faculty o f the Louisiana State University and
Agricultural and Mechanical College In partial fulfillment of the
Requirements for the degree o f Doctor of Philosophy
in
The Interdepartmental Program in Engineering Science
byParviz S Razi
B. S., College of Science and Technology, Tehran, Iran. 1972 MBA. Louisiana State University, Baton Rouge, 1980 M. S., Louisiana State University, Baton Rouge, 1983
August. 1999
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UMI Number: 9945734
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ACKNOWLEDGEMENT
The author wishes to express sincere appreciation to Dr. Aravamudhan Raman for his
continuous advice and encouragement during the preparation and advancement o f this
project. In addition, special thanks are expressed to Dr. Portier, Institute for
Environmental Studies, for his support o f this research and to Dr. Collier, Chemical
Engineering Department, for the use o f the extruder and other facilities. Appreciation is
also extended to Dr. Pang, Dr. McCarley, Dr. Schulz and Dr. Joshi for their valuable
advice. Special thanks to my very patient wife, Manijeh, and to my two sons, Sam and
Keyan, for their patience and loyal support.
ii
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TABLE OF CONTENTS
ACKNOWLEDGMENT.........................................................................................................ii
LIST OF TABLES.................................................................................................................. v
LIST OF FIGURES................................................................................................................ vi
LIST OF NOMENCLATURE.............................................................................................. ix
ABSTRACT.............................................................................................................................x
CHAPTER IINTRODUCTION....................................................................................................... 1
1.1. Mechanical Properties o f Wood-Polymer Composite.................... 21.2. Effect o f Coupling Agents................................................................41.3. Fracture and Fracture Toughness......................................................6
1.3.1. K[C...........................................................................................6
1.3.2. Crack Opening Displacement (COD)..................................71.3.3. The Crack Extension Force, G ............................................8
1.3.4. J Integral................................................................................ 91.3.5. R Curve................................................................................12
1.4. Objectives..........................................................................................21
CHAPTER 2EXPERIMENTAL PROCEDURES...................................................................... 22
2 .1 . Common Characterization Techniques Used................................ 222.1.1. Scanning Electron Microscopy......................................... 222.1.2 Tensile Testing...................................................................222.1.3. Instrumented Impact Testing............................................. 22
2.2. Experiments Pertaining to Mechanical Properties........................232.2.1 Preparation o f Wood-Polymer Composites Samples 23
2.3 Experiments Pertaining to Effect of Coupling Agentsand Surface Modification................................................................242.3.1. Preparation o f Wood-Polymer Duplex Samples............ 25
2.4 Experiments Pertaining to Impact Fracture Properties................. 262.4.1. Preparation o f Composites Samples................................. 26
2.5. Experiments Pertaining to Fracture Resistance............................ 27
CHAPTER 3RESULTS AND DISCUSSION: STATIC MECHANICALPROPERTIES OF PINE CHIP - HDPE COMPOSITES INTENSION AND BENDING................................................................................... 36
3.1. Tensile Mechanical Properties.......................................................363.2. 3-Point Bending Testing................................................................. 363.3. Nail Removal Testing......................................................................37
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CHAPTER 4RESULTS AND DISCUSSION: INTERFACE MODIFICATIONAND INTERFACE BONDING IN PINE WOOD - HDPE COMPOSITES 48
4 .1. The Influence o f Pre-treatment in NaOH or H2SO4 on theBond Strength Between Wood and Polymer..............................48
4.2. The Influence o f Coupling Agents on the Bond StrengthBetween Wood and Polymer........................................................ 49
4.3. The Influence o f Surface Morphology o f Woodstockand he Bond Strength Between Wood and Polymer.................. 50
CHAPTER 5RESULTS AND DISCUSSION: IMPACT FRACTUREPROPERTIES OF PINE CHIP - HDPE COMPOSITES....................................58
5.1. Instrumented Impact Testing.......................................................... 585.2 Experimental Results.......................................................................595.3 Energy to Cause Failure..................................................................645.4 Mechanism for Energy Absorption and Fracture
Morphology.................................................................................... 6 6
CHAPTER 6RESULTS AND DISCUSSION: FRACTURE RESISTANCEAND FRACTURE TOUGHNESS OF PINE CHIP - HDPE COMPOSITES...73
6 .1. Kic Determination Using ASTM Standard Procedure..................736.2. JIc Determination Using ASTM Standard Procedure..................736.3. R Curve Determination................................................................... 746.4. Analysis o f Data.............................................................................. 746.5. The critical Stress Intensity Factor for Particulate Filled
Composites..................................................................................... 78
CHAPTER 7CONCLUSIONS AND SUMMARY .................................................................. 87
CHAPTER 8
SUGGESTIONS FOR FUTURE WORK............................................................. 928 .1. Evaluation o f Krif,com and K[C for Composites with 60
vol. % Pine Wood Chips................................................................928.2. Modeling o f Crack Growth and Delamination of the Wood
Chip - polymer Interface................................................................928.3. Fatigue and Creep Characterization...............................................928.4. Effect o f Environmental Variables................................................ 938.5. Damping Characteristics................................................................. 93
BIBLIOGRAPHY.................................................................................................................94
VITA...................................................................................................................................... 99
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LIST OF TABLES
Table 1 Mechanical properties o f southern pine (\vood)-polvethylene (polymer)composites with various wood chip sizes and concentrations...................40
Table 2. Summary of different wood surface treatments and experimentalcoupling strengths between woodstock and polymer in southern pine- high density polyethylene coupled samples................................................52
Table 3. Drop weight impact fracture properties of southern pine (wood) -polyethylene ( polymer) composites with various wood chip sizes and concentrations................................................................................................ 61
Table 4. The test result summary' for the K[C tests (as per ASTM E399-83) for pine-HDPE composite sample, (medium size wood chips in 50%
volume concentration)...................................................................................81
Table 5 The test result summary for the Ji0 tests (as per ASTM E813-89) for pine-HDPE composite sample, (medium size wood chips in 50% volume concentration)...................................................................................81
Table 6 Comparison of the Knr.com and K[C.com obtained using ASTM E561-76Tand Kq obtained using ASTM E399-83.......................................................81
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LIST OF FIGURES
Figure 1 . Crack opening displacement.............................................................................. 7
Figure 2. (a) Elastic body containing a crack o f length 2a under load P: (b) load Pversus displacement. e, diagram........................................................................8
Figure 3. Load-displacement and compliance-crack length curves............................... 10
Figure 4. Definition of the J integral.................................................................................10
Figure 5. Physical interpolation of the./ integral. The./ integral represents thedifference in potential energy (shaded area) of identical bodies containing cracks of length a and a - da ........................................................ 1 1
Figure 6 . R curve for (a) brittle and (b) ductile material..............................................13
Figure 7. Definition of K in terms of crack parameters.................................................. 14
Figure 8 . Definition of J for a physically deforming material.........................................15
Figure 9. Steps in the crack growth process..................................................................... 16
Figure 10. A J-R curve........................................................................................................ 17
Figure 11 The morphology of pine (wood) chips used to produce WPCs; (a) small,(b) medium and (c) large...............................................................................29
Figure 12. Schematic diagram of an adapter used for the production of compositesamples of pine (wood)-PE............................................................................31
Figure 13. The resultant composite sample used for tensile testing............................. 32
Figure 14. Schematic diagram of (a) the drop weight impact tester (b) condition at the instant of impact, (c) specimen holder and (d) specimen to be fitted between the holder plates in (c)..................................................................... 33
Figure 15. The dimensions of compact tension specimens used for K[C and J[Cdetermination................................................................................................... 34
Figure 16. The dimensions of compact tension specimens used for R curvedetermination..................................................................................................35
Figure 17. Effects o f pine (wood) chip size and concentration on UTS of pine PEcomposites...................................................................................................... 41
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Figure 18.
Figure 19-1.
Figure 19-2.
Figure 19-3.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Effects o f pine (wood) chip size and concentration on break strain ofpine PE composites........................................................................................ 42
Effects o f pine (wood) chip size and concentration on peak load of pine PE composites.................................................................................................43
Effects o f pine (wood) chip size and concentration on MOR of pine PE composites...................................................................................................... 44
Effects o f pine (wood) chip size and concentration on stiffness o f pine PE composites................................................................................................45
The effects o f pine (wood) chip size and concentration on nail pull-out force...................................................................................................................46
The morphology o f the fracture surface of a stretched and broken sample............................................................................................................... 47
The surface morphology of untreated pine (wood) sample.........................53
The surface morphology of pine samples treated in the solutions of (a) NaOH and (b) FTSO-t...................................................................................... 54
The surface morphology of pine (wood) samples treated with NaOH and then with (a) CA1, (b) CA2, and (c) CA3.....................................................56
The surface morphology of pine (wood) samples treated with H;SO.i and then with (a) CA1, (b) CA2, and (c) CA3.....................................................57
Example o f load vs. deflection and energy vs. deflection curves in an instrumented drop weight impact test for a sample with large pine (wood) chips and 60% concentration............................................................ 58
Example o f load vs. time and energy vs. time curves in an instrumented drop weight impact test fora sample with large pine (wood) chips and 60% concentration.......................................................................................... 59
Effect o f pine (wood) chip size and concentration on Maximum load sustained...........................................................................................................62
SEM fractograph of a WPC specimen with large pine (wood) chips and 60% volume concentration. One large pine (wood) chip with a layer o f peeled polymer hanging on it (top) is seen....................................................63
Effect o f pine (wood) chip size and concentration on deflection at maximum load sustained................................................................................6 8
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Figure 3 1. Effect of pine (wood) chip size and concentration on energy to maximumload sustained.................................................................................................. 69
Figure 32. Effect of pine (wood) chip size and concentration on total energy tofailure by full tup penetration.........................................................................70
Figure 33-1. The morphology o f fractured surfaces o f samples broken in drop weightimpact test: specimen with 60% volume concentration of small pine (wood) chips (a) top surface (b) bottom surface..........................................71
Figure 33-2. The morphology o f fractured surfaces o f samples broken in drop weightimpact test: specimen with 60% volume concentration of large pine (wood) chips (a) top surface, and (b) bottom surface................................. 72
Figure 34. Load vs. crack opening displacement curve for the pine-polyethylenecomposite sample, (medium size pine (wood) chips in 50% volume concentration)..................................................................................................82
Figure 35. True Stress and engineering stress vs. a crack length plot for pine-polvethvlene composite sample, (medium size pine (wood) chips in 50% volume concentration).....................................................................................82
Figure 36. Engineering FQ vs. crack length plot for the pine-polyethvlene compositesample, (medium size pine (wood) chips in 50% volume concentration)..83
Figure 37. True FQ vs. crack length plot for the pine-polyethylene composite sample.(medium size pine (wood) chips in 50% volume concentration)............... 83
Figure 38. Slope of FQtnie vs. crack length plot for the pine-polvethylene compositesample, (medium size pine (wood) chips in 50% volume concentration)..84
Figure 39. Resistance curve. KJ'[,-ng/E vs. crack length plot for the pine-polyethylenecomposite sample, (medium size pine (wood) chips in 50% volume concentration)...................................................................................................85
Figure 40. Slope of R curve vs. crack length plot for the pine-polyethylenecomposite sample, (medium size pine (wood) chips in 50% volume concentration)...................................................................................................8 6
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LIST OF NOMENCLATURE
PE Polyethylene
HDPE H igh Density PE
PP Polypropylene
WPC Wood-Polvmer Composite
WPEC Wood-Polvethvlene Composite
CA Coupling Agent
UTS Ultimate Tensile Strength
MOR Modulus of Rupture
LEFM Linear Elastic Fracture Mechanics
EPFM Elasric-PIasdc Fracture Mechanics
ESEM Environmental Scanning Electron Microscope
CTOD Crack Tip Opening Displacement
FEM Finite Element Method
K,c Critical Stress Intensity Factor
COD Crack Opening Displacement
Knie Stress Intensity Factor Calculated Using Unbroken Ligament Dimension Ahead of the Crack Tip (i.e., W-a) Instead of ’\V \
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ABSTRACT
Processing o f recycled wood chips, recently produced after extraction o f creosote from
telephone posts and railroad crossties, and combining them with a recycled polymer for the
synthesis o f novel composite materials would initiate a new trend toward preservation o f natural
resources. The challenge is taken in this work to produce and study the properties o f such
materials for further advancement o f science.
In the present study, addition o f pine wood chips to HDPE reduced the tensile strength
and break strain in tensile loading. Smaller wood chips generally resulted in smaller reductions.
Peak load, modulus o f rupture and stiffness were slightly higher at 40% pine wood chip
concentration in the composite than the polymer alone: they later decreased with increasing
concentration o f wood chips. Optimal mechanical properties in these composites were
produced by pine wood chips smaller than 0.125 inch in size, at around 40 vol. % in
concentration. Pre-treatment od wood chips in a suitable solurion o f NaOH caused an increase
in the coupling strength between protruding wood fibers and polymer. Such a treatment
followed by a second one with vinyltrimethoxysilane was found to be the best for obtaining
maximum bonding strength. Impact testing o f the prepared samples showed that more fracture
resistant wood-polymer composites were those with larger wood chips at the higher concentration
range o f 50 to 60 vol. %. In contrast composites with 60% fine wood chips would fail easily at
energy' levels far less than those required to break the polymer alone.
Variation o f (K<.ng)2/E vs. crack length, the R curve, indicates three regions. The point
o f transition from region I (elastic) to region II is considered as a critical point o f fracture
process initiation, Ki,t.com. The transition from the state o f stable crack growth, region II. to
region III is considered to be at the point o f instability. Likewise, the point o f inflection on the
plot o f K,rae vs. crack length, corresponding to a large change in slope, indicates also the point o f
instability. Kcng corresponding to this critical point o f inflection is proposed to be K[c, the real
fracture toughness o f these composites.
x
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CHAPTER 1
INTRODUCTION
Wood and discarded plastics, which present challenges for solid waste disposal,
have long been recognized as potential recyclable materials. Recently, recycled wood
chips were produced by the process o f extraction of creosote from telephone posts and
railroad crossties. Wood chips produced by this process provides us with an abundant
source of cellulosic waste material. Consideration has been given to making use of
these wood chips by combining them with polymer for the synthesis o f novel composite
materials.
Wood-polymer composites (WPCs) have elicited considerable attention due to
their improved properties. These properties range from increased strength to
dimensional stability and to resistance to biodeterioration, Katsurada et al., (1983).
Additionally, the development o f novel WPC materials provides the opportunity for the
utilization of abundant cellulosic wastes and recycled plastics. Traditionally, WPCs are
obtained by impregnating wood with liquid monomers, and then polymerizing those
monomers in situ, using catalvsis-heat or radiation processes. Because of the materials
involved and the production process, the properties o f WPCs should largely depend on
the nature and properties of wood, polymer and the polymerization process involved,
Lauke et al. (1985) and Okajimaet al. (1970).
Above facts have attracted much research toward the WPCs since 1960. Their
physical and mechanical properties, water and humidity absorption, as well as
dimensional stability have been profoundly investigated by varying the factors
mentioned before. It has been found that WPC exhibited improved strength properties,
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dimensional stability and resistance to biodeterioration. Manrich et al. (1989) and the
extent of these improvement in properties indeed was found to be directly related to the
nature of wood, polymer and the processing.
Till now, WPCs used in research and industry were produced from sawdust and
fine wood particles obtained as by-products in wood processing. Prior studies have
focused on moldable viscose composites, Sain et al. (1994). As a result, in some
processes the mixtures o f polymer and wood chips were kneaded, Otaigbe (1991), after
mixing for further reduction in particle size. However, the composites made of coarse
wood chips and polymer matrix have not been developed and their properties not
explored. Above facts can have a profound effect on the industry today and on the use
of wood-polymer composites. A better understanding is needed for the development of
more usable materials made of polymer and wood.
1.1. Mechanical Properties of Wood-Polymer Composites
There is a growing body of literature covering a wide variety of subjects in the
polymer-wood composite area. Wood-polymer composites are obtained by
impregnating wood with liquid monomers, and then polymerizing those monomers in
situ, using catalysis-heat or radiation processes. The mechanical properties of WPC are
expected to depend on properties of the wood, polymer, the polymerization process and
the interface bonding, Otaigbe et al. (1985).
WPCs have been the subject of much research recently. Their physical and
mechanical properties, water and humidity absorption, as well as dimensional stability
and biodeterioration, have been investigated by varying several factors. Due to the
profound effect of the interface between wood and polymer on the mechanical
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properties, the bonding between wood and polymer, and the influences of different
coupling agents on the properties of the interfaces have also been the topic o f many
research efforts, Kenega et al. (1962).
The study of melt rheology of composites o f polypropylene filled with wood
flour, at filler concentrations of 3-20 wt%, has indicated an increase of melt viscosity
and a decrease o f polymer’s elasticity due to addition of filler and increase of its
concentration, respectively, Maiti et al. (1989). In an update review paper by Wright
and Mathias (1993) new tests and techniques developed for wood-polymer composites
were described. Processability and viscoelasticity properties of composites of wood and
polystyrene were studied by Hon, et al. (1993), where the addition of benzyl chloride as
plastisizer in the polyblend was investigated. This study also revealed the existence of a
single damping peak for the composite regardless o f the blending ratio. Other studies
have concentrated on the thermoplasticization of wood, which can be achieved by (a)
etherification, (b) esterification, and (c) grafting reaction, Hon et al. (1989)]. Khan et
al. (1992:45) studied the effect of combination of monomers in radiation induced wood-
plastic composite and later studied the effect of other additives on these composites,
Khan et al. (1992:49). Using balsa wood and copolymer containing ethyl a-
(hydroxymethyl) acrylate (EHMA)-styrene mixtures, Wright et al. (1993) developed
lightweight composites with improved dimensional stability and good mechanical
properties o f balsa wood. Other researchers studied the thermal properties, Murthy et
al. (1982), o f tropical wood - polymer composites using oxygen index,
thermogravimetry, differential scanning calorimetry, and elemental analysis. This
group also analyzed the dynamic mechanical properties of their composites, Yap et al.
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(1991). Khan et al (1993)] also studied the physio-mechanical properties o f vvood-
plastics composites. Infrared spectroscopy (IR) is widely used to elucidate chemical
structures, enabling functional groups and linkages to be identified. Khan, et al.
(1991:45) studied and observed the graft polymerization o f wood components and
acrylonitrile and styrene. However, highly improved spectra o f wood - polymer
composites have been obtained and studied using Fourier Transformed Infrared (FTIR)
spectrophotometers, compared to the conventional dispersive IR instruments.
1.2. Effect of Coupling Agents
Mechanical properties o f composites are a direct function of the interface
bonding between the strengthening fiber or particulates and the matrix. Composites o f
wood and polymers are not exceptions. Recently, through an investigation on the
mechanical behavior o f wood-polyethylene WPECs, it was observed that fracture of the
WPECs was caused by delamination of wood chips from polymer matrix, Razi et al.
(1997). Since the interface between the wood and the polymer matrix is often the
shortfall of WPCs, the bonding strength on the interface plays a key role in determining
the mechanical properties of the WPCs, Ebewele et al. (1991). Studies have shown that
the tensile properties o f a WPC can be improved by the addition of a compatibilizer or a
coupling agent to enhance the bonding force at the interface, or by the application of a
thin polymer layer on the outer surface of the composite material in order to decrease or
eliminate possible sites for the initiation of cracks on the surface, Maldas et al. (1989).
Coupling agents are hybrid organic-inorganic compounds that bridge the
interface between the wood fiber and polymeric matrix. An adhesion promoter, based
on maleic anhydride modified polypropylene, was found to behave as a true coupling
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agent, i.e., impact strength and elongation value increased significantly, while the
elastic modulus remained unchanged, Dalvag et al. (1985). Isocyanates also were used
to improve the mechanical properties of the composites based on several polyolefins
(PP, LDPE, HDPE) and wood pulp fibers, Coran et al. (1982). Zadorecki, et al. (1986)
found that some coupling agents (trichloro-s-triazine and di-methylolmelamine)
produced covalent bonds between cellulosic materials and polymer matrix leading to
the modified performance and reduced sensitivity to water, Bataille et al. (1990). This
idea has been proved by the research of Maldas, et al. (1989), in which phthalic
anhydride was selected as a coupling agent for wood fiber-polystyrene composites and
it was found that the benzene ring of phthalic anhydride can interact with the benzene
ring of polystyrene and an anhydride group can link to the -OH group of cellulose.
Another strategy to enhance the interfacial adhesion in WPCs is through surface
modification of the wood. Corona discharge treatment on polymer has been shown to
improve the adhesion in polyethylene/cellulose composites, Dong et al. (1993).
Compatibility between wood pulp fibers and thermoplastics may be improved by the
surface grafting of a polymeric segment having a solubility parameter similar to the host
polymeric matrix, Kokta et al. (1983). Recently, 0 2-, Ar- and NH3-pIasma treatment
was found to be able to improve the wettability and adhesion of silicone rubber, Lai et
al. (1996).
Traditionally, the effect of coupling agents on the bonding behavior in
composites is evaluated by testing the mechanical properties of the composites. An
enhancement o f the properties indicates the positive effect o f the coupling agent and the
surface treatment. This method, however, does not reveal the influence of other
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variables and could lead to erroneous deductions. There is a need for a more direct
measurement of bonding strength and characterization of bonding in these composites.
1.3. Fracture and Fracture Toughness
One o f the important properties for failure safe design is the fracture toughness
of composite material. Traditionally linear elastic fracture mechanics (LEFM)
parameters and elastic - plastic fracture mechanics (EPFM) have been used to
characterize the fracture resistance o f composites.
In order to facilitate the following discussion, a brief description of a few
important fracture parameters are described.
1.3.1. Kic:
Kjc is the critical stress intensity factor under conditions o f plane strain, which
is characterized by small-scale plasticity at the crack tip. The material is fully
constrained in the thickness direction. When determined under this condition, K[C will
be a material constant. Thus, when one needs to characterize materials by their
tenacity, in the same way that one characterizes materials by their ultimate tensile
strength or tensile yield strength, only valid Kjc data should be considered.
Kc is the critical stress intensity factor under conditions o f plane stress, which is
characterized by large plasticity at the crack tip. In this case, the through-thickness
constraint is negligible. Kc value can be up to two times greater than the Kic values for
the same material. In general, Krc depends on temperature T, strain rate, and on
metallurgical variables.
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1.3.2. Crack Opening Displacement (COD):
The development o f a plastic zone at the crack tip results in a displacement of
the faces without crack extension. The relative displacement of opposite crack edges is
called the crack opening displacement, Figure 1. It is suggested that when this
displacement at the crack tip reaches a critical value 5C, fracture ensues.
Figure 1. Crack opening displacement
One important point about COD is that theoretically, Sc can be computed for
both elastic and plastic materials. It allows one to treat fracture under plastic condition.
The only drawback is that we cannot define a single critical COD value for a given
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material in a manner equivalent to that o f Kic, as the COD value will be affected by the
geometry o f the test specimen.
1.3.3. The C rack Extension Force, G: The elastic strain energy release rate with
respect to the crack area or the crack extension force G is a parameter that can be used
to characterize the fracture condition in a manner similar to the stress intensity factor K.
G can be interpreted as a generalized force.
P
P
i I. JX dP
Ir 't
IIIX^ — de
----------►
Figure 2. (a) Elastic body containing a crack of length 2a under load P; (b) load Pversus displacement, e, diagram
If we consider an elastic body o f uniform thickness B containing a through
thickness crack o f length 2a and let the body to be loaded as shown in Figure 2, with
increasing load P, the displacement e o f the loading point increases, as shown. The
potential energy stored in the body is
Ui= V-l P e before crack extension and
U2 - Vi (P - S P ) ( e + 8 e ) after the crack extension.
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The change in the potential energy, U2 -Ui, is given by the difference in the
areas o f the two crosshatched regions I in the Figure. The crack extension force, G, per
unit length is
G = lim^_x) S U SA where SA = B Sa
It is convenient to evaluate G in term of compliance c o f the material defined
as
e = c P
Ignoring the higher-order product terms, and by applying further manipulations,
we can write:
G = VziP2 B )(8c 5a)
From this relation we see that G is independent o f the rigidity of the
surrounding structure or the test machine. It depends only on the change in compliance
of the cracked member due to crack extension. Thus, to obtain G for a material, all we
need to do is to determine the compliance of the specimen as a function of crack length
and measure the gradient o f the resultant curve, dc da, at the appropriate initial crack
length, Figure 3.
1.3.4. /Integral:
The J integral is defined as a line integral, independent of path, along a curve T
surrounding the crack tip. Mathematically, it is shown that
•/ = I r ( W d y - T (5u / dx ) ds)
where W is the strain energy density function, T is the traction vector perpendicular to
the surface T, u is the displacement in the x direction and ds is an arc along T, Figure 4.
9
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Disphicement,
(dafdc)O
aCrack Length, a
Figure 3. Load-displacement and compliance-crack length curves, Paris et al.(1979).
y
>- X
PlasticZone
Figure 4. Definition of the J integral, Rice and Rosengren (1968).
From a physical point o f view, the J integral represents the difference in the
potential energy of identical bodies containing cracks of length a and a + da, that is,
J = - ( U B ) (d U/ d a )
where U is the potential energy, a the crack length, and B the plate thickness, Figure 5.
>o
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a+da
A
Figure 5. Physical interpolation o f the J integral. The J integral represents the difference in potential energy (shaded area) o f identical bodies containing cracks o f length a and a + da
U is equal to area under the load versus displacement curve. Figure shows this
interpretation, where the shaded area is dU = J B da. J measures the critical energy
associated with the initiation o f crack growth accompanied by substantial plastic
deformation.
The path independence o f the J integral, together with this interpretation in
terms of energy, makes it a powerful analytical tool. The J integral is path independent
in the case of material behaving elastically, linear or non-linear. When there occurs
extensive plastic deformation, the current practice is to assume that the plastic yielding
can be described by deformation theory of plasticity. According to this theory, stresses
and strains are functions only o f the point of measurement and not of the path taken to
get to that point. As in the case o f slow, stable crack growth, there will be relaxation of
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stresses at the crack tip, so there will be a violation o f this postulate. Thus, the use o f J
integral should be limited to the initiation of crack propagation, by stable or unstable
processes.
1.3.5. R Curve:
The R curve characterizes the resistance of a material to fracture during slow
and stable propagation o f a crack. An R curve graphically represents this resistance to
crack propagation o f a material as a function of a crack growth. With increasing load in
a cracked structure, the crack extension force G at the crack tip also increases, Figure 6 .
However, the material at the crack tip presents a resistance R to the crack
growth. The failure will occur when the rate of change o f the crack extension force
(5G/3a) equals the rate o f change o f this resistance to crack growth in the material
(5R/da). The resistance o f the material to crack growth, R, increases with an increase in
plastic zone size. The plastic zone size increases non-linearly with a; thus, R will also
be expected to increase non-linearly with a. G increases linearly with a. Figure 6
above shows the instability criterion: the point of tangency between the curves G
versus a and R versus a. Figure also shows the R curve for a brittle material and the
R curve for a ductile material. Crack extension occurs for G > R. Considering the G
line for a stress <x’; the crack in this material, at this stress cF, will grow only from a0
to a ' since G > R for a <a '. G < R for a > a ’ and the crack does not extend beyond a
As the load is increased, the G-line position changes, as indicated in the Figure. When
G becomes tangent to R, unstable fracture ensues. The R curve for a brittle material is a
“square” curve and the crack does not extend at all until the contact is reached, at which
point G = GC and the unstable fracture follows.
12
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Gc
a
&o
aa — ►
Figure 6 . R curve for (a) brittle and (b) ductile material, Clark (1969).
At this point a comparison of these methods is offered. The most important
consideration in the application of the LEFM’s K analysis is that the test material be
essentially linear elastic. This application uses the principle of a unique linear elastic
crack tip field. This field, which has a unique stress and strain distribution, is
characterized by a single parameter, K, the crack tip stress intensity factor, which
determines the magnitude o f the field, Figure 7. This uniqueness provides a method for
directly correlating the test results, which measure fracture properties with the fracture
behavior of the structural components.
13
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2 - "/ cos—sin —cos —V2 «r 2 2 2
Q\3 = 2̂3 = 0^33 = 0 (plane stress)^33 = K0 „ +0 = ) (plane strain) Then: K is the Intensity of the
Elastic Field Surrounding the Crack Tip
if w < r « Planner Dimension1 k 2w = j
6 k a Q
K = a cjjj V (7t . a) where a = crack lengtha = specimen geometry or compliance
Figure 7. Definition o f K in terms of crack parameters, Begley et al. (1972)..
If large-scale plastic stresses and strains are encountered, the K parameter is no
longer a good characterization o f the crack tip field. This limitation restricts the use o f
LEFM and its fracture toughness characterization to higher strength and lower
toughness material, ASTM E399-83, ASTM (1995), and a method for extending these
principles to include larger scale plasticity in materials should also be considered.
Hutchinson (1968) and Rice and Rosengren (1968)] developed the plastic crack
tip field stress-strain analysis. They showed the existence o f a unique stress and strain
distribution with a single characterizing parameter, J, to describe the magnitude of these
stresses and strains, Figure 8 .
ElasticZone
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if D<r«PIanar Dimensions, then: J is the Intensity o f the Plastic Field Surrounding the Crack Tip for Power Law Hardening Material
e£'0
Figure 8. Definition of J for a physically deforming material, Rice (1968).
The parameter J came from the path independent J integral developed by Rice
(1968); it could be used to characterize the fracture toughness and sub-critical crack
growth for cases o f large scale plasticity in a way that is analogous to the use o f K for
linear elastic cases.
The development of the crack tip field equations for large scale plasticity
effectively has extended the capability o f fracture mechanics from the linear elastic
regime into the elastic plastic regime where J now has replaced K as the parameter to
characterize fracture type behavior in more ductile and plastic material. In an analogy
to the linear elastic toughness Kic, the elastic-plastic fracture toughness is labeled J[C,
Begley et al. (1972). A complete description o f the ductile fracture process includes
four steps from a sharp crack tip starting point: the crack tip blunting during initial
stress, the initiation o f stable crack growth, the continued stable crack growth, and the
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ductile instability, as shown in Figure 9. One of these steps, the initiation o f the tearing
crack from the blunted crack tip, is taken as the point to specify Jrc.
Start Sharp Crack Tip(Fatigue Precrack
Step 1: Crack Tip Blunting ^
Step 2- Initiation of Stable Crack ---------------------------Growth '
S t e p 3 . Continued Stable Crack Growth
Step 4: Ductile Instability v
Figure 9. Steps in the crack growth process
The entire process o f ductile cracking is better described by the crack growth
resistance curve, R curve, where a driving force is plotted as a function of crack
extension, Figure 10. For the ductile fracture case J can be plotted against physical
crack extension, Landes et al. (1977).
The R curve could then be used to describe the initiation o f the ductile cracking,
Jic and the process o f stable crack advance. The method for determination of Jic has
been standardized in ASTM E 813-89.
16
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Step 4: Ductile InstabilityStep 3: Stable Crack Growth
a
Step 2: Initiation of Stable Crack Growth
M
Step 1: Crack Tip Blunting
&O
Start: Sharp Crack
Crack Extension
Figure 10. A J-R curve, Clarke (1979).
Although the EPFM methodology extends the capability o f the fracture
characterization well beyond elastic regime, there are also limitations to consider, Paris
et al. (1972). The concept of a unique stress and strain field in plastic zone requires that
the field should not be disturbed by structural or specimen boundaries. A specific
requirement is that specimen dimensions be greater than M J/ao, where <To is the flow
stress and M is a constant developed to satisfy the stress field requirement, Clarke et al.
(1979).
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Hutchinson and Paris (1979) showed that the concept o f a J field is valid for the
growing crack, if certain conditions can be maintained. These were quantified by Shih
and labeled “conditions for J-controlled crack growth.”, Shih et al. (1980). They
include M = 25 and
© = (b/JXdJ/da) > 5
Aa/b < 0.1
where b is remaining uncracked ligament length.
In developing an R curve for stability analysis, very often an extensive amount
of crack growth is needed to establish an instability point. The restriction in above
equations limits the amount o f the crack extension so that many times this intersection
cannot be reached. Specimen sizes are often limited by material availability and there
may be a need to exceed these limits. However, R curves tend to be geometry-
dependent when crack growth exceeds the limits. A geometry independent R curve for
greater amounts o f crack growth has been developed by Ernst (1983) in which he
suggested a modified J parameter, Jm, which can be used to characterize the R curve
behavior. Experimental results showed that geometry independence was maintained in
the R curve behavior for crack growth well in excess o f the limits o f the above equation.
As a result o f all o f the above developments, there is a much extensive list o f
parameters available to characterize fracture toughness of composite materials. Among
the different fracture parameters, critical stress intensity factor (Kic), critical J-integral
(Jic), critical crack tip opening displacement (CTODc), fracture energy (Gf) and the R-
curve analysis have been used mainly by researchers to describe the fracture behavior of
particulate-filled composites. The resistance curve methods are used to characterize the
18
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resistance to fracture o f slow stable crack extension in the material and provides a
toughness record as the crack is driven stablv into the fracture process zone, caused by
an increase in the applied loads.
Fracture study toward the understanding of the near tip behavior of material
ahead of cracks in particulate composites consisting o f semi-rigid polyhedral particles
in a soft matrix has shown a strong surface influence on the lowest eigenvalue at the
free surface. Smith et al. (1990).
The particulate-filled polymer-concrete composites have been studied for their
fracture characteristics. Cook and Crookham (1978) observed that K[C and stiffness
decreased in polvmer-portland cement concrete (PPCC) matrix composites, regardless
of polymer type, mix and treatment. Polymer impregnation, on the other hand,
increased the fracture toughness and failure strain. Polymer impregnated concrete was
found to be notch sensitive with BQc having a limiting value between the notch-to-depth
ratio of 0.35 and 0.42, beyond which Ktc generally decreased. Alezksa and Beaumont
[1975] have observed that the work to fracture was enhanced in the concrete system
when impregnated with an acrylic polymer.
The strain energy release rate (fracture energy) for polyamine-modified urea-
formaldehyde-polymer-bonded wood joints at crack initiation (G[C) and arrest (Gca)
were calculated from the observed loads at crack initiation and arrest. High fracture
energy means that a large amount of energy is required to initiate a crack, and this
suggests the existence o f a good joint. Alezksa and Beaumont [1975], Microcracking
such as debonding between fillers and matrix in composite material is an important
form of damage. Zhou and Lu (1991) studied the damage process induced by
19
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microcracks, its nucleation and extension process in particle-filled composite materials.
They derived a damage variable D from an analysis of the microcrack system, which
was a function o f microcrack size and density.
Zdenek et al. (1990) developed a random particle model for fracture of brittle
particle-filled composite materials. In this model the particles were assumed to be
elastic and have only axial interactions, as in a truss. The idea of particle simulation
was originally proposed by Cundall (1971) and Serrano and Rodrigues (1973). These
works dealt with rigid particles that interact by friction and simulate the behavior of
granular solids such as sand. An extension of this method was introduced by
Zubelewicz and Bazant (1987). In their model they neglected the shear and bending
interaction o f neighboring particles throughout their contact layers. It was verified that
random truss yielded a realistic strain-softening curve, but the study did not include the
fracture mechanics, nor the size effect aspects.
Polymer - matrix composites are normally not brittle and fracture parameters
determined using LEFM and initial notch depth methods do not represent the fracture
properties of these systems. Due to non-linear slow crack growth prior to peak stress
intensity in polymer composite materials, models with two or more parameters may be
more suitable to explain the fracture process. In order to establish the fracture criteria
and fracture toughness model for polymer composites made with polyethylene and pine
(wood) particles, it needs to be shown that tests on specimens with significantly
different dimensions and notch depth produce nearly similar results within experimental
errors.
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1.4. Objectives
Previous studies have shown that WPCs exhibit improved strength, dimensional
stability, and resistance to biodeterioration. The effect of wood particle size and its
concentration on the properties and the effect o f coupling agents on the interface
bonding, as well as fracture mechanism and fracture toughness of the WPCs have not
been thoroughly studied. Thus, the objectives of the present research are:
(i) to investigate and increase our understanding of the mechanical properties of
WPCs:
(il) to investigate the effects of coupling agents on the interface bonding in
WPCs and the development o f a direct method for wood-polvmer bonding
force measurement:
Cui) to study their impact fracture properties, and
(iv) to define fracture resistance behavior and fracture toughness of these
particulate-filled polvmer-matrix composites.
21
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CHAPTER 2
EXPERIMENTAL PROCEDURES
2.1. Common Characterization Techniques Used
In order to determine the mechanical properties and surface characteristics of
pine-HDPE composites, following characterization techniques were utilized.:
2.1.1. Scanning Electron Microscopy
A scanning electron microscope (SEM) and an environmental scanning electron
microscope (ESEM) were utilized to observe the fracture surfaces sample at room
temperature in a low vacuum o f about 5 mm Hg. Additionally an SEM was used to
observe the surface morphologies o f untreated and treated wood pieces and also the
fractured interface on polymer after the bending, tensile, impact and resistance tests.
Fracture surfaces were also viewed with a low power stereoscope to study the features of
fracture under different modes o f fracture.
2.1.2. Tensile Testing
A universal hydraulic tensile test unit MTS 810 was used for the tensile testing.
This unit was used for the mechanical property characterization, surface bonding force
and fracture toughness determination.
2.1.3. Instrumented Impact Testing
Impact fracture properties o f the WPCs were determined using an instrumented
Dynatup Impact Tester, Model 8225 . The instrumented impact testing was carried out to
determine damage resistance, maximum load sustained under impact, deflection to
maximum load, energy to maximum impact load, and total energy to fracture under
multiaxial high speed deformation.
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2.2. Experiments Pertaining To M echanical Properties
Mechanical properties of pine (\vood)-polyethvlene (polymer) were obtained
using specimen prepared using materials and preparation techniques as described below.
Polymer: High-density polyethylene (HDPE) 'EA60-007' in powder form, provided by
PAXON Chemical Company, Baton Rouge Louisiana, was selected in order to obtain
good homogeneity and uniformity o f the composites. Pelletized high density
polyethylene provided by Exxon Chemical Company was also selected as the polymer
base for the composites used to study the effect of coupling agents and surface
modification.
Wood: Southern pine (genus and species) was chosen in this study because it is widely
used in the production of creosote-preserved telephone post. Small wood chips with
various sizes and shapes were prepared using a commercial chipper. First, wood strips of
size 0.5”x l”x8” were cut from 2”xl0”x8" boards along the grain direction and then fed
into the chipper from which wood chips o f random sizes were produced. Then the wood
chips were soaked in 1 N sodium hydroxide (NaOH) solution for 24 hours, drained and
dried in an oven at 70° C for 48 hours.
2.2.1. Preparation of Wood-Poly mer Composites Samples
Three stage sifters with mesh sizes o f 0.5” , 0.25” , and 0.125” were used to sort
out the chips and discard the wood chips that could not pass through the sieve with the
largest mesh. Samples entrapped between the sieves of 0.5” and 0.25” mesh size were
designated as "large”, between the sieves o f 0.25” and 0.125” mesh size as "medium",
and those passing through the sieve with 0.125” opening as "‘small” . Figure 12 (a), (b)
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and (c) show the morphology o f the "small’’, "medium” and "large” wood chips,
respectively.
Rectangular WPC samples were prepared in a 5” x 13” x 0.75” steel mold with
removable lid. The mold was filled with pre-mixed wood and polymer and then heated in
a constant temperature furnace. The mold was then heated in a constant temperature
furnace at 155 °C for 3 hours. After the melting of the resin a pressure of 500 psi was
applied for 3 minutes, and the block was subsequently cooled to room temperature.
Resultant rectangular samples were hvdraulically pressed out of the molds and were later
cut into specific dimensions for testing.
The mechanical properties of the WPCs were determined by tensile testing, 3-
point bending and nail removal testing on an MTS tensile tester. All tests were
performed following guidelines specified in relevant ASTM standards. The samples for
tensile and bend tests were cut in compliance with ASTM D 1037-89 (for wood-base
fiber and particle panel materials) specification [ASTM 1989]. The strain rate in tensile
testing and the rate of head movement in bend testing were 0.2 inch/min. Prior to the
mechanical tests, conditioning treatment was undertaken on all the samples at 23±1°C
and 50% humidity for at least 12 hours. Four identical samples were tested in every
experiment and consistent results were averaged and reported.
2.3. Experiments Pertaining to Effect of Coupling Agents and SurfaceModification
Three different coupling agents (CA), provided by SIGMA-ALDR1CH Scientific,
were chosen. Following is a list of their names and their molecular formulas:
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OCHj
CA1: vinvltrimethoxvsilane: CH,0— Si—CH=CH2.
OCH3
O OCH,: : i
CA2: 3-(trim ethoxysilyl)propyi methacrylate: H2C=C—C—OCH2 CH2 CHt—Si—OCH3,! 1
CH3 OCHj
OCHj
CAS: 3-glycidoxypropvltrim ethoxysilane: CH,0— Si—CH2 CH: CH2OCH3.
OCH,
2.3.1. Preparation of Wood-Polymer Duplex Samples
The southern pine wood board was cut into rectangular plates of 2.5 cm x 2.5 cm x
15 cm and turned by lathe machine into round stocks of 2 cm dowels and 15 cm length.
ASTM 400 grit sand paper was used to polish the end sides of the woodstocks.
Two kinds of surface treatment including treating in 1 N sodium hydroxide
(NaOH) or 1 N sulfuric acid (H2SO4 ) solution and/or in coupling agents were provided on
polished samples. In the former treatment, samples were soaked in the solution (NaOH
or H:SO.t) for 24 hours, drained and then kept in an oven at 103° C for 24 hours to dry.
In the latter treatment, the samples were soaked in the coupling agent for 3 hours and
then dried in the oven at 103° C for 24 hours. All studied samples with different
treatments are listed in Table 1.
To produce wood-polymer interface for the measurement of bonding force
between the wood and the polymer, a Berstorff twin screw extruder model ZE25X28D
was used. An adapter, as shown in Figure 13, was machined such that its one end fitted
the entrance of the extruder and a woodstock could be inserted into it through the other
2 5
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end. During operation, the output temperature o f the extruder was kept at 215° C and the
pressure on discharge at the contact point was maintained between 0.7 and 0.8 MPa. The
melt polymer as well as the woodstock were pushed out through the end o f the adapter
where the woodstock was inserted. After the polymer was pushed through 2/3 length of
the adapter, the adapter was removed from the extruder and cooled down in a water bath
without wetting the sample inside. Then, the composite sample was pressed out using a
hydraulic jack from the side of polymer. The resultant duplex sample with a single
wood-polymer interface is shown in Figure 14. Two holes were drilled one on each side
o f the duplex sample in order to pin the sample for the tensile testing. The strain rate
used in tension was 0.2 in/min. and the stress at the point of fracture was considered as a
measure of the coupling force between the woodstock and the PE. Four identical samples
for each treatment were tested to obtain consistency and the average value of the
corresponding bonding force. An SEM was used to observe the surface morphologies of
untreated and treated wood pieces and also the fractured interface on polymer after the
tensile test.
2.4. Experiments Pertaining to Impact Fracture Properties
Vinyltrimethoxysilane, 98%, (CA1), was chosen as the working coupling agent
for this experiment due to its good performance.
2.4.1. Preparation of Composites Samples
Samples were prepared following the procedure described in the section 2.1.
However samples for these experiments, once chosen from among the blocks with most
uniform distribution o f wood chips, were sorted out and cut into 4” x 4” x 0.37” plates for
testing.
2b
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Impact fracture properties of the WPCs were determined using an instrumented
Dynatup Impact Tester, Model 8225 . The instrumented impact testing was carried out to
determine damage resistance, maximum load sustained under impact, deflection to
maximum load, energy to maximum impact load, and total energy to fracture under multi-
axial high speed deformation. Test specifications given in ASTM 3763-86 were followed,
which require an unsupported region beneath the specimen that is six times the tup diameter
in order for the failure mode to be multidimensional. The configuration (Figure 15) also
allows the falling tup to completely penetrate the specimen, relinquishing less than 33% of
its energy in the process to fracture of the specimen. A total cross-head hammer weight of
7.463 lb. was used in this experiment. The original rest position of this weight from the
point of impact on the specimen determines the velocity of the tup and the initial impact
energy of the test according to the kinetic energy equation v2 = 2gh, where v is the velocity,
g is the gravitational acceleration, and h is the free fall distance to the point of impact
During the fracture test, the instruments crosshead was raised to a maximum height of 48
inches to allow for the complete penetration of the tup through the specimens. Four
identical samples were tested in every experiment and nearlv-consistent results were
averaged. The scatter in the results obtained from different specimens for all properties
ranged between 2.3% and 4.6%.
2.5. Experiments Pertaining to Fracture Resistance
Rectangular WPC samples were prepared in a 5” x 13” x 5” steel mold with
removable lid. NaOH-treated medium size wood chips were first soaked in the coupling
agent and dried in a furnace at 70°C for 24 hours. They were then weighed and mixed with
polymer resin in suitable proportion to yield 50 vol.% o f wood chips in the composite and
27
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loaded in the mold. The mold was then heated in a constant temperature furnace at 155°C
for 3 hours. After the melting of the resin a pressure of 500 psi was applied for 3 minutes,
and the block was subsequently cooled to room temperature. Resultant rectangular samples
were hvdraulically pressed out o f the molds and cleared. Compact tension specimen
samples of size 2.5” x 2.4” x I” were cut from the block and the ones with the most uniform
distribution of wood chips were sorted out to be prepared as per ASTM E 399 83, [ASTM
1995]. For R-curve determination compact tension specimens of size* 4.5” x 6” x 1”
dimension were cut from the main block and the ones with the most uniform distribution of
wood chips were sorted out to be prepared for testing as per ASTM E 561-76 [ASTM 1995],
Straight-through notches were cut through the samples with crack plane orientation of S-T
as given in ASTM E 399-83. Final sample geometry for K[C and J[C determination is shown
in Figure 15 and for R-curve determination in Figure 16. The initial crack extensions for R
curve samples were produced by the edge of a razorblade to 0.213-inch depth. Due to the
randomized nature of the method used to prepare samples, as described earlier, wood
particles are assumed to be randomly distributed in the matrix with no preferred orientation.
Initial fatigue cycling was used as prescribed in the standards in order to initiate and locate
the pre-crack at the root of the notch.
’ Note that the dimensions o f specimens used in this study do not conform to the recommendations given in the standard.
28
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(b)
Figure 11 The morphology o f pine (wood) chips used to produce WPCs; (a) small, (b) medium and (c) large
29
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(C)
Figure 1
30
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j .U cm
h H
u > oHO cm [D
0.6 cn- ^eei r:ia re- oc-d
16.0 cn!.0 cn
i.0cn iO tubing for* wood dowels
ro copper tubing For support
Figure 12. Schematic diagram of an adapter used for the production of composite samples of pine (vvood)-PE
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Figure 13. The resultant composite sample used for tensile testing
32
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L
~ ^ 3 ~
tt
J
(a) (b)
(d)
(c)
Figure 14. Schematic diagram o f (a) the drop weight impact tester (b) condition at the instant of impact, (c) specimen holder and (d) specimen to be fitted between the holder plates in (c)
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Figure 15. The dimensions o f compact tension specimens used for KIc and J[c determination
34
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c
Figure 16. The dimensions of compact tension specimens used for R curve determination
35
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CHAPTER3
RESULTS AND DISCUSSION:STATIC MECHANICAL PROPERTIES OF PINE CHIP - HDPE COMPOSITES
IN TENSION AND BENDING
3.1. Tensile Mechanical Properties
Tensile mechanical properties o f the WPCs were studied initially as a part o f
this investigation. The average tensile properties obtained are listed in Table 1. Figure
17 and 18 show the experimental results o f ultimate tensile strength (UTS) and break
strain. Regardless of wood chip size in the composites, there was a sharp decrease in
UTS of the composites in the range o f 0 to 40% wood chip concentration and no
significant further change in this property in the range o f 40% to 60% wood chip
concentration. Also, a smaller decrease in UTS was found in the composites with
smaller wood chips. The break strain changed in a similar manner for all composites. It
showed significant reduction in the range of 0 to 40% chip concentration and no further
change thereafter. It can be concluded from these results that the introduction of wood
chips to polymer significantly deteriorated the tensile properties. The experimental
results suggest that tensile applications of composites with more than 40% wood chips
concentration should be limited.
3.2. 3-Point Bending Testing
The peak load, modulus o f rupture (MOR) and stiffness o f WPCs were
determined by 3-point bend testing, as shown in Figure 19-1, 19-2 and 19-3. In the
concentration range between 0 and 40%, there were trivial changes in peak load and
MOR, but stiffness increased significantly. Between 40% and 50% wood chip
concentration, a slight decrease in peak load and MOR was found for the composites
36
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with small and medium wood chips. But, there was a serious decrease in these
parameters for the composite with large wood chips. In the same concentration range,
stiffness was found to increase slightly for the composite with small wood chips,
decrease slightly with medium wood chips and decrease appreciably with large wood
chips. Afterwards, these three parameters decrease slightly for the composites with
small and medium wood chips, but no further change was found for large wood chips.
3.3. Nail Removal Testing
Figure 20 shows the experimental results of nail-removal testing. The nail-
removal characteristic is very important if the present composite is to be used as a
construction material. It is found that in the concentration range between 0 and 50% the
nail pull-out force is increased by the increasing wood chips concentration (except for a
small decrease for the composite with large wood chips in the concentration range
between 40% and 50%). The highest increase in the pull-out-force occurs in the small
wood chip composite with around 50% wood chip concentration. In the range of 50%
to 60% wood chip concentration, the experimental results show a decrease in the nail
pull-out force for all composites.
Thus, the mechanical properties o f the WPCs are found to vary with
concentration and size o f the wood chips in the composites. In these composites, wood
chips are found encapsulated in a matrix o f flexible polymer and aligned parallel to the
surface by the compressive force applied to the composite after melting. As a result,
there is no binding between two adjacent wood chips and the thickness o f the polymer
ligament between two adjacent wood chips would depend on the wood chip
concentration in the mixture. In the process o f external tension, the difference in tensile
37
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deformation between wood chips and polymer matrix would result in the separation or
cracking along the interface. The initiation and propagation of cracks cause the
degradation o f tensile strength and elongation.
The morphology o f fracture surface of a stretched and broken sample is shown
in Figure 21. it could be inferred from the features observed that fracture o f the tensed
sample occurred by the initiation of cracks due to debonding at the interface and then
propagation of the cracks along the interfaces. Based on this fact, it can be inferred that
the tensile properties o f the composite largely depend on the bonding forces between
the wood and polymer, which have been deduced, from the present results, to be lower
than the tensile strengths of individual wood and polymer respectively (otherwise the
latter would have broken first). For the composite with larger wood chips in the same
concentration range, more severe degradation in tensile properties was found (Figure
18-1) due to larger deformation mismatch on the wood-polvmer interface and larger
continuous interface area over which the crack can propagate.
The reason for the bonding strength on interface to be lower than the strengths
of individual polymer and wood is more than likely the presence of residual stress on
the interfaces and the weak bonding between the wood and polymer. The composite
with fine wood chips is expected to have lower residual stress on the interface and
possess higher UTS, compared with the ones with medium and large wood chips. This
has been confirmed by experimental results indicating that for WPC with smaller chips,
there is a smaller decreasing rate of UTS, Figure 17. It has been suggested that the
tensile properties of WPC can be improved by the addition of a compatibilizer or a
coupling agent to enhance the bonding force between the wood chips and the polymer
38
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matrix, or by the development of a thin polymer layer on the surface o f the composite
material in order to decrease or eliminate possible sites for the initiation of cracks on the
surface, Maldas et ai. (1990).
In the compression condition of 3-point-bending, wood chips in the composites
tend to compress against each other as well as against the polymer matrix. Because of
the bending strain, the polymer matrix will not fracture before a wood chip starts sliding
with respect to adjacent chips. The wood chips in polymer produce extra internal
frictional forces that restrict deformation. Large external compressive forces are
required to initiate sliding. As a result, the encapsulated wood chips in polymer matrix
act as obstacles to compressive deformation o f bulk composite material, w hich explains
the enhanced properties during the compression of the composite with low wood chip
concentration. However, the compressive properties o f composites would deteriorate in
the high concentration range since excessive amounts of wood chips, particularly with
large size, would cause brittleness of the bulk composite.
As shown in Figure 20, smallest wood chips result in largest improvement in
nail pull-out-force of the polymeric composite. It is found from the experimental results
that the addition of wood chips in polymer matrix produces improvement in MOR due
to the extra obstacle to plastic deformation by the wood chips. Also, it has been seen
that the composites with small wood chips obtain largest improvement in MOR. When
a nail is forced into the composite, it produces a compressive strain field in its vicinity.
The matrix with higher MOR will provide larger vertical holding force onto the nail,
which is equivalent to producing a higher friction force on the nail during the pull-out
process. As a result, the nail pull-out-force of the composites with wood chip
39
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concentration less than a certain value (50% in this research) increases with increasing
wood chip concentration, compared with that o f plain polymer base. Also, small wood
chips create high modification in the nail pull-out force. However, high concentration
(>50% in this research ) o f wood chip in composite appears to make the structure more
brittle, as stated above, resulting in the decreased nail pull-out force.
Considering the overall correlation between chip size, its concentration and the
mechanical properties of WPCs. the suggested parameters to produce the composite
should be wood chip size of <0.125'" and wood chip concentration of around 40%.
Specific surface treatments for improving the bonding and interfacial properties are
addressed in the next section.
Table 1Mechanical properties o f southern pine (wood)-polyethvlene (polymer) composites with
various wood chip sizes and concentrations
Wood Chip
Size/Concentration
UTS
(psi)
±5.2%
% Strain at
Break
±6.1%
Peak Load
(lb)
±5.5
MOR
(psi)
±~.5%
Stiffness ij
psi ( 10J) j
±6.1% |
i
Nail Pull
out Force
(lb)
-5.4%
0% 2918 3.9 80 3626 165 iI
28.1
Small 40% 1933 1.07 81.5 40243,5 i
30.8
<0.125 in 50% 1578 1.03 77.2 3742 323 | 31.9
60% 1000 1.05 71 2950 280 j 28.9
Medium 40% 1330 1.13 76.6 3386 304 !1 29 2
>0.125 and 50% 1206 0.72 69.2 2958 291 j 29.5
<0.25 in 60% 921 1.07 62.3 2397 283 | 28.8
Large 40% 1151 1.07 73.4 3419 269 | 29.9
>0.25 and 50% 941 1.03 51.8 2066 245 | 29.4
<0.5 in 60% 750 0.70 51.1 1776 239 j 22.3
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Specimens with small wood chips
350C
3000 --------------------------------------------------------------------------------
2500 • ’ * — --------------------------------------------------
2000 -----------------
1500 -----------------------------------------1000 i---------------------------------------------500 ------------------------------------------------------------------------------------
20 30 40 50 60 70
Wood Concentration
Specimens with medium wood chips
2500 ,
3000 £ --------------------------------------------------------------------------------------------------
2500 j------------ ^ -----=----------------------------------------------------------------------------
"3* a.<nH-
W ood Chip Concentration
200Q 4—
1500 t - ~X------1000 I -
500 t -
o i-
t r r
500
0
Specimens with large wood chips
-XT
20 30 40 50
wood Chip Concentraoon
Figure 17. Effect o f pine (wood) chip size and concentration on UTS o f Pine-HDPE composites.
41
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Specim ens with sm all w ood ch ips
Figure 18.
m 4 F—<x>
® 3(9
•£ 2m *
0 --------------------------------------------------------------------------------0% 10% 20% 30% 40% 50% 60% 70%
Wood Chip C oncentration
(Q<Uk.meo•f 2<u
5 -
4 C—3 ------
V)
S pecim ens with m edium w ood chips
Q i--------------------------------------------------------------------------------0% 10% 20% 30% 40% 50% 60% 70%
Wood C oncentration
<n 4 F— « L - - .
5 3
■§ 2 (03? '
0
Specim ens with large w ood chips
0% 10% 20% 30% 40% 50% 60% 70%Wood Chip C oncentration
Effect of pine (wood) chip size and concentration on break strain of Pine- HDPE composites.
42
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Figure 19-1
Specimens with small wood chips
90
a 80 £-1 70 -
60 f-<s oQ- 50
40
t -
0% 10% 20% 30% 40% 50%Wood Chip Concentration
60% 70%
90
15 7001 60 aoo- 50
40
Specimens with medium wood chips
§ : [ " i .
0% 20% 40% 60% 80%Wood Chips Concentration
Specimens with large wood chips
90
1 80H 70 - o^ 60(30)Q- 50
400% 20% 40% 60%
Wood Chip Concentration80%
. Effects o f pine (wood) chip size and concentration on peak load of pine- HDPE composites
43
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Specimens with small wood chips
5000
4000
J. 3000
O 2000 stooo
00% 10% 20% 30% 40% 50%
Wood Chip Concentration60% 70%
Spemens with medium wood chips
5000 ,
4000 £-----------------------------------------------------
\ 3000 i -------------------------- ---------------------------a. ! * * 3O 2000 f --------------------------------------------------------- -s ‘
1000 \---------------------------------------------------------
0% 20% 40% 60% 80%Wood Chip Concentration
Specimens with large wood chips
5000
4000
q. 3000
O 2000s
1000
0
- I
0% 20% 40% 60%Wood Chip Concentration
80%
Figure 19-2. Effects of pine (wood) chip size and concentration on MOR of pine- HDPE composites
44
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Specimens with samll wood chips
— 390 n0 340 —
•S 290 - &« 240 —VI0)1 190 ~33 .140 —
0% 20% 40% 60% 80%Wood Chip Concentration
_ 340C l
° 290X«A& 240(A(A
£ 1903=
Specimens with medium wood chips
140 —
7 ^ 4 - - I -
0% 20% 40% 60% 80%Wood Chip Concentration
Specimens with large wood chips
_ 340c l<
Z 290 -------------------------------------------------------------------------
% 240 --------------------- ------------------ — I — J ----------------(A - *(A<H 1 9 0 -------------------------------------------------------------------------
i e * '» 140 -------------------------------------------------------------------------
0% 20% 40% 60% 80%Wood Chip Concentration
Figure 19-3. Effects of pine (wood) chip size and concentration on stiffness of pine- HDPE composites in bending
45
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S pecim ens w ith sm all wood chips
36_ 34 ---------------§ 32 -5 30 =- o3O. 26"5 24 -z 22
200% 10% 20% 30% 40% 50% 60% 70%
W ood Chip C oncentration
Specim ens w ith m edium w ood chipst
32
S’ 30¥ 28= 26 3Q. 24I 22
200% 10% 20% 30% 40% 50% 60% 70%
Wood Chip Concentration
S pecim ens w ith large w ood chips
1 - • i
1 1 -
10% 20% 30% 40% 50% 60% 70%
W ood Chip C oncentration
Figure 20. Effects o f pine (wood) chip size and concentration on nail removal force of pine-HDPE composites
46
<oz
3432302826242220
0%
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Figure 21. The morphology o f the fracture surface of a stretched and broken specimen.
47
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CHAPTER 4
RESULTS AND DISCUSSION:INTERFACE MODIFICATION AND INTERFACE BONDING IN PINE WOOD
- HDPE COMPOSITES
4.1 The Influence of Pre-treatment in NaOH or H2SO4 on the Bond StrengthBetween Wood and Polymer
The experimental data for bond strength under each treatment condition is
shown in Table 2. A significant increase in coupling strength between the woodstock
and polymer is found for the sample in which the woodstock was treated in the solution
o f NaOH, comparing sample 5 with sample 1, and samples 7, 8 , 9 with samples 2, 3, 4,
respectively. The treatment with the solution of H2SO4 shows detrimental effect on the
bonding strength, comparing sample 6 with sample 1 and samples 1 0 , 1 1 , 1 2 with
samples 2, 3, 4. Sample 7 with NaOH first treatment and using CA 1 next shows the
highest value of coupling strength, w'hile H2SO4 solution treated sample with CA2
further treatment show's the lowest one in the value.
Pre-treatment in the solutions of NaOH and H2SO4 can also change the surface
morphology of woodstock. The surface morphology of untreated wood sample is
shown in Figure 22 w'hich exhibits the cellulose property w'ith tiny fibers protruding out
of the wood matrix. The treatment of woodstock in solution of NaOH produces
straight, well aligned and unidirectional wood fibers on the original wood surface,
Figures 23 (a). How'ever, the treatment in solution of H2SO4 produces curved,
unaligned and scattered wood fibers, Figure 23 (b). In addition to interfacial acid/base
interaction force, the propagated wood fibers penetrating into melt PE during extruding
process can produce extra mechanical bonding after polymerization. The improvement
in bond strength by the solution o f NaOH and the decline by the solution of H2SO4 have
4S
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been verified by the experimental results. Because of acid/base interaction (even
though not dominant) and, more importantly, mechanical bonding through wood fibers,
the pre-treatment o f wood in the solution o f basicity can be a consideration of merit to
enhance the bond strength between the wood and PE. This fact has been supported by
comparing experimental data of samples 7, 8 and 9 with those of samples 2, 3 and 4.
On the other hand, it is concluded that pretreatment in the acidic solution should be
avoided due to its detrimental effect, comparing experimental results of samples 10, 11
and 12 with those o f samples 2, 3 and 4.
4.2 The Influence of Coupling Agents on the Bond Strength Between Wood andPolymer
From Table 2, it is found that CA1 has significant influence on the bond
strength, while the influence of CA2 and CA3 on bond strength is trivial.
Usually, coupling agents which are expected to produce some functional groups
on the wood surface can be used as surface modification materials. The chemical
interactions among the components on the interface rely on the chemical bonding
interactions of functional groups in the polymer with complementary functional groups
produced on the treated wood surface. It is known that processing of materials with
coupling agents which increases surface functional groups and surface roughness or
eliminate surface defects on the wood can improve interfacial adherence, Maiti et al.
(1989).
In brief, the coupling agents modify the coupling strength between wood and
polymer by increasing the adherence and bonding between these two materials.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.3 The Influence of Surface Morphology of W oodstock on the Bond StrengthBetween Wood and Polymer
It has been found that the protruding wood fibers are beneficial for the
improvement of bond strength between woodstock and PE due to the formation of
mechanical bonding. The treatment in the solution o f NaOH produces sharp, well
aligned and unidirectional wood fibers on the surface o f woodstocks which increases
the bonding strength between woodstock and polymer, comparing sample 5 with sample
1. Under this condition, further improvement can be created by treating with coupling
agents, comparing samples 7, 8 and 9 with sample 5. The surface morphologies o f
samples 7, 8 and 9 are given in Figure 24 (a), (b) and (c). It is found that the common
feature of these three surfaces is well grown and oriented wood fibers. Coupling agents
may not have dominant influence on the surface morphology, but enhance the bonding
strength by possible chemical reactions between wood and polymer, as mentioned
above.
In contrast, the treatment in the solution of H2 SO4 produces curved, unaligned
and scattered wood fibers which decreases the bonding strength between wood and
polymer, comparing sample 6 with sample I. Further treatment in CA1 restores the
bonding strength back to the value of untreated sample, comparing sample 1 0 with
sample 6 , but treatments in CA2 and CA3 weaken the bonding strength, comparing
samples 11 and 12 with sample 6 , due to the special interactions between CA2, CA3
and polymer, as discussed above. Figure 25 (a), (b) and (c) show the surface
morphologies of samples 10, 11 and 12. It is found that a relatively thick and flat layer
on the surface of wood is formed after treatments in H 2 S O 4 and coupling agents CA1
and CA2, which may be attributed to the reaction between H2SO4 and the coupling
50
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agents. The formed layer covers the surface of woodstock completely in sample 11
(Figure 25 (b)) and prevents penetration of wood fibers into polymer. Thus, the
treatment in the solution of H2SO4 worsens the coupling strength between wood and
polymer, especially in sample 1 1 .
The typical fracture interface on PE in sample 1 includes distributed tiny pores.
It is found through careful observation that these pores are produced due to wood fiber
pulling-out from polymer matrix during the tensile process. The corresponding low
bonding strength tested on sample 1 is more than likely attributed to the weak adherence
between wood and polymer.
Comparing with that o f sample 1, the fracture surface of sample 7 shows the
broken wood fibers embedded in PE matrix. The consequential improvement in
bonding strength indicates better adherence between wood and polymer caused by pre
treatment (solution of NaOH) and application o f coupling agent (CA1). This
observation reveals that good adherence of penetrating wood fibers into PE matrix
strengthens the mechanical bonding between wood and polymer. As a result, the
coupling strength between the woodstock and polymer increases significantly, as
verified by experimental data.
51
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Reproduced
with perm
ission of the
copyright ow
ner. Further
reproduction prohibited
without
permission.
Table 2Summary of different wood surface treatments and experimental coupling strengths between woodstock and polymer in southern pine-high density polyethylene coupled
samples
Sample No. NaOH H2S04 CA1 CA2 CA3 Bonding Strength*
(MPa)
1 1.36
2 Y 2.06
3 Y 1.50
4 Y 1.37
5 Y 2.51
6 Y 1.05
7 Y Y 3.28
8 Y Y 2.75
9 Y Y 2.61
10 Y Y 1.36
11 Y Y 0,593
12 Y Y 0.828
Y: corresponding treatment
*:fracture stress at the wood-polymer interface
Figure 22. The surface morphology o f untreated pine (wood)) sample.
53
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(b)
Figure 23. The surface morphology of pine samples treated in the solutions of (a) NaOH and (b) H2S 0 4
54
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(a)
(b)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(c)
Figure 24. The surface morphology of pine (wood) samples treated with NaOH and then with (a) CA1, (b) CA2, and (c) CAS
I j MJ HD 4 8nn S 009 0 3 P 06001
(a)
56
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fc)
Figure 25. The surface morphology o f pine (wood) samples treated with H2 SO4 and then with (a) CA1, (b) CA2, and (c) CA3
5 7
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CHAPTER 5
RESULTS AND DISCUSSION: IMPACT FRACTURE PROPERTIES O F PINE CHIP - HDPE COMPOSITES
5.1. Instrumented Impact Testing
Figure 26 depicts typical curves obtained for load vs. deflection and energy vs.
deflection for a pine chip-HDPE composite specimen subjected to tup velocity and impact
energy of 14.22 ft/s (4.33 M/S) and 23.45 ft-Ib.(31.77 J), respectively. The highest point
on the curve, designated the maximum load, corresponds to the onset of material damage
or complete failure. The x-coordinate of this point corresponds to the deflection to
maximum load, which is the distance the impactor travels from the point of impact on the
specimen to the point of maximum load. A more damage resistant material will have a
higher peak; in other words, the value of the maximum load is a function o f the damage
resistance of a material. The energy value corresponding to its deflection point is the
energy to maximum load and represents the amount of energy that is absorbed by material
to be deformed prior to damage initiation.
j2L o a d -r
.Energy
I o k O0.40 °0 04 0 16 0.20
Ch 1 aetlection finch iQ.2i 0.32 0.360.00
Figure 26. Load vs. deflection and energy vs. deflection curves for specimen with small pine (wood) chips and 60% concentration.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 27 shows sample plots of load vs. time and energy vs. time during the test
for wood-polvmer composites. The highest point on the load-time plot also represents the
maximum load. The energy value corresponding to the same x-coordinate as the
maximum load on the energv-time plot is the energy to maximum load, which is the
energy absorbed by the specimen up to the point o f maximum load. The values
corresponding to the maxima and their locations can be compared for different composites
to ascertain their fracture resistances. Critical values corresponding to the peak in these
plots were obtained from impact tests conducted on different WPC samples. The average
impact fracture properties obtained are listed in Table 5 for WPC samples with various
sizes and volume fractions o f wood chips.
Load
P
OQ0.00 0 25 0.50 2.00 2.25
Figure 27. Load vs. time and energy vs. time curves for a specimen with small pine (wood) chips and 60% concentration.
Results indicate that low velocity impact properties of the wood-polymer
composites vary with the concentration and size of the wood chips in the composites.
5.2. Experimental Results
Figure 28 shows plots of comparative values for the maximum load in the drop
weight test for the pine-HDPE WP composites. The experimental results indicated that
regardless of wood chip size and its concentration in the composites, there was an increase
59
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initially in the maximum load sustained by all composites with increase in wood content.
This is in agreement with the resistance to sliding offered by the wood chips. Composites
containing small wood chips showed consistent increase in the maximum load by about
40% up to 50% concentration and then the value dropped sharply to a level 25% below that
of the polyethylene matrix itself. Composites made of medium wood chips exhibited an
increase in maximum load by about 50% up to 40% concentration, beyond which there
was a small decrease by about 10%. Composites with large wood chips, on the contrary,
showed a steady increase in the maximum load with increase in wood fraction. The
highest value for the maximum load was exhibited by samples made o f large wood chips
with 60% concentration. These samples sustained a maximum load slightly more than
twice that of the polyethylene itself, improving it by about 130%
In wood-poiymer composites, wood chips are individually encapsulated in a matrix
of flexible polymer and are mostly aligned parallel to the surface by the compressive force
applied to the composite after melting o f the polymer. The average thickness of the
polymer layer between two adjacent wood chips would depend on the wood chip size and
concentration in the mixture. Higher concentration of wood chips in composite would
generally result in a thinner polymer layer between wood particles. With the advent of
external forces, separation or cracking along the generally weak polymer-wood interface
would result. The initiation and propagation of resultant cracks due to delamination
contributes to a weakening of the composite. Crack propagation would be facilitated if the
particles are aligned preferentially.
The morphology of a typical fracture surface of an impact failure sample is shown
in Figure 29. It can be inferred from the features observed that the fracture of the impact
60
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sample occurred by the initiation of cracks due to debonding at the interface and
propagation of resultant cracks along the weak interface. This observation implies that the
impact properties of the composite partially depend on the bonding forces between the
wood and polymer.
Table 3Drop weight impact fracture properties o f southern pine (wood l-polyethvlene
(polymer) composites with various wood chip sizes and concentrations.
Average' A verage A verage A verage
Wood Chip Max. Load Deflection a t Max. Energy to Max. Total Ener
Size/Concentration Load Load
Vol% (lb.) (inch) (Jt-lh.) (ft-lb.)
-5.4% - 3.9% =3.5% =4.3%
0% 278 0.290 4.1 9.8
Small 40% 348 0.255 4.6 7.3
<0.125 in 50% 395 0.200 4.2 10.9
60% 210 0.160 1.7 3.9
Medium 40% 610 0.250 4.5 10.8
>0.125 and 50% 378 0.220 4.9 9.4
<0.25 in 60% 376 0.145 3.0 7.1
Large 40% 441 0.270 4.1 9.7
>0.25 and 50% 530 0.195 4 2 7.6
<0.5 in 60% 649 0.190 5.0 21.4
* The values given are the average o f consistent data in four tests for each case. The maximum
deviation (scatter in results) is indicated for each property. The exact deviation for each value
is shown by bars in the corresponding plot in Figures 5. 7. 8, and 9 (Note: I ft.lb = 1.356 J)
61
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Specimens with small wood chips
| too ------------------------------------------------------------------tm0)< 0 -----------------------------------------------------------------------------
0% 10% 20% 30% 40% 50% 60% 70%Wood Chip Concentration
Specimens with medium wood chips
■O 500 - ^o Xe 400 :---------------E _ 300X AE 200
I* 100WO< o
0% 10% 20% 30% 40% 50% 60% 70%Wood Chip Concentration
Specimens with large wood chips
■g 800 ••0 700 --------------------------------------------------------E 600 ---------------------------------------------------------------1 _ 500 ~ rX £ 400 ................................ — ............ ...............E 300 c . ~ ' --------------------------------------------Si 200 -------------------------------------------------------------------| 100 -----------------------------------< 0 --------------------------------------------------------
0% 10% 20% 30% 40% 50% 60% 70%Wood Chip Concentration
Figure 28. Effect o f pine (wood) chip size and concentration on maximum load sustained under impact
62
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Figure 29. SEM ffactograph of a WPC specimen with large wood chips and 60% volume concentration. One large wood chip with a layer of peeled polymer hanging on it (top) is seen.
Results obtained from impact fracture tests indicate that for the same concentration
o f wood chips, fewer larger particles would offer better resistance to compressive
deformation than large number o f fine particles in the particularly for concentrations
beyond 50%. The impact properties o f composites improve in the higher wood chip
concentration range of larger particles, since perhaps larger amounts o f wood chips would
offer more obstacles for compressive deformation, crack growth and fracture. On the
contrary, finer particles will not be able to resist the deformation o f the matrix, and though
their introduction into the composite strengthens it initially at lower concentration, easy
crack propagation seems to result at higher concentrations (at ~ 60%) contributing to a
63
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reduction in the maximum load sustained, to a level lower than that o f the matrix polymer
itself.
The deflection at maximum load. Figure 30, shows a consistent decrease of
deflection at maximum load for all composites as wood chip concentration is increased.
The deflection at maximum load starts to level off for samples with large wood chips at
concentrations in excess of 50%. From this general result it can be deduced that impact
fracture would initiate in general more easily with less deflection when the wood fraction
is increased in this kind of WPCs. Only in the case of composites with large size wood
chips does reduced deflection appear to be an indication of an increase in rigidity, since the
maximum load sustained (damage resistance) keeps on increasing with increasing wood
fraction.
53 . Energy to Cause Failure
Figures 31 and 32 show the results for the energy to maximum load and total
energy to failure under the given set of experimental conditions. The total energy to
failure represents the energy that the material would absorb until full penetration of the
impactor. These parameters are determined from measuring the relevant areas under the
load-deflection curve by the data analysis program in the computer attached to the impact
tester. Evaluation of this parameter begins at the initial contact between the impactor and
the sample and continues beyond until complete penetration by the tup is achieved. Figure
31 indicates for small and medium chips that the energy to maximum load remains nearly
the same up to 50% wood chips addition to polymer for all the composites studied.
Addition of more small or medium wood chips leads to a deterioration of this value and
there is a recognizable drop in the range of concentration 50 to 60%. Addition of large
64
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wood chips, on the contrary, showed a noticeable rise in the energy in the range of 50% to
60%. This is attributed to the increase in maximum load at nearly constant deflection in
this range of composition.
The total energy absorbed by the wood-polvmer composites for total penetration
by the tup indicates likewise little or no improvement relative to plain polymer up to 50%
wood concentration (Figure 32). The addition o f wood chip particles of all sizes in the
composites seems not to affect the total energy significantly up to 50% concentration of
wood chips, but a rather large drop by about 60% is seen for the composites with small
wood particles in the 50 to 60% concentration range. Only the composites with the large
wood chips show an increase (nearly double) in the concentration range of 50% to 60%.
Considering the larger external force required for the slipping of the wood chips in the
matrix at 60% concentration (Fig 28) and the fact that inclusion of larger wood chips
would present larger contact surfaces with larger friction energy to overcome, one can
explain the increase of energy to maximum load and the total energy absorbed in these
WPC composites with large wood chips at higher concentrations in the 50 to 60% range.
Further work is needed to ascertain whether any other factors influenced the drastic
increase in the impact fracture energy encountered in the composites containing large
wood chips and paradoxically the large reduction in fracture energy in composites with the
small wood chips in the 50 to 60% concentration range. Nevertheless, it is clearly
demonstrated that whereas the WPCs containing 60% small wood chips would fail more
easily than the matrix polymer itself, those containing 60% large wood chips would resist
impact fracture far more efficiently (at least four times better than the analogous
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
composites with small wood chips) and those containing medium size wood chips would
show minimal effect.
5.4. Mechanism for Energy Absorption and Fracture Morphology
Earlier works involving the evaluation of tensile properties of WPC composites
indicated that the addition of 60% large wood chips to polymer matrix deteriorated the
tensile properties of these composites, Razi et al. (1997). In contrast, as we have shown
here, the impact properties of these composites seem to be improved drastically for
samples with 60% concentration o f large wood chips. The initial failure mechanism
during the tensile failure was due to the debonding at the wood chip - polymer interfaces
and consequent crack initiation. Once initiated, the crack grows more easily over the
entire interface of wood chip and polymer as the specimen is loaded in tension. The only
material resisting the complete separation into two pieces, the polymer matrix at the cross
section, experiences higher level of stress due to the reduced effective area, and, as a result,
fails easily. During the impact test, however, wood chips would start sliding in the soft
polymer matrix more easily and concurrently debonding would occur due to the difference
in strain rates of wood and polymer. Smaller wood chips can be expected to slide more
easily, though in this process they may enable easy crack propagation through
reorientation and alignment o f the delaminated interfaces. In samples with 60%
concentration o f large wood chips, however, there seems to be a greater resistance for
sliding and when sliding is initiated there would also be a possibility of chip-to-chip
interference. This interference of large wood chips would cause increased resistance to
deformation and necessitate higher level of stress and energy to cause fracture. Such
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interference does not seem to occur in composites with small wood chips and these appear
to sustain easy crack extension in a continuous manner.
Figures 33-1 and 33-2 show the fracture features at the top and the bottom faces of
tup-penetrated samples containing 60% small and large size wood particles. The
photographs in figure 33-1 corresponding to the specimen containing small wood chips
show a perfectly circular penetration hole both at the top and bottom faces. The surface of
the hole in the penetrated region is also rather smooth and clear-cut These features are
indicative of least resistance to fracture and ability of the material to be punched rather
smoothly. On the contrary, in the case o f an analogous specimen with large wood chips
the hole is somewhat elongated and oval-shaped, and its surface in the penetration zone is
also rugged and non-uniform. Individual wood chips can also be seen in the photograph of
the hole taken at the bottom surface of the specimen, Figure 33-2. Considering that the
specimen plate bends downward when the hit is made and because of tensile stressing
cracks that would formed at the bottom surface, the features observed are indicative of
increased resistance offered for the propagation of the crack and a more tear-type hole
being formed. These observations strengthen the discussion given earlier that the coarse
chips at 60% concentration would enable an increase in the resistance to fracture, whereas
analogous composites with 60% fine chips would break very easily.
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Effect of Large Wood Chip C oncentration on A verage Deflection a t Maximum Load
~ 0 .4 -eo —= .££ V 0 .3 c rU <sQl o ? J 0 2 - a E§> E 0.1 — 2 x 0) (B
o -0% 10% 20% 30% 40% 50% 60% 70%
Wood Chip C oncentration
Effect of M edium Wood Chip C oncentration on Average Deflection a t Maximum Load
0.4c *o.2 g 0.3U _1 .2c E 0 2« 3° EI » °'1 £ 0
• I - ___
0% 10% 20% 30% 40% 50% 60% 70%
Wood Chip C oncentration
Effect of Sm all Wood Chip C oncentration on Average Deflection at Maximum Load
0.4
•2 <o 0.3 u o 0>« | 0.2 » 1S’ ra 0.12 s 4>< 0
0% 10% 20% 30% 40% 50% 60% 70%
Wood Chip C oncentration
Figure 30. Effect of pine (wood) chip size and concentration on deflection at maximum load sustained
68
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Specimens with small wood chips
c
4
3
2
1
010% 20% 40% 70%0% 30% 50% 60%
Wood Chip Concentration
Specimens with medium wood chips
6n o — 5
4
3OS 3
2P 3
1
020% 30% 40% 70% i10% 50% 60%0%
Wood Chip Concentration
Specim es with large wood chips
_ 6 3 ? 52* -a 40) (ISc oin _i -j
§ O ,2 e 1o> 2 1 -
< « 10 ■
0% 10% 20% 30% 40% 50% 60% 70%
Wood Chip Concentration
Figure 31. Effect of pine (wood) chip size and concentration on energy to maximum load sustained
69
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Specimens with sm all wood chips
g> 10 -o • c £ a 114 3 8 __ ^
I -
<10%0% 20% 40% 50% 70%30% 60%
Wood Chip Concentration
Specimens with medium wood chips
2 12>»S’<u ~ c £
— £12 1 o>m5v><
10 -r.
£ 4
.....................-I.
0% 10% 20% 30% 40% 50%Wood Chip Concentration
60% 70%
Specimens with large wood chips
25 r
= £ MJ - T- £
10 -e-
<40% 50% 70%0% 10% 60%20% 30%
Wood Chip Concentration
Figure 32. Effect of pine (wood) chip size and concentration on total energy to failure by tup penetration
70
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(b)
Figure 33-1. The morphology o f fractured surfaces of samples broken in drop weight impact test: specimen with 60% volume concentration of small wood chips (a) top surface (b) bottom surface
71
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(b)
Figure 33-2. The morphology o f fractured surfaces of samples broken in drop weight impact test: specimen with 60% volume concentration o f large wood chips (a) top surface, and (b) bottom surface.
72
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CHAPTER 6
RESULTS AND DISCUSSION:FRACTURE RESISTANCE AND FRACTURE TOUGHNESS OF PINE CHIP -
HDPE COMPOSITES
Standard procedures described in various ASTM methods were used to
characterize the fracture properties of wood-polymer samples in compact tension type
specimens. The applicability of these procedures to analyze the fracture behavior o f the
wood chip reinforced polvmer-matrix composites is probed into in this chapter. An effort
is made to define the fracture toughness Krc for such composites.
6.1. Kic Determination Using ASTM Standard Procedure
The fiacture toughness, Kic, was determined following the methodology outlined in
ASTM E 399-83, ASTM (1995). The compact tension specimen, C(T)(S-T), used, its size
and configuration are shown in Figure 16. The pre-crack extension o f the notch tip
produced by fatigue was 3.6 mm. Pretest compliance was 0.0056562 mm2/N. Ramp rate
was chosen to be 40 N/sec. The summary o f test results is given in Table 4. As
recommended by the standard, displacement measurements on the load line were carried
out by a double-cantilever clip-in displacement gage. Each experiment was repeated at
least three times with three different samples and the results obtained were averaged. The
average results were then reported.
6.2. Jic Determination Using ASTM Standard Procedure
The Jic measure of fracture toughness determination followed the methodology
outlined in ASTM E 813-89, ASTM (1995). Similar compact tension specimens, C(TXS-
T,), as used in Krc determination described above, were also used in these evaluations. The
summary of test results for Jic determination is given in Table 5. Displacement
73
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measurements on the load line were again obtained by a double-cantilever clip-in
displacement gage
6 J . R curve Determination
The resistance R-curve was obtained by applying uniaxial tensile loading on the
compact tension specimens. Slightly larger samples were prepared for these tests due to
the fact that the results were being analyzed to the fullest extent o f material resistance.
Larger samples provided more material and longer test time to study the reaction of
samples to the test variables. For these series of tests, the methodology outlined in ASTM
E 561-76T, ASTM (1995), was followed. Figure 17 shows the size and configuration of
the compact tension specimens, C(T)(S-T), used for R curve determination. The cross
head movement speed was set at 0.1-inch per minute. The crack length was measured
directly by a walking-microscope set up to move parallel to the movement of the crack tip.
Load values were taken corresponding to different values o f preset crack extension. The R
curve was developed subsequently as a plot of K7E vs. crack length a.
6.4. Analysis of Data
Fracture properties of the WPCs were determined using a 810 MTS universal
servo-hydraulic test system in the tensile mode. The K[C and J[C experiments were
performed and controlled by the Teststar software added to the MTS machine.
Using the computerized standard test procedure and the software supplied by
MTS, fracture toughness, K[c. and J[C, were determined and the results are reported in
Tables 4 and 5 as already mentioned. Since these standard tests were designed
originally for metals with limited ductility and since R*; > 0.1, the test results obtained
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were characterized as not valid. This was expected for the wood - polymer-matrix
composites due to the step-wise crack growth in these materials.
The plot o f load applied vs. crack front displacement is shown in Figure 34. The
graph indicates an initial rise in the load, which represents the elastic deformation of the
materials. Beyond the maximum load point, where the plastic deformation is expected
to start in the polymer, the load decreases smoothly to the failure point. Figure 35
represents plots of stress-equivalents, what can be designated loosely as uniform
engineering stress and true stress values calculated using B. W in the former case and
B.fW-a) in the later case for the area carrying the load vs. crack extension. It should be
noted that this designation is purely arbitrary and simple for it is well known that the
stress ahead of the crack tip is not uniform and is a function o f the radial distance away
from the crack tip, Rice (1968), [see also Figures 7 and 8].
Engineering stress curve shows a steady decrease past the maximum stress and
continues this trend to failure. The graph for true stress, on the contrary, continues to
increase past the elastic zone to a maximum stress corresponding to a crack length of
about 2 inches. The true stress decreases continuously beyond this point up to complete
failure. This can be interpreted to mean that as the crack grows, wood chips in the
composite are offering resistance initially to crack growth. Under this condition fast
crack growth and complete rupture are not realized until crack grows to a level of
instability initiating faster fracture. In order to determine the point o f instability and the
resultant faster growth, the true stress was felt to be a better indicator. The plot in Fig
35 indicates the fact that the true stress keeps on increasing beyond the initial fracture
initiation point and after reaching a peak level begins to decrease. This point where the
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true stress begins to decrease can be construed as the point of instability. To determine
the critical points corresponding to the initial crack growth initiation point and the
critical point o f instability for onset of fast fracture, the engineering stress intensity
factor, (obtained by using instantaneous load and initial sample dimensions B and
vt> throughout in the formula for K[ in the ASTM E 561-76T standard), and the true
stress intensity factor * obtained by the use o f similar formula using the remnant
segment, W-a, instead o f W, were plotted against crack length, Figures 36 and 37.
Whereas K^g increases somewhat in a parabolic manner past the elastic limit to reach
peak value of around 1800 psiVin, variation o f Ktrue with crack tip displacement shows
very interesting trend. Contrary to Keng, Kuue reaches very high values of the order of
million psiVin. It increases in a slow manner initially and then, after some growth, it
increases rather rapidly reaching values in the order o f several million psiVin. There is
an inflection in the curve past the crack length of -2 in. The variation of Ktxue vs. crack
extension thus indicates that the point where the Ktrue begins to rise rather sharply could
be considered as the point o f onset of instability. This point is around 2.2 in. of crack
length, in the present case. Corresponding to the point o f intersection of tangents to the
sloping points o f the curve with low and high slopes, respectively.
Further analysis was carried out by determining the slope of K^e vs. crack tip
displacement plot at various locations and a plot was made of dKtme/da vs. crack length,
a, Figure 38. This plot is similar to Ktmc vs. crack length plot, Figure 37. Two different
speeds of crack growth are indicated by the different slopes in the plots. At the point
where the low slope corresponding to the low values begins to change to high values
* Note that the designation Keng and Ktrue are used loosely here to denote K values calculated using
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corresponding to high slope, at the point of inflection, the critical point (c) of instability
can be located. This can be taken as the point where again the lines corresponding to
the two different slopes intersect, i.e. at about 2.2 in. o f crack length. The Keng value
corresponding to this point o f inflection can then be taken to represent the true fracture
toughness K[c for the composite material, Krc.com-
In essence, it is found that in these composite materials the crack is propagating
in a sub-critical mode up to the point where the proposed instability is reached. The
crack growth up to this point is either under steady resistance offered to crack growth
by the strengthening particles (wood chip) or under progressively decreasing resistance
until the point o f instability is reached. The analysis o f resistance to crack growth will
define the exact characteristics o f the crack propagation in sub-critical mode prior to
and after the point o f instability is reached. This behavior is analogous to crack
propagation in stress corrosion cracking and in fatigue loading. In those cases the initial
critical stress intensity factor at which crack propagation would be initiated is defined as
Krscc and K[fat respectively. In the case of composites, an equivalent point
corresponding to an initial critical stress intensity factor value can be designated as
stress intensity factor for crack growth (fracture) initiation, Kiif.com. This value would
correspond to the normal K[C value derived by following the procedure given in ASTM
E 399-83, ASTM (1995). That is, by giving 5% offset to the initial slope to the elastic
part o f the curve and drawing a straight line, the intersection point of the offset line with
the plot in the crack growth region will define the K[if.cotn in the case of composites.
constant W all the time for the former and the non-cracked ligament o f dimension w-a instead o f w for the latter in the formula for stress intensity factor.
77
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6.5. The Critical Stress Intensity Factor for Particulate Filled Composites
Based on the above discussion the K[c that is determined following the ASTM E
399-83 and designated to be Kq is actually the Knf.com as defined here. The variation of
Kime and/or of dKtnie / da vs. crack length, a , is more appropriate to find the point of
instability and the fracture toughness Kjc for composites.
From the data collected in the low slope region up to the point o f inflection and
beyond, the resistance for crack growth can be studied using a method analogous to that
given in ASTM standard 561-76T, ASTM (1995). The analysis in this curve is based
on K2/E. Variation of this value against crack length is given as the fracture resistance
of material. The use of engineering and true K values obtained in this curve shows also
interesting results.
Figure 39 shows the R curve obtained following a procedure similar to the one
given in the ASTM standard. The standard has K^r values for K in K2/E , Hetrzberg
(1996). Various (Keng)2/E values corresponding to the different crack lengths were
plotted against respective crack lengths. The plot indicates that as the load increases
initially, i.e. in the elastic region, (Keng)_/E increases sharply. Subsequent increase with
increasing crack length in the sub-critical crack growth region occurs rather slowly.
The plot further indicates that when the point o f instability is reached, (Keng)2/E seems
to have reached a value very close to the highest attainable in that region. Thus the
resistance curve plot may be interpreted to have three different slopes corresponding to
three different regions. Region I is where the crack is growing barely amidst a lot of
resistance, when the slope is high. After the point of initial fracture (crack growth)
initiation in sub-critical mode the slope seems to be reduced to much lower level in
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region II o f sub-critical growth. At the point o f instability, the slope changes again and
approaches zero, the value of region III. The fracture occurs first with near zero
resistance in region III.
The slope o f the plot K2/E vs. crack length, a , that is d(K2/E )id(a) vs. crack
length, a , is shown in the Fig 40. Representing K by the simple equation:
K = Y . a . y f MK 2 = Y2 .cr2 .rr.a (1)
K 2 <7 = i ' .cr.— -X .aE E
Since cr (2)
£then:
K 2 , = Y c r .s .K .a (3)E
Where cr in equation (1) is applied stress, e is strain, a is crack length, BC is stress
intensity factor, E is modulus of elasticity and Y is the (compliance) calibration factor.
The or in the final equation (3) can be considered as the resistance stress corresponding
to the force o f resistance offered by the material for crack propagation.
The variation of slope of K2/E vs. a, i.e. the resistance criterion is clearly
indicated in Figure 40. Normalizing the data in the Y-axis to vary between 1 and 0 (of
slope), i.e. 100 to 0% o f resistance to fracture, in the initial elastic region I, the
resistance to crack growth can be considered to be 100%, for the crack is not growing.
This resistance is maintained for a short while and there is a rather large sharp decrease
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in the value to about 30%. Where the drop occurs, it signifies the initiation of sub-
critical crack growth due to a considerable drop in resistance to crack growth. Further
crack growth in region II the sub-critical crack extension region, occurs under
continuously decreasing resistance, until the latter reaches the value close to zero when
the instability sets in. The latter critical point corresponds to the total loss of resistance
and easy crack propagation beyond that point under zero resistance. The K^g value of
the initial point corresponding to the large drop in resistance would correspond to the
value of Kuf.com, and that o f the point where resistance reaches close to zero would
correspond to the value of k[c.COm- Thus the resistance curve indicates the two critical
points very clearly. The values of Kur.com and Kfe.com obtained from the crack growth
extension study are given in Table 6. Comparing these values to those given in Table 4,
it can be found that the values of KufCOm given in Table 6 are close to the K(c or Kq
obtained by using methodology given by the standard. Thus the standard procedure for
determining K[C according to ASTM E 399-83 enables the determination of the initial
critical stress intensity factor for fracture initiation in sub-critical mode and not the
fracture toughness o f the composite. The latter is obtained from the fracture resistance
curve from the point o f instability, wherefrom further crack propagation should be
about zero resistance. The point o f instability is more clearly defined in the Ktmc vs. a
and/or dK^Jda vs. a plots at the point of inflection, as already outlined. Thus the FQ-ng
value corresponding to the point of inflection gives K[c more accurately.
The results o f the study gives the following data for the 50 vol.% medium size
pine wood chip polyethylene composite samples:
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fC[lt-= 1209 psiVin (42.00 N/mmA1.5), Kfc = 1766 psiVin (61.36 N/mtnA1.5), and
Drop in resistance upon sub-critical crack growth initiation ~ 75%
The fracture surface analysis using SEM essentially indicated a brittle type
fracture process. This was evident from the flat nature of the fracture surface. Particles
protruding from the surface were indicative of delamination at the interface and the
crack moving around the particles.
Table 4:The test result summary for the K[C tests (as per ASTM E399-83) for pine-HDPE
composite sample (medium size wood chips in 50% volume concentration)
Kic Test ResultP5 1329.02 NPQ 1329.02 NPmax 961.5 NKQ 39.0274 N/mmA 1.5RA2 0.999879 Correlation CoefficientRsc 0.335604 Strength RatioKQ * KIc Not valid according to
ASTM E399-83
Table 5:The test result summary for the Jic tests (as per ASTM E813-89) for pine-HDPE
composite sample, (medium size wood chips in 50% volume concentration)
___________________ Jic Test Result ______________________JQ 2.78789 N-mm/mmA2Jic ^ JQ____________ ___________________ Not valid according to ASTM 813-89
Table 6:Comparison of the Kiif.COm K[cxom obtained using ASTM E561-76T and Kq obtained
using ASTM E399-83.
ASTM E399-83 ASTM E561-76TK o * K , c Knfcom K[c.com
1120.5 psiVin (39.02 N/mnr’i .5 ) 1209 psiVin (42.00 N/mmA1.5) 1766 psiVin (61.36 N/mmA1.5)
81
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400
300
200
100
0 0.2 0.4 0.6 0.8 1 1.2 |
Crack Opeming Displacement (in)
Figure 34. Load vs. crack opening displacement curve for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)
</3c .
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Crack Length (in)
Figure 35. True Stress and engineering stress vs. a crack length plot for pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)
82
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200018001600140012001000800600400200
02.50 0.5 I 1.5 2 3
Crack Length (in)
Figure 36. Engineering Ki vs. crack length plot for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)
5000 4500 4000 3500 3000
to' 2500 \ 2000 'Z 15005 1000* 500
2.51 1.5
Crack Length (in)
0.5
Figure 37. True Kt vs. crack length plot for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)
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30000
25000
20000
15000
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Figure 38. Slope of K|lrue vs. crack length plot for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)
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Crack Length (in)Figure 39. Resistance curve, K2icng/E vs. crack length plot for the pine-polyethylene
composite sample, (medium size pine (wood) chips in 50% volume concentration)
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Slope of R curve vs. crack length plot for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)
CHAPTER 7
CONCLUSIONS AND SUMMARY
Based on the results o f the present study the following conclusions are drawn:
1. Addition of wood chips to polymer reduces the UTS and break strain in tensile
loading o f the resultant composites. Smaller wood chips generally result in smaller
reductions.
2. The peak load, MOR and stiffness in bending tests o f the pine-PE composites have
been found to have increased at 40% wood chip concentration; they then decrease
with additional concentration of wood chips. The pace of change in mechanical
properties of composites depends on the wood chip size, the smallest chip size
showing largest increases.
3. In tensile testing, fracture of samples is caused by delamination of wood chips from
polymer matrix. The debonding at the interface between wood chips and polymer
matrix may have been caused by residual stress left at the interface after
compression o f the composite during preparation of the specimens.
4. Nail pull-out-force is affected by chip size and concentration. It increases with
wood concentration up to 50% and then decreases. Smaller chip size results in
larger nail pull-out-force.
5. According to the experimental results, the required wood chip size and
concentration for optimal mechanical properties are size smaller than 0.125” and
concentyration around 40vol.%, respectively.
6. Pre-treatment in the solution of NaOH is beneficial to increase the coupling strength
between negatively charged (due to OIT) wood and positively charged (due to H*)
87
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PE due perhaps to interaction o f acidity/basicity, but predominantly due to the
modification of surface morphology.
7. Protruding wood fibers after treatment are found to be able to form mechanical
bonding at the interface between wood and polymer.
8. Treatment of wood first with NaOH and after drying with vinyltrimethoxysilane is
the best treatment for obtaining maximum bonding strength at the interface and
mechanical properties resultant therefrom. Treatment in H2SO4 increases acidity of
wood and also scatters the fibers on wood surface, weakens the bond, and hence,
should be avoided.
9. Wood chips in the composite act as obstacles for impact-induced compressive-type
deformation.
10. The buried wood chips would rub against each other and produce internal friction
forces that would restrict their sliding in the matrix and, as a result, resist cause damage
under compressive-type impacts
11. Higher concentration of large size wood chips yields higher maximum load, higher
energy to maximum load and total energy absorbed prior to complete failure compared
to analogous samples with medium or small size chips. More rigid wood-polymer
composites are those with larger wood chips at the higher concentration ranges of 50 to
60 vol.% . On the contrary, pine-HDPE WPCs with 60% fine wood chips are the
weakest under impact-type loading and would fail easily at energy levels far less than
those required to break the polymer alone.
12. Compared to which increases in somewhat parabolic manner with respect to
crack growth the corresponding to the true stress sustained by the un-cracked
88
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segment increases slowly initially, but rather fast after the crack has grown
considerably.
13. Variation of K ^ / E vs. crack length, the R curve, indicates three regions of
different slopes-region I where the slope is high, region II where the slope is
medium, and region III where the slope is 0.
14. The slopes at various points on the R curve plotted against a, that is d(K2eng/E)/da
vs. a plot yields the true fracture resistance plot. From the initial high value o f the
region I there is a sharp drop at the transition from region I to region II. Slopes at
points on the curve in region II steadily drop and reach a value very close to zero
corresponding to near loss of all resistance and the onset o f crack growth in region
III. Region III crack propagation can be construed to be under near zero resistance.
15. The point o f transition from region I to region II corresponding to a large drop in
fracture resistance is considered as a critical point o f fracture process initiation in
the sub-critical mode. In region II, the crack propagates in a sub-critical manner
16. The transition from region II to region III, corresponding to the onset o f near zero
resistance, can be considered to be the point of instability.
17. The first critical point at low K[ corresponding to initiation of fracture in the sub-
critical mode, is designated as K[if,COm. The 2nd corresponding to the point of
instability is considered as Kjc, the fracture toughness of the composite
18. The standard procedure given in ASTM E 399-83 to determine the fracture
toughness Kic or Kq yields Kn̂ m- K[C can be derived from the resistance curve
corresponding to the point of onset of zero resistance.
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19. The variation o f with crack length is a true indicator of the onset of crack
instability. The point o f inflection corresponding to a large change in slope
indicates the point of instability. The engineering stress intensity
corresponding to this critical point o f inflection would be K[c, the fracture toughness
o f the composite.
The salient results obtained can be summarized as follows:
Composite plates of wood chips and high-density polyethylene (HDPE) matrix were
prepared for study in a 5” x 13” x 0.75” steel mold with removable lid by first filling it with
pre-mixed wood and polymer and then heating the mold in a furnace. Resultant
rectangular samples were later cut into specific dimensions for testing. This procedure
resulted in polymer-wood com posite samples with w ood chips aligned somewhat parallel
to the surface by the compressive force applied to the com posite during solidification and
sample removal, especially in samples with low wood chip concentration.
Mechanical properties o f these samples were characterized later. It was found
that the addition o f wood chips to polymer reduces the UTS and break strain in tensile
loading of the resultant composites and smaller wood chips generally resulted in smaller
reductions. In tensile testing, fracture o f samples is caused by delamination of wood
chips from polymer matrix. Nail pull-out-force was found to increase with wood chip
concentration up to 50% and then it decreased with 60% wood chip concentration.
Smaller chip size results in larger nail pull-out-force.
Optimal mechanical properties are attained in composites with wood chips of
size smaller than 0.125” and in concentration around 40 vol. %.
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Treatment of wood first with NaOH and after drying with vinyltrimethoxysilane,
is the best treatment for obtaining maximum bonding strength at the interface and
resultant mechanical properties
Under impact type loading, more rigid wood-polymer composites are those with
larger wood chips at the higher concentration ranges o f 50 to 60 vol.%. On the contrary,
pine-HDPE WPCs with 60% fine wood chips are the weakest and would fail easily at
energy levels far less than those required to break the polymer alone.
A method to obtain stress intensity fracture toughness of polymer-matrix
composites is not well defined in the literature. Since this class of composite material
can propagate a crack slowly and sustain reduced loads even after the crack has
advanced considerably, the standard procedure prescribed in ASTM E 399-83 is no
longer applicable. Evaluation of a resistance curve following the procedure given in
ASTM 651-76T indicates that the fracture toughness Korean be correlated to a point of
onset of near zero resistance to fracture. Analysis in this work indicates that this point
approximately coincides with the point o f inflection in the K ^ e vs. crack length, a,.
curve. It is found from further analysis arrived at in this study that the procedure given
in ASTM E 399-83 can be used to determine the stress intensity for fracture initiation
designated as KnriCom, whereas the fQcng value corresponding to the point o f inflection in
the Kitruc vs. crack length, «, curve would closely derive the Kic value of the composite.
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CHAPTER 8
SUGGESTIONS FOR FUTURE WORK
In this work the mechanical properties, bonding strength, impact fracture and
static fracture behavior o f pine-HDPE composites were studied. Conditions to obtain
optimum properties have been discussed. However, there are other areas where future
work can be concentrated to better understand the behavior o f these composites in order
to utilize them.
8.1. Evaluation o f Kijfcom and K[C for composites with 60 vol. % pine wood chips
In view of the fact that the composites with 60 vol. % large and small pine wood chips show
interesting impact properties, their Knf.com and K[C should be determined further using the
procedure adopted in this work. The differences in characteristics between slow and fast
crack growth processes could be better understood through such evaluations and
comparisons.
8.2 Modeling of Crack Growth and Delamination o f the Wood Chip-PolymerInterface
Modeling of fracture and crack growth in polymer (viscoplastic)-matrix
composite materials and understanding the effect o f interface strength and brittle
particles imbedded in them can positively help the potential utilization of these materials.
Such a study could help design material with better-engineered fracture properties. Pine
chip- HDPE composites fall into this category o f materials and such a model could
further lead to methods to improve the fracture resistance of these materials. A clearer
understanding of the delamination process in polymer-matrix composites would enable
development of methods and processes that would eliminate the delamination or at least
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delay its onset to higher stress levels. Nucleation and growth o f voids in the polymer
matrix and at the wood-polvmer interface should be studied more carefully.
8.3. Fatigue and Creep Characterization.
During the mechanical characterization of pine-HDPE composites, the need for
better understanding the fatigue properties of these composites developed. Likewise
stress relaxation under constant and constrained strain-type loading and creep strain
development under constant loading should be studied. These properties would be o f
interest toward structural application o f these composites.
8.4. Effect of Environmental Variables.
Wood-polymer composites have been the subject o f much research during last
decade. Their water and humidity absorption as well as dimensional stability and
biodeterioration have been investigated. These studies have been concentrated mainly on
composites developed by liquid monomers impregnation into wood and their subsequent
in situ polymerization. It is o f interest to find the effect o f varying environmental
moisture and temperature on these materials with different sized wood chips and
correlate them to applications in the industry.
8.5. Damping Characteristics
One of the important properties of polymers and hence of polymer-based
composites is their vibration damping and mechanical energy absorption characteristic.
This property needs to be studied in pine-chip - PE composites as well. It is possible that
on account of this property they can be utilized in panels and sidings of structures that
would be subject to oscillation and vibration. They could find similar application in
acoustic damping devices and structures as well.
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VITA
Parviz Sharif Razi was bom in 1951. He graduated from College o f Science
and Technology o f Tehran, Iran, earning his bachelor o f science degree in Mechanical
Engineering (ME) in 1972. He served two years of military service in the Iranian
Navy as an officer and then joined the Navy's engineering department where he
practiced his engineering skills. He started his graduate studies in 1977 at Louisiana
State University where he received his master of business administration (1980) and
master o f science in mechanical engineering (1983) degrees earlier. He worked as a
project engineer/designer and project manager during his professional career. He is
currently a candidate for the degree o f Doctor of Philosophy in Engineering Science
from the Louisiana State University.
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DOCTORAL EXAMINATION AND DISSERTATION REPORT
Candidate: Parviz S. Razi
Major Field: Engineering Science
Ti-tlfi of Dissertation: Pine—Polyethylene (Wood—Polymer) Composites:Synthesis and Mechanical Behavior Characterization
Approved:
iduate School
EXAMINING COMMITTEE:
X
Date of Examination:
March 25, 1999
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