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Joints in composite materials
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REVIEW ARTICLE
on
STRUCTURAL JOINTS IN POLYMERIC COMPOSITE MATERIALS
PREPARED FOR
PERMANENT SCIENTIFIC COMMITTEE 56 FOR STRUCTURAL & CONSTRUCTION ENGINEERING
BY
HOSSAM EL-DIN MOHAMMED SALLAM, Ph.D.
January, 2004
STRUCTURAL JOINTS IN POLYMERIC COMPOSITE MATERIALS
BY
HOSSAM EL-DIN MOHAMMED SALLAM, Ph.D.
CONTENTS
1. INTRODUCTION
1
2. MECHANICALLY FASTENED JOINTS 9 2.1 Stress Concentration Factor in Composite Materials
Containing an Open Circular Hole. 11
2.2 Failure Modes in Mechanically Fastened Joints 13 3. BONDED JOINTS 21
3. 1. Stresses in Adhesive Joints 25 3.2 Standard Mechanical Test Procedures 28 3.3. Fracture Mechanics Considerations 30 3.4. Fatigue behavior and Durability of Adhesive lap joints 35 3.5 Prevention of Peeling failure in Adhesive Joints 35 3.6 Adhesive Bonded Dissimilar Materials 37 3.7 Fusion Bonding for Joining Thermoplastic Matrix Composites 40
4. SNAP JOINT IN COMPOSITE STRUCTURES 44 5. HYBRID JOINTS 46
5.1. Hybrid Joints in Plated Beams 47 6. SUMMARY 50
6.1 Mechanical Joints (Bolts or Rivets) 50 6.2 Adhesive Bonding 50 6.3 Hybrid joints 51
7. REFERENCE 52
1. INTRODUCTION
The permanent assembly of individual manufactured components is an important aspect
of fabrication and construction. There are many ways for accomplishing this including the
use of fastenings such as bolts and rivets, the use of adhesives, and by soldering, brazing
and welding [1]. Joint design is dependent upon the nature of materials to be joined as
well as the method of joining. The first cast iron bridge, at Ironbridge, in Shropshire,
England, was erected in 1777-1779 by casting massive half-arches and assembling the
mortise and tenon joints, with fit-on parts and tapered pegs, see Fig. 1-1.
Figure 1-1. The first cast iron bridge in Shropshire.
This mode of construction was the only one known to the designer, and is closely based
on the design of timber structures. The only concession to the new structural material was
in the use of large scale, single piece, curved members. Rolled bars of T- and L-sections
did not become available until after Napoleonic wars and the first I-beams, for light
loading only, were made in Paris in 1847. Heavy I-beams only become available in 1860
after the invention of mild steel. Fastening was originally by slotting a circular cylinder,
cast as an integral of the beam, over a cylindrical column, cast with a retaining lip, but
fish plates and bolting were introduced at early stage [2,3].
1
Steel “nailing” was developed and primarily used for fastening sheet metal to structural
steel for metal roof decks and claddings. Research undertaken at the University of
Toronto has shown that the existing technology of steel nailing also can be used to
connect structural steelwork, particularly hollow structural section steel members [4].
Cold-formed structural members may be joined with bolted connections that are designed
with the aid of applicable national design standards, such as American Iron and Steel
Institute (AISI). Winter [5] categorized the failure of bolted connections into four separate
modes, described as end tear-out, bearing, net-section fracture, and bolt shear. Recently
Rogers and Hancock [6] found AISI specification cannot be used to accurately predict the
failure modes of thin cold-formed sheet-steel bolted connections that are loaded in shear.
It is sometimes necessary to use combination (hybrid) joints in steel construction. For
example, high-strength bolts can be used in combination with welds, or rivets can be used
in combination with bolts. The need for a hybrid joint can rise for a number different
reasons. For example, the load demands on an existing bolted joint may change with time,
necessitating renovation of that joint. If the geometry does not permit the addition of more
bolts, welds can be added to the connection in order to increase its capacity [7]. Bonded
structures can be of two types based on purely adhesive or an adhesive/mechanical
connections. These types of connections include bonded-welded, bonded-riveted, and
bonded-screwed connections [8]. The concept hybrid joint is also used in structural
timber by injecting a resin into the gap between the connector and the wood to improve
the performance of bolted and dowelled joints [9]. On the other hand, steel bolts/screws
were used as shear connectors to joint steel/timber beam to concrete slab [10-12].
Elshafie [12] also used concrete shear key (dovetail) as shear connectors in timber-
concrete composite joints.
It is convenient now to define an adhesive as a polymeric material which, when applied to
surfaces, can join them together and resist separation. The structural members of the joint,
which are joined together by the adhesive, are the adherends, a word first used by de
Bruyne in 1939 [see Ref. 3]. Fracture mechanics has become a very popular tool for
2
characterization of adhesive metal joints in recent years [13]. Furthermore, Conrad et al.
[14] studied the effect of droplet diameter and droplet spacing on mode I fracture
toughness of a discontinuous wood-adhesive bonds. The fracture mechanics concept is
also used to understand the basic dentin adhesion mechanisms [15].
Advanced composite materials and structures have undergone rapid development over the
past four decades [16]. The majority of advanced composite materials are filamentary
with continuous fibers. As such, their behavior and the behavior of structures made from
them are more complicated than that of monolithic materials and their structures.
Advanced composite materials can be divided into classes in various manners. One
simple classification scheme is to separate them according to the reinforcement forms-
particulate-reinforced, fiber-reinforced, or laminar composites. Fiber-reinforced
composites, can be further divided into those containing discontinuous or continuous
fibers. Polymers, ceramics, and metals are all used as matrix materials, depending on the
particular requirements. The matrix holds the fibers together in structural unit and protects
them from external damage, transfers and distributes the applied loads to the fibers, and in
many cases contributes some needed property such as ductility, toughness, or electrical
insulation.
The structure of polymers consists of long molecules with a backbone of carbon atoms
linked by covalent bonds. In non-crystalline or amorphous polymers the molecular chains
have an entirely random orientation and are cross-linked occasionally by a few strong
covalent bonds and numerous but weaker van der Waals bonds. These weaker bonds
break as the temperature reaches a value known as the glass transition temperature, Tg,
characteristic for each polymer. Below Tg the polymer behaves as a linear elastic solid.
Creep becomes increasingly significant as the temperature increases and, above Tg, the
polymer deforms in a viscous manner under load. In crystalline polymers the molecules
are oriented along preferred directions, bringing with them optical and mechanical
anisotropy. Polymers are described as being either thermosets (e.g., epoxy, polyester,
3
phenolic) or thermoplastics (e.g., polyimide, polysulfone, polyetheretherketone,
polyphenylene sulfide). Ceramics, such as glasses, cement & concrete, and engineering
ceramics ( Al2O3, SiC, Si3N4, ZrO2), have a wide range of engineering applications. The
strong ionic and covalent nature of the bonding in most ceramics leads to a stable crystal
structure with a high melting point and high stiffness. Many ceramic materials have very
high elastic moduli and strengths, but the advantages of these properties bestow are often
outweighed by their highly brittle nature, which leads to low and unpredictable failure
stress resulting from the presence of flaws. Metal matrix composites typically comprise a
light metallic alloy matrix, usually based on aluminum, magnesium or titanium alloys.
The major reinforcing elements used in composites are glass, carbon/graphite, organic,
and ceramic. Both metal and ceramic materials have properties closer to those of likely
reinforcements and this leads to a different choice of properties for which these composite
systems are optimized. With polymer matrix composites, it is almost true to say that the
properties of the composite are essentially those of the fibers, with little contribution from
the properties of the matrix.
Fiber Reinforced Polymer (FRP), earlier limited to aerospace structures, are gaining
acceptance in civil engineering applications also. FRP plates, sheets, rebars, and strands
have been increasingly used in the construction industry due to their superior properties.
One of the potential structural applications of FRP composites with concrete or steel
structures is strengthening of RC or steel beams with unidirectional fiber-composite
sheets/plates bonded on their tension faces via the use of epoxy adhesives. Growing
maintenance and durability problems in transportation infrastructure have led researchers
to explore FRP box-girder, I-beam, or other shapes in bridges [17-19]. There are
numerous examples of completed all-composite new bridges and several more are under
construction, see Fig. 1-2. FRP materials could play an important role for the construction
industry of tomorrow.
4
Aberfeldy Footbridge in Scotland Bonds Mill Lifting Bridge near Gloucester
Figure 1-2. Fully composite bridges [20].
Due to the relatively low weight and high strength of FRP girders and decks it has proven
to be efficient to replace old bridges that no longer meet today’s requirements with FRP
alternatives [21-26]. While design concepts vary, several systems employ a modular
system to build up the bridge deck, meaning that composite profiles are transversely
joined to form the required bridge span or occasionally width [22]. Kumar et al. [23]
installed an all-composite bridge deck made up of FRP. The materials used for the
fabrication of this 9.144 m long by 2.743 m wide deck were 76.2 mm pultruded square
hollow glass and carbon FRP (GFRP and CFRP) tubes of varying lengths, Fig. 1-3.
These tubes were bonded using an epoxy adhesive and mechanically fastened using
screws in seven different layers to form the bridge deck with tubes running both
longitudinal and transverse to the traffic direction. The cross-section of the deck was in
the form of four identical I-beams running along the length of the bridge. They [23]
concluded that, all-composite bridge decks made of pultruded glass and carbon tubes are
judged to be a suitable replacement for short span bridges made of conventional materials.
In fact, the better fatigue performance of FRP was one of the main reasons suggested by
design engineers to replace conventional materials in some applications [24].
5
Figure 1-3. Experimental setup for the bridge deck test
One promising structural form is frames, where replacement of steel members is by
standard pultruded composite profiles of similar shapes [27,28]. The two most sensible
production techniques, pultrusion and filament winding, yield products with considerable
anisotropy in their properties. This results in considerable difficulties in making joints.
Drilling for bolts tends to sever the load-carrying fibers, and the lack of isotropy
precludes other load paths. Welding is impossible[20]?!!(is it), and although adhesives
can be used, the design of efficient and durable adhesive joints is not trivial. The most
cost-effective way to manufacture constant-cross-section composite profiles is pultrusion,
since this process is highly automated and uses low-cost forms of raw material. The
nature of the fiber reinforcement lends itself to novel forms. Pultrusions are difficult to
bend into conventional reinforcement shapes without seriously weakening the element at
the bend. So why not make use of the fibers’ flexibility to form the links before adding
resin to stiffen the material? There is research in Germany into the use of a variety of
textile processes, including the double-layer and Raschel knitting techniques (Figure 1-4)
which naturally give three dimensional structures [29]. The performance advantages of
three-dimensionally (3D) reinforced textile composites (as compared to traditional 2D
ones) have been well documented [30]. These advantages include full suppression of
delamination, lower notch sensitivity, and better fatigue and impact properties. 3D woven
6
composites can be easily machined into parts, very similar to metals, without considerable
effect on their properties.
Figure 1-4. Double-knit textile reinforcement [31]
. Just because FRP’s have good potential, they cannot be used in every structure [32-37].
One thing that needs to be considered is the beam-to-column connections. Mosalam [32]
concluded from an evaluation of available information that a majority of connections
research and manufacturers’ information was mimicking ‘steel-like’ details, i.e. they
copied the connection practice of steel frames, and that this wrongly ignored the effect of
difference in mechanical properties. The development of his Universal Connector (UC)
[33] was therefore presented as the first positive step in correcting this deficiency of
current connection design details. It has the conventional leg-angle shape of steel cleats,
but with side ribs to substantially increase its stiffness and strength. Appropriate
placement of the E-glass fiber reinforcement has been achieved by manufacturing the UC
with the resin transfer method. Cleated connections are the most favoured method for
pultruded frames due to the unsuitability of composite materials for conventional steel
endplate or welded joints [34,35]. Mottram and zheng [34,35] have tested both
connections consisting of FRP materials and those of steel. The conclusions from this test
were that the steel option worked better and was recommended in the article.
7
Although many FRP structures can be molded in single pieces to simplify the
manufacturing process and to reduce assembly coast, they cannot be completely exempted
from the joining process. Both mechanical fastening and adhesive bonding are commonly
used in structure assemblies. Each technique has its own advantages and disadvantages in
terms of function and cost. In terms of strength, different techniques also lead to different
damage modes and failure strengths. Based on the high in-plane strengths of FRP
materials, mechanical fastening is commonly used in assembling composite laminates.
However, mechanical fastening can cause high stress concentration due to structural
discontinuity. Adhesive bonding, although free from structural discontinuity, is primarily
based on the shearing strength of the adherend, the adhesive and the bonding between
them. These shearing strengths are not greater than the in-plane strengths and are limited
to the bonding surface, instead of through the thickness. As the joining components
become thicker, a much larger bonding area is required, not only to achieve a higher
joining strength but also to reduce the high stress concentration around the bonding ends.
Since mechanical fastening is actually a three-dimensional technique, it is superior to
two-dimensional adhesive bonding in joining thick composite laminates [36].
The main objective of this review article is to present a sound background regarding the
development of the understanding of the mechanical behavior and the failure modes of
the bolted, adhesive, and hybrid joints in polymeric composite materials. Section 2
presents a review of mechanically fastened joints in FRP including various modes of
failure. In Section 3, the review article discusses the bonded joints in FRP and dissimilar
materials. Section 4 presents a review of snap joints in FRP. Hybrid joints in composite
materials are introduced in section 5.
8
2. MECHANICALLY FASTENED JOINTS
Mechanical joints are used when repeated disassembly and reassembly is
required or when surface preparation is not practical. Mechanical joints require
that bolt or rivet holes are drilled into the composite, that reduced the net cross
sectional area of the structure and introduce localized stress concentration.
These stress concentrations can cause ply delamination since they will include
through thickness tensile and shear stresses. Mechanical joints add weight to
the structure from the added weight of the bolt or rivet. They also pose a risk
for corrosion since the laminate and fastener may comprise dissimilar materials
and moisture can be trapped in the crevices inherent in such joints. However
mechanical joints can be readily inspected before assembly and while in
service. Examples of two typical bolted joints are the single lap joint and
double strap joints as shown in Fig.2-1.
PiPiPi
P D W
e
Pt
Bolted Single Lap
P P
Axial tch
Bolted Double Strap
P D W
e
Pt
Bolted Single Lap
P D W
e
Pt
Bolted Single Lap
P P
Axial tch
Bolted Double Strap
P P
Axial tch
Bolted Double Strap
Figure 2-1. Basic types of mechanical joints.
The single lap joint is the simplest and most weight efficient but the load
results in a moment due to off-set load. The double lap joint eliminate the
moment but adds additional weight from the straps and additional bolt.
9
A circular hole in tensioned FRP plates may be classified into three types as
follows [38]: (1) open-hole tension: the FRP plates were subjected to uniaxial
tension with no constraint imposed on the hole, (2) filled-hole tension: a bolt, φ
= dB, was inserted inside the hole, φ = dH, with/without a clamp-up load. A
washer, φ = dW, was inserted between a bolt head and tail and the FRP plate to
distribute the clamp-up load, as shown in Fig. 2-2. The FRP plates were
subjected to uniaxial tension, and (3) bolted joint, bolt-loaded hole: double-lap
bolted joints were subjected to a uniaxial load with/without clamp-up load.
Dw
W
Load Cell
Rigid Washer
Washer Nut Bolt
Tab Tab
W
D
Specime
Load Cell
Strain Gauge Washer
(b) (a) (c)
Figure 2-2. Geometries of the (a, b) specimens for open- and filled-hole
tension tests, and (c) bolted joint test setup.
In the case of open-hole tension, the mode of failure mainly depends on the
fiber/matrix interface. A weak interface results in longitudinal crack
propagation along the interface, while a strong interface results in transverse
crack propagation across fibers leading to premature composite failure.
However, an interface of intermediate strength leads to optimum composite
performance between these extreme conditions. Based on the specimen
geometry and the interfacial strength of unidirectional FRP, the crack
emanating from notches, such as circular holes, may grow parallel to the
loading and fiber direction, i.e. notch insensitive [39]. A schematic description
of the damage pattern for a composite laminate is shown in Fig. 2-3. In the case
of Fiber Breakage & Matrix Cracking mode of failure, damage is localized to the
10
stress concentration areas before final failure. Edge Delamination mode of
failure may occur in both open- and filled-hole specimens in an early loading
stage. The edge delamination grew throughout the width of the open-hole
specimens. In addition, a Fiber-Matrix Splitting mode occurred along the zero
degree plies (in the direction of the applied load) emanating from the edge of
the hole [38].
Fiber Breakage Matrix Cracking
Fiber-Matrix Splitting
Delamination
Figure 2-3. Typical tensile failure modes in composite laminates containing a circular hole.
2.1 Stress Concentration Factor in Composite Materials Containing an
Open Circular Hole The susceptibility of composite materials due to effects of stress concentrations
such as those caused by notches, holes, etc., is much less than for metals.
Failure in long fiber-reinforced laminated composite structures containing
stress concentration areas, such as circular holes or bolt-loaded holes, has been
one of the technological issues by many researchers during the last decades
[38-42]. The mechanisms of such failures are significantly affected by fiber
orientation, relative strength of the matrix and fiber and the bond strength
between them. Open holes, notches, scratches, inclusions, and so on, all
produce concentrations of strain and stress. Stress concentrations can seriously
weaken brittle materials and can shorten the fatigue life of both ductile and
brittle materials. But a ductile material under static load can redistribute stress
by yielding without fracture. Thus, although strain concentration factor persists,
11
stress concentration factor (SCF) decreases markedly. The theoretical SCF for
an infinite orthotropic plate containing an open circular hole, , can be
calculated from the following relation [43]:
KT∞
12
1112
22
11T G
E υ
EE
2 1K +
−+=∞ (2-1)
Where E11, E22, ν12, and G12 are the elastic constants for an orthotropic plate,
i.e. E11 & E22, are longitudinal and transverse elastic modulus respectively, and
ν12, and G12 are the principal in plane Poisson’s ratio and shear modulus
respectively. It is worth to note that, in the case of isotropic plate, i.e. E = E11 =
E22, and G = G12 = E/2(1+ν), will be equal to the value obtained from the
will known relation = 1+2(D/ρ)
KT∞
KT∞ 0.5 = 3, where D and ρ are the depth and the
radius of the notch respectively. The inverse of isotropic finite width correction
(FWC) factors for open circular hole can be written, according to [44], as
3.
1
)1(2)1(3
. αα
−+−
=
=
∞−
IsoT
T
KK
FWCIso
(2-2)
Where α = dH/w, and KT is the finite SCF. The inverse of orthotropic FWC
factor equals [45]:
( )KK
FWC M K MT
T OrthTIso
∞− ∞
= + − −
.. ( ) ( ) ( )1 60 5 3 1α α 2 (2-3)
&
( )M
FWCIso=
− −−1 8 1 1
2
1
2.
( )α
− (2-4)
Finally, SCF based on net section instead of the whole width, KTn, can be
calculated as follows:
K KTn T= −( )1 α (2-5)
12
2.2 Failure Modes in Mechanically Fastened Joints
There are six basic failure modes in mechanically fastened joints in FRP
[46,38]: net-tension, shear-out, transverse splitting, cleavage, bearing, and pull-
through as shown in Fig. 2-4. The bearing stress, σb is the load, P divided by
the projected transverse cross sectional area of the hole σb = P /(dHt). The
shearout stress is determined by the longitudinal shear surfaces, and is given as
σSO = P /(2et). The net section stress is σN = P /[(W-dH)t]. The transverse
splitting stress is a localized stress normal to the applied load. The gross stress
defined as σ = P /[Wt] is used to rate the effectiveness of the joint. Joint
efficiency is the ratio of the gross section stress at failure to the strength of the
laminate in the gross section. For metals single fastener joints can have
efficiencies as high as 80%. Polymer matrix-fiber composite laminates have
efficiencies generally less than 50% due to their strength anisotropy and
inability to redistribute stress. Net-tension failure is associated with matrix
and fiber tension failures due to stress concentrations. Bearing failure leads to
an elongation of the hole. Shear-out failure can be regarded as a special case
of bearing failure. Shear-out and bearing failures result primarily from the
shear and compression failures of fiber and matrix. Cleave failures are
associated with both an inadequate end distance (e) and too few transverse
plies. Pull-through failure occurs mainly with countersunk fasteners or when
the plate thickness (t) to hole diameter (dH) ratio, t/dH, is sufficiently high to
precipitate failure. Li et al [47] found two others failure modes, i.e., bending
induced cross-section failure and rivet cap penetration failure. Further, they
[47] found that, failure modes may change with increasing loading rate and
rivet rotation decreases with increasing loading rate. Generally, the total energy
absorption of composite riveted joints increases with increasing loading rate,
except for joints used countersunk rivets. It was also shown that rivet rotation is
significant in some joint designs, which could damage the laminate and thus
reduce the joint strength.
13
(b) (d) (e) (c) (a) (f)
P P
Pull-out
P P P
Figure 2-4. Failure Modes (a) Net-Tension, (b) Shear-Out, (c) Transverse splitting,
(d) Cleavage, (e) Bearing, and (f) Pull-out.
Net section failures can be prevented by increasing the ratio of the plate width
to hole diameter, W/dH. Generally, W/dH > 6 is sufficient to prevent net section
failures. Shearout failures can be eliminated the ratio, e/dH to 3 or greater.
Transverse splitting is rare but will occur if there is a high fraction of the fibers
in the load direction such as would be the case in a unidirectional composite.
Bearing failure is the preferred failure mode since the joined members are not
catastrophically separated. Bearing failures are associated with localized hole
damage such as local delamination and matrix crazing. The experimental
observations of the effects of joint geometry, ply-orientation, lay-up, t/dH, dH to
plate width (w) ratio dH/w, e/dH, and clump-up load, i.e. through-thickness
pressure, on the joint behavior were reviewed by Camanho and Matthews [46].
The effect of e/dH on the SCFs of unidirectional CFRP single-bolt tension
joints with α = 0.2, i.e. w/dH = 5, was studied numerically by Sallam [48].
SCFs were calculated based on equivalent, shear, and longitudinal stresses. The
calculated SCFs based on equivalent stress are very high due to the high
compressive stress ahead of the loaded-bolt. SCFs decrease with increasing
e/dH up to e/dH equals three, as shown in Fig. 2-5. After that, the effect of e/dH
is insignificant.
14
1
11
21
31
41
0 2 4 6 8 10e/dH
Stre
ss c
once
ntra
tion
fact
or
GPaE11 = 165E22 = 11E33 = 11G12 = 5.3G13 = 5.3G23 = 3.9ν12 = 0.26ν13 = 0.5ν23 = 0.5
σEq.
τXY
σYY
Based on
α = 0.2
Figure 2-5. SCFs of unidirectional CFRP single-bolt tension joints.
Sallam [48] found through an experimental investigation that, the failure mode
of all tested bolt-loaded unidirectional CFRP plates is shear-out, regardless the
values of dH, dB, e, and w or their ratios. This is due to the weakness in the
bond between the fiber and matrix or in the shear strength of the matrix
compared with the tensile strength of the fiber. Load-bolt displacement
responses of all tested bolt-loaded unidirectional CFRP plates are shown in Fig.
2-6. The static failure is defined as the maximum load, Pult, achieved during the
test to failure [49]. As shown in the Fig. 2-6, the stiffness of the joints is not a
function of the values of dH, dB, e, and w, whereas, the ultimate load is
dependent on the contact area alone, i.e. the value of dB. This observation is
acceptable only in the case of shear-out failure in unidirectional composite
plate, i.e. lower stiffness and strength in transverse direction.
0
2
4
0 2 4 6Bolt displacement, mm
Load
, kN
Test # dH dB e w 1 7 6 10 20 2 10 6 11 30 3 10 6 25 30 4 10 6 25 50 5 13 6 17 50 6 13 6 25 50 7 10 8 30 30 8 10 8 30 50 9 13 8 25 50
Figure 2-6. Load-bolt displacement curves of unidirectional CFRP plates.
15
Aktas and Karakuzu [50] found experimentally and numerically that, the
failure mode of the unidirectional composite pinned joint is shear-out except
for the cases of e/dH and w/dH ratios of 4 and 5 respectively. For those values
the mode is bearing mode. When failure caused by the pin continues up to 2–3
mm from the free end of the plate in the direction of loading, the plate tears
immediately. Net-tension mode occurs in the following situations: fiber
orientation ranged between 30o and 60o, w/dH = 2 –3, and e/dH = 4. Although
the failure mode is the same, the propagation of failure is a bit different for
each criterion, because the angle of first failure area is different in each case, as
shown in Fig. 2-7 .
(a) (b) Figure 2-7. Ultimate failure of specimen for (a) E/D, and (b) W/D series.
Furthermore, Karakuzu and his co-workers [51,52] studied the effect of joint
geometry and ply orientation on failure strength and failure mode of composite
laminates with a pin-loaded hole. They concluded that, there is a definite
dependence of bearing strength on stacking sequence and joint geometry. The
lay-up [0/±45]s laminate was found to have higher bearing strength than the
lay-up [90/±45]s laminate. Ultimate strengths are sensitive to w/dH and e/dH
values in a larger range compared to the laminate tensile strength. Increasing
the end distance increased the bearing strength of the joint until a critical end
16
distance was reached; any increase of the end distance beyond that value did
not result in a corresponding increase in the strength of the joint. Pin bearing
strength decreases with decreasing w/dH ratio. As the width of the specimen
decreases, there is a value where the failure changes from the bearing to net-
tension failure mode. Maximum bearing strength is reached when the edge
distance ratio, e/dH, is equal to or greater than 3, and the side distance ratio,
w/dH, is equal to or greater than 4 for the [90/±45]s laminate. For the [0/±45]s
laminate, the critical e/dH and w/dH ratios are 3. In net-tension and shear-out
failure, the reduction in load is both greater and more sudden. But in bearing
failure load doesn’t drop suddenly. Therefore, joints that fail in the bearing
mode are stronger and safer joints although this depends on selected design
rule. The net-tension strength of a single-hole joint is dependent on ply
orientation and specimen width. The effect of a change in the ratio of specimen
width to hole diameter is least for [90/±45]s laminate and most marked for
[0/±45]s laminate. The shear strength of single-hole joints has been shown to
be strongly dependent on the ply orientations within the laminate and the edge
distance. For the [0/±45]s laminate, the maximum shear stress at failure is
reduced to 50% with increasing edge distance to diameter ratio. Whereas for
[90/±45]s the effect of a change in the ratio of edge distance to diameter is of
about 66% loss in strengths. It is evident from the load–displacement curves
that the [(±45)3]S laminate with small w/dH fails in a more sudden fashion than
the [(0/90)3]S laminate. For this reason, the use of mechanically fastened joints
in [(±45)3]S laminates with small w/dH is not recommended. Turvey [53] found
that, both the initial stiffness and the ultimate load depend on the w/dH and e/dH
for pultruded GRP plate. It is worth to mention that, the reinforcement of
pultruded GRP plate is in two forms, viz. E-glass roving (unidirectional fiber
bundles) and Continuous Filament Mat (CFM). The CFM provides the
transverse stiffness and strength. Further, Turvey found that the failure mode
changed from bearing to cleavage to shear-out with decreasing e/dH with
constant w/dH. However, the failure mode changed from bearing to tension
with decreasing w/dH with constant e/dH. In drilling operations, high axial
17
forces are induced locally in the material at the tip of the drill because of the
stationary tool center. In composite materials, the high axial contact force
between the drill and the composite may cause separation of piles at the exit.
This effect can also be observed when sawing with a core drill. At the entrance
of the hole, delamination may occur if the cutting angle is too small and the
cutting edge is wedged between two laminates [54], Fig. 2-8. Zachrisson et al.
[55] developed a new method (KTH-method) which gives defect free holes.
Failure analysis of glass woven roving composites of bolted connections with
drilled and moulded-in holes were examined by Lin & Tsai [56]. They found
that, laminates with moulded-in holes are stronger when the (e/dH =1).
Lamina
Interface (a) (b)
Figure 2-8. Delamination at (a) the exit and (b) the entrance sides of the hole.
The influence of bolt spacing (i.e. pitch distance, row spacing, end distance,
and bolt diameter) and the degree of laminate anisotropy on the bolt load
distribution and failure prediction in multi-fastened composite joint was
investigated numerically by Sergeeva et al.[57] and Chutima and Blackie [58].
Failure loads for two laminate lay-ups [10/60/30] and [25/50/25], with fastener
diameter specified as D= 8 mm and varying spacing, S, are shown in Fig. 2-9.
There is no notable interaction among the stress concentrators for the case of
distantly placed fasteners, and the relationship between bolt spacing and failure
load is approximately linear. As the fastener positions are closer to each other
and the high stress areas near the fastener holes merge, the joint strength starts
to decrease. The results indicate that the optimal bolt spacing may be different
18
for different laminate lay-ups: in the case of the quasi-isotropic lay-up
[25/50/25], the maximum joint strength is achieved at S/D = 3, while for the
[10/60/30] laminate the optimal fastener spacing corresponds to S/D=3.5.
Figure 2-9. Variation of the failure load for different fastener spacing [57].
The effect of edge geometry (i.e. round or rectangular, see Fig. 2-10), stacking
sequence, and clamping force on failure modes in mechanically fastened joints
in FRP was studied by several researchers [59,60] The lateral clamping
pressure suppresses the onset of delamination and continuously suppresses the
propagation of interlaminar cracks. The failure mode changes from a
catastrophic mode to a progressive one. Consequently, the lateral clamping
pressure increases both the delamination and ultimate failure strengths of
bolted joints in composite materials [59].
(a) (b) (c)
Figure 2-10. Types of the failure modes: (A) Net-tension failure mode
(B) Shear-out failure mode (C) Bearing failure mode [60].
19
It was found that, either bolt-to-washer or bolt-to-hole clearance can affect the
initial failure loads but not the ultimate failure loads [61, 62]. Starikov and
Schon [63] found that the amount of load carried by the part with ordinary
holes compared to the part with countersunk holes increased during fatigue
loading. The contact stresses in the cylindrical part of the holes are probably
larger in the plate with countersunk holes than in the part with ordinary holes
since the cylindrical part of the hole is less for countersunk holes than for
ordinary holes. This probably results in more fatigue damage in countersunk
holes than in ordinary holes and the applied load is redistributed to the plate
with ordinary holes. Figure 2-11 shows joints with protruding head bolts and
countersunk head bolts that failed by bolt failure [64]. In all cases of bolt
failure, the bolt failed at the threads. It can be seen that extensive bearing
damage existed at the hole before the bolt failed. The bearing damage at the
shear plane appears more extensive in the countersunk case, which is most
likely due to the bearing load being taken almost entirely by the cylindrical
portion of the hole in the countersunk laminate.
(a) protruding head bolt
(b) countersunk head bolt
Figure 2-11. Final failure of protruding head and countersunk head bolts.
20
3. BONDED JOINTS
The key advantage of adhesive bonded joints over other joint approaches, e.g.
mechanical fasteners (bolting or riveting), is that it enables the development of
large, cost-effective, and highly integrated structures [37]. Adhesively bonded
joints have high structural efficiency and are used extensively to join
composites in advanced aerospace structures. Bonded joints can be between
two composite laminates or between a composite laminate and a metal/concrete
structure. Adhesively bonded joints can distribute load over a much wider area
than mechanical joints. Since no holes are required, the risk of local
delamination is practically eliminated. Compared to bolted joints with weight
of the joint is significantly reduced. On the other hand adhesively bonded joints
cannot be disassembled without destroying the substrate. Some adhesives are
susceptible to degradation by temperature and humidity. The most critical
drawback for safety critical structure such as airframes is their inspectability.
Critical joints may require ultrasonic inspection over their entire area.
Corrosion can be problem in carbon fiber composite to steel/aluminum joints
due to galvanic action. In such cases an intermediate insulating layer can be
used. Some of the most common types of adhesively bonded joints are
illustrated in Fig. 3-1. In addition to the lap and strap joints that are used in
bolted joints additional types are possible with adhesive bonding such as the
stepped joints and scarf joints [3,65,66].
Single-lap joints are efficient at transferring in-plane shear, but they should not
be used for compression loads unless the joint is stabilized because the
eccentricity increases as the load increases in compression. Adhesively bonded
doublers transfer load through the adhesive in the same manner as other
structural joints. The adhesive between the flanges of a stiffener and a skin is
loaded similarly to a bonded doubler case. The single-strap butt joint has two
disadvantages caused by the eccentricity in which the members butt together.
These disadvantages are (1) high bending moment in the splice plate and (2)
high adhesive peel stress, which may lead to either premature failure of the
21
Single lap
Double lap
Double strap
Double scarf
Single scarf
Double stepped lap
Stepped lap
Figure 3-1. Common adhesive joint configurations.
joint or delamination of the composite members. Thick adherends in single-lap
joints should have the overlap ends tapered. This reduces adhesive peel stress
so that the adhesive shear strength can be fully developed. The taper also
reduces the magnitude of the peak in the shear stress distribution, and when
both overlap ends are tapered, the strength of the joint increases.
Double-lap and double-strap joints are balanced stiffness designs: that is, the
stiffness of a strap or splice sheet should be equal or slightly higher than one-
half of the central sheet stiffness, and the two straps have equal stiffness.
Again, these straps and splice plates should be tapered at the ends to reduce the
adhesive peel stress. A maximum taper tip thickness of 0.5 mm for the splice
22
sheets. Another detail that reduces the adhesive peel stress and the peak shear
stress is locally thickening of the adhesive at the tips of the splice plates and
doublers. The stress condition at the end of the central sheet remains constant
with these modifications, but it is possible to increase the adhesive thickness at
this point by slightly tapering the end of the central sheet. Tension loads on the
joints do not generate adhesive peel stresses at the end of the central sheet
because these are compression stresses.
The stepped-lap bonded composite joint strength is improved when the
adherend stiffness is balanced, that is, a constant stiffness joint. The tip and end
thicknesses should be limited. The length and number of steps are additional
design variables, and the number of steps has the most effect on joint strength.
Step-lap joints have been used to bond composite to metal adhesively where
high magnitude concentrated loads are transferred, and the composite cannot
withstand the high local stresses associated with mechanical fasteners.
Adhesively bonded stepped-lap joints of composite to metal can be found in the
tails of the F-14, F-15, F-16, and F-18 and in the wings of F-18 aircraft. Scarf
joints are usually considered uniformly strained. In practice, however, the tips
of the adherends have a finite thickness and they must be analyzed as an
approximation to a stepped-lap joint with a fine grid over the length of the
overlap. In scarf joints that have adherends with different stiffnesses. there is a
tendency for the thin tip of the stiffer adherend to fail in fatigue [16].
Adhesively bonded composite joints have the following three basic failure
modes:
1. The failure is in the adherend outside of the joint, which fails at 100% of the adherend strength. This is the strongest joint. There is no adhesive failure.
2. The failure is caused by the shear strength of either the adhesive or the
laminate/adhesive interface. This is the next strongest joint.
3. The failure is caused by peel loads either as failure of the adhesive or as delamination of the adherend.
23
For the first failure mode, the strength of the joint is proportional to the
adherend thickness and no bending of the adherend is assumed. The loading
can be tension, compression, or in-plane shear. For the second failure mode, the
shear strength of the bonded joint is proportional to the square root of the
laminate thickness. For the last failure mode, the peel strength is proportional
to the quarter-power of the laminate thickness.
New joint designs were proposed for adhesive bonding of thick multi-layered
composite adherends by Bahei-El-Din and Dvorak [67] to reduce or eliminate
the failure modes associated with delamination and tensile and/or shear failure
of the surface plies that are often observed in lap joints, and provide for a better
stress distribution in the adhesive. In contrast to lap-joint designs, which
transfer in-plane tensile stresses and other loads from the adherends to doubler
plates by out-of-plane shearing of the surface plies, the new joint
configurations transfer most of the load by in-plane shear and normal stresses,
through bonded inserts or interlocking interfaces which have the same
thickness as the laminate adherends, as shown in Fig. 3-2. Doublers will
transfer a calculated percentage of the load.
b=15
bδ=
15 c,g d,h
b,f X1A
l2
3tδ d=9
l1 l=70δ=0.2 3td
Dimensions in mm X3
tA=30
td=3
td=3
Section A-A
X1
a b
fe
A
Figure 3-2. New design with diamond-shaped inserts [67]
24
Avila and Bueno [37] addressed the advantages and disadvantages of the wavy-
lap joint proposed by Zeng and Sun [68], Fig. 3-3, and they modified it. The
results showed an average increase on loading to failure close to 41%. This fact
could be due to the compressive stress field developed inside the wavy-lap
joint. In addition, this stress field distribution can also be the reason for the
adherent delamination observed on the wavy-lap joints. So far, the modiffed
wavy-lap joint seems to lead to stronger joints.
12o
24.6
227.8
8.40
101.6
6.15 6.15 Tab
12o Adhesive
25.40
2.80
Figure 3-3. Wavy-lap joint main dimensions.
3. 1. Stresses in Adhesive Joints
The single lap joint, in which two sheets are joined together with an overlay, is
one of the most common joints encountered in practice. The joint is easy to
make and the results are sensitive to both adhesive quality and adherend
surface preparation. The simplest analysis considers the adherends to be rigid
and the adhesive to deform only in shear. If the width of the joint is b, the
length λ, and the load P, then the shear stress τ is given by: τ = P/(bλ).
However, the adherend tensile stress will decrease linearly to zero over the
joint length from A to B, as shown in Fig. 3-4(a). In Fig. 3-4(b) is shown a
similar joint but in which the adherends are now elastic (i.e. deformable). For
the upper adherend، the tensile stress is a maximum at A and falls to zero at B.
Thus, the tensile strain at A is larger than that at B and this strain must
progressively reduce over the length λ. The converse is true for the lower
adherend. Thus, assuming continuity of the adhesive/adherend interface, the
uniformly sheared parallelograms of adhesive shown in Fig. 3-4(a) become
25
distorted to the shapes given in Fig. 3-4(b). This phenomenon is called
differential shear. Essentially, this is what Volkersen analyzed in 1938 (the
first one proposed a simple shear lag model based on the assumption of one-
dimensional bar-like adherends, i.e. the adhesive deforms only in shear, while
the adherend deforms only in tension) [see Refs. 3 and 69]. The linear elastic
analysis by others and the elasto-plastic analysis (Hart-Smith’s solution)
including the end effects of lap joints are reviewed in Ref. 3. There are many
different forms of structural joint, but most transmit essentially collinear loads
such as the lap joints. More complex configurations exist, such as corner or T-
joints, but these are very difficult to analyze.
(a) L
B A P
P
x
x
τ
B A
L
P
(b)
τ
P
Figure 3-4. Exaggerated deformations in loaded single-lap joint:
(a) with rigid adherends; (b) with elastic adherends.
In general, failure takes place in the adhesive (called cohesive failure) rather
than between the adhesive and the adherend or substrate (called adhesive
failure). Adhesive failure is often due to environmental degradation and cannot
be generally analyzed, although it is possible to treat specific cases, providing
simplifying assumptions are made. Peel loads are the greatest enemy of the
26
designer of bonded joints. Wherever possible, the adhesive should be loaded in
shear so that peel and cleavage stresses are avoided. Increasing the width of lap
or peel joints increases the strength pro rata, whereas increasing the length is
beneficial only for very short overlaps. However, the benefits to be gained from
having a large area of lightly stressed material in the middle of the joint,
especially when creep and fatigue and faulty manufacture need to be taken into
account. Figure 3-5 shows various ways of joining sheet loaded in tension. The
“loading'” rating is indicative of how well the joint will withstand the applied
load and the “cost” is an estimate of the assembly, cutting and purchasing
costs. Great accuracy is not claimed for the numbers quoted as these will vary
with adherend material and thickness and the choice of adhesive.
Figure 3-5. Joining of sheet materials in tension; the higher the loadrating, the stronger is the joint.
10 8
2 2
10 4
12 4
10 2
7 3
15 7
15 1
10 2
1 1
Rating Load Cost
27
3.2 Standard Mechanical Test Procedures
Since adhesives were first used, there has always been a need to define a series
of tests which can be carried out in order to quantify (or at least qualify) their
suitability in technology. Various tests have been proposed and some are still in
use today for structural adhesives which have been developed from the fabric
and wood-working industries. The wide variety of standard test procedures as
listed by the International Standards Organization (ISO), European Standards
(EN), American Society for Testing and Materials (ASTM), British Standards
Institution (BSI) and other organizations are essentially for testing adhesives
and surface treatments rather than joints. Unfortunately, most if not all of these
standard tests consist of joints in which the adhesive stresses are far from
uniform [3]. Let us therefore examine the reasons why we might need to carry
out any form of test on an adhesive, other than the not unworthy cause of
curiosity. The most commonly used test is the single-lap joint illustrated in
Fig. 3-6(a). These dimensions are as specified by ASTM D 1002 (very similar
to ISO 4587) which also specifies the adherend materials. It is recommended
that the specimens be cut from a 177mm wide bonded plate since this gives the
most representative results. The outer strips should be discarded. Note that this
joint is automatically misaligned before it is placed in the testing machine.
Some laboratories bond tabs at the ends to improve alignment, as shown in
Fig. 3-6(b). Even so, under load the joint will bend as shown in Fig. 3-6(c),
giving rise to large transverse peel stresses in the adhesive layer. As known, the
adhesive shear stress is nonuniform owing to differential straining in the
adherend and other factors: this is illustrated schematically in Fig. 3-6(d)
together with the associated adhesive peel (transverse) stresses in Fig. 3-6(e).
Even though it is still recommended in ASTM D 1002 and similar standards
that the results be given as the average shear stress at failure (i.e. load divided
by bond area), it has long been recognized that this average shear stress bears
little relationship to what is actually happening in a joint, especially when
geometric, adhesive and adherend nonlinearities become significant.
28
Figure 3-6. Single-lap joint test piece to ASTM D 1002 (dimensions in mm).
Transverse adhesive stress (acting across the bond-line thickness)
(e)
F F
(c)
Average shear Stress = F / Area
(d)
Adhesive Shear stress
F F
(b) Alignment tabs
F F
101.6 63.5 12.7±0.25
25.4
(a) Single-lap joint
Grib area
1.62±0.125 F
F
29
The advantages of the single-lap test are that it is simple, cheap, uses a standard
tensile testing machine, and there are a lot of data available for comparison. Its
main disadvantage is that the reported average shear stress is not an intrinsic
adhesive property. Curiously, this very disadvantage is also an advantage since
no-one really believes that the average shear stress means anything
fundamental. In other tests with apparently more precisely controlled
conditions, it is common for misleadingly definite values to be quoted. Also,
the complex stress situation in the single-lap test makes it quite representative
of many structural applications and loading situations. This test is widely used,
often abused, but remains one of the most trusted standards. Nevertheless,
there have been many attempts to improve the single-lap test. These include the
laminated assembly (ASTM D 3165) and sometimes wrongly referred to as a
double-lap joint.
3.3. Fracture Mechanics Considerations
Adhesive joint studies proposed in the literature usually fall into two classes:
those based on stress analysis and those based on fracture mechanics. Fracture
mechanics has become a very popular tool for the characterization of adhesive
joints in recent years. Fracture mechanics tests are routinely conducted by
industry during materials development and have also found extensive
application in fatigue and durability studies over the past 20 years. More
recently, fracture mechanics data has been used to predict the impact failure
response of, for example, the impact wedge peel test and currently fracture
mechanics data are finding application in structural impact studies via the use
of cohesive zone models. The use of Linear Elastic Fracture Mechanics
(LEFM) tests to measure the mode I adhesive fracture energy, GIC; of adhesive
joints dates back to the work of Ripling and co-workers in the 1960s who
developed a mode I test method to measure the toughness of structural bonds
between metallic substrates. This work led to the publication of an ASTM
standard in 1973 [see Ref 70]. Since then, the many developments in the
application of fracture mechanics to, for example, FRP composites has created
30
great potential for the development of a new test protocol for structural
adhesive joints. It was against this background that a technical committee of
the European Structural Integrity Society (ESIS) commenced work on
structural adhesives test methods in 1997. The new protocol accommodated the
use of both metallic and FRP composite substrates. During the course of the
wide ranging program involving ten test laboratories, modifications were made
to both the experimental and analytical procedures compared to the original
ASTM standard of 1973. For example, a new corrected beam theory analysis
for the tapered double cantilever beam was developed and a correction for
system compliance and additional validity checks were built into the
experimental procedure. Following these modifications, the revised protocol
was submitted to the British Standards Institution for consideration as a British
Standard (under the direction of the ‘‘Adhesives Standards Policy Committee
PRI/52’’) and was accepted and subsequently published in 2001. It is intended
that this document should also be published as a European standard. Blackman
et al. [70] described the stages in the development of this new protocol and the
modifications made in the light of the results from the round-robin tests. The
results from the inter-laboratory round-robin highlighted the importance of
correcting for system compliance effects if accurate and reproducible results
are to be obtained. The values of GIC deduced were shown to be independent of
test geometry but dependent upon the substrate material used to make the
joints. Additional studies have shown that the substrate dependence was due to
the cured adhesive in the different joints possessing different values of glass
transition temperature. The existence of pre-bond moisture in the CFRP
substrates and variations in heat-up rate during cure were both shown to affect
the Tg of the cured epoxy-paste adhesive employed in the present work. The
pre-bond moisture effect was however, much more important than the heat-up
rate effect but both would need to be considered when optimizing joints for
toughness with the present adhesive.
31
There are three types of delamination test to obtain pure mode I loading, pure
mode II, and mixed mode I/II loading [71]. For the mode I Double Cantilever
Beam (DCB), Fig. 3-7(a), if it is assumed that the compliance CI is a function
of (a + |∆|)3 where “a” is the crack length and ∆ is a correction parameter, a
first strain energy release rate expression is
)∆2b(a3PδG I +
= (3-1)
where P is the applied load, δ the crack opening displacement, b the specimen
width. ∆ is determined from the plot of C1/3 versus a. Another compliance
expression is the following:
C = k an (3-2)
where n is the slope of the ln(C) versus ln(a) curve. Therefore, the
energy release rate expression becomes
2abnPδGI = (3-3)
For the mode II End Notched Flexure (ENF) test, Fig. 3-7(b), a first energy
release rate formulation comes from beam theory:
f32
2
II Eh16bP9aG = (3-4)
where P is the applied load, Ef is the flexural modulus such that
o3
3
f C4bhLE = (3-5)
32
in which C0 is the compliance obtained experimentally from a three-point
bending test in the elastic range on the uncracked specimen. Another
expression involving � may be used:
)32(29
33
2
aLbPaGII +
=δ (3-6)
If we assume that the compliance determined experimentally can be written as
C = C0 + ma3 (3-7)
the energy release rate is expressed in the form
2bP3maG
22
II = (3-8)
The Mixed Mode Bending (MMB) test, Fig. 3-7(c), is the superposition of the
two previous tests, and it allows any combination of mode I and mode II
loadings to be obtained. It consists of a three point bending test, in which the
load P is applied by means of a lever with a central fulcrum, at a distance c
from this fulcrum. The load bends the MMB specimen at the fulcrum and at the
same time pulls the delamination open, which gives a combination of mode II
(bending) and mode I (crack opening) loadings. The mixed mode ratio I/II can
be modified by changing the lever length c. An MMB test analysis is based on
beam theory. The expressions for mode I and mode II energy release rate
components are
Eh16bP9aG
,EhbP12aG
32
2II
2
II
32
2I
2
I
=
= (3-9)
33
PI and PII are the pure mode loading components:
P,L
LcP P,4L
L3cP III
+
=
−
= (3-10)
where c is the lever length, and ¸ is half of the distance between lower loading
points. The mixed mode ratio is defined by
L/3c ,LcL3c
34
GG 2
II
I ≥
+−
= (3-11)
It is independent of the crack length a and it only depends on the lever length c.
Figure 3-7. Test configuration of three different types of delamination test.
L c P
2h
a
L L
(c) MMB test configuration
(a) DCB test configuration.
δ
a P
P
(b) ENF test configuration.
δ L L
P
a
34
3.4. Fatigue behavior and Durability of Adhesive lap joints
The response of bonded joints to fatigue loading has been extensively
researched in the last years concluding that the failure generally initiates at the
adhesive [72,73] for moderate stress levels. The fatigue life is strongly
influenced by the profile of the edges of the joint. The initiation of small cracks
at the edges points represent the major part of the fatigue life. The static and
fatigue properties of polymeric adhesives can be influenced by centered defects
[73,74] and environmental conditions such as: temperature, moisture and
chemical agents [72]. It has been shown that thermal effects, whether due to
mismatch of the adherends or to adhesive contraction by temperature of cure,
lead to significant changes in the stress state of lap joints. The temperature Tg
of the adhesive should be above the maximum temperature expected in service.
It has been also verified that the strength of joints usually decreases in presence
of humidity and time of exposure. Also the debonding of adhesives caused by
cleavage of cracks increases with the water presence in form of liquid or vapor.
This is the result of the hydrophilic nature of adhesives, which is caused by the
polar groups, needed to confer adhesive properties on polymeric materials.
Therefore chemical degradation of the adhesive, substrate and chemical bonds
across the interface is possible as a result of interaction with water. The water
can enter in the adhesive and then attack it by diffusion through the adhesive
and adherent, and finally transport along the interface and move by capillary
action through cracks in the adhesive. The moisture can not only affect the
adhesive but also the mechanical behavior of the adherend composite. However
no significant effect of moisture was observed on the static strength and fatigue
crack growth of laminate composites.
3.5 Prevention of Peeling failure in Adhesive Joints
Where peel is encountered with lap joints, or even to counter the peel loads
inherent in loading even in the double-lap joint, various techniques can be
used; these are illustrated in Fig. 3-8. Of these, the best is probably the positive
constraint of the joint end by riveting, bolting or spot welding [3]. However,
35
the arrangement shown in Fig. 3-8(d) is also common and is used in the
construction of nearly all motor car doors, the joint being called a clinch or
hem-flange join. Where lap joints are subjected to bending moments a rivet,
bolt or spot weld near the end A, where the joint tends to open in peel, should
be used if possible, as shown in Fig. 3-9(a). At the other end B, the adhesive is
in compression and failure will not occur there. Alternatively, an overhanging
reinforcement plate may be used, as shown in Fig. 3-9(b). This has the
advantages of reducing the bending strain in the upper adherend, thus
strengthening the joint, and transferring the load further into the joint [3].
Rivet, bolt or spot weld
Increase stiffness
(a)
Increase area
(b)
Bead end (if possible)
Peel action
(c) (d)
Figure 3-8. Techniques for combating peel.
36
Reinforcement
(b) A
B
(a) A
B
Figure 3-9. Combating bending loads in lap joints using (a) a rivet, and (b) overhanging reinforcement.
3.6 Adhesive Bonded Dissimilar Materials
The use of adhesive bonded structure is significantly increasing recent years. It
is well known that mechanical properties of the bonded structure depend on the
surface conditions of the substrates. Accordingly, many studies have been
made on the surface treatment of polymer, ceramic, and metallic substrates to
improve the mechanical properties of FRP/metal, FRP/concrete, steel/concrete,
metal/metal and metal/ceramic bonded structures [75-84]. One appealing way
to bond composite parts to steel structures is to mold a steel edge into the
composite, and then weld this assembly to the steel structure via this edge. This
requires that the steel can successfully be molded into the composite during the
manufacturing. The strength of these joints can be increased [76] by using
perforated steel, as patented by Unden and Ridder [77]. The perforations will
decrease the elastic mismatch between the stiff steel and the relatively
compliant FRP, and as an added benefit provide mechanical interlocking.
37
Co-cured joining method [78], which is regarded as an adhesively bonded
joining method, is an efficient joining technique because both the curing and
joining processes for the composite structures can be achieved simultaneously.
The co-cured joining method requires neither an adhesive nor a surface
treatment of the composite adherend because the excess resin, which is
extracted from composite materials during consolidation, accomplishes the co-
cured joining process. Since the adhesive of a co-cured joint is the same
material as the resin of the composite adherend, the analysis and design of the
co-cured joint for composite structures are simpler than those of an adhesively
bonded joint, which uses an additional adhesive.
The manufacturing process of the specimens of co-cured single and double lap
joints of the plate type with steel and composite adherends was introduced by
Shin et al. [78]. They found that, the initial failure mechanism of the co-cured
single lap joint was interfacial failure between steel and composite adherends
and the failure mechanism of the co-cured double lap joint was cohesive failure
by delamination at the first ply of the composite adherend. Out-of-plane tensile
and shear stresses played an important role of the interfacial failure of the co-
cured single lap joint and only an out-of-plane shear stress played a most
important role of the cohesive failure of the co-cured double lap joint.
Dvorak et al. [79] explored a new approach for the joining of thick, woven E-
glass/vinyl-ester composite laminated plates to steel or other composite plates,
with applications in naval ship structures. Adhesive is applied along through-
thickness contoured interfaces, employing tongue-and-groove geometry. They
found that adhesively bonded tongue-and-groove joints between steel and
composite plates loaded in monotonically increasing longitudinal tension are
stronger than conventional strap joints even in relatively thin plates.
Melogranaa et al. [80] studied tongue-and-groove geometries, see Fig. 3-10, in
thin composite laminated plates. Adherends were comprised of 2.7 mm thick
38
stainless steel and 1.6 mm thick T700/SE84HT carbon fiber/epoxy composites.
The adhesive used was the two-component epoxy paste Hysol 9430. They
compared their thin tongue-and-groove joints to conventional single lap joints
manufactured with the same materials and similar dimensions. The tongue-and-
groove specimens with large aspect ratio tongues were stronger than the
conventional single lap joints.
Wgroove
Figure 3-10. The general geometric parameters for a tongue-and-groove joint.
Rmax
L=25.4[1.0]
tg=0.1 [0.039]
w=2
5.4
[1.0
]
Rtip
T700 carbon/epoxy AL–6XN steel
So far, FRP has mostly been used for strengthening concrete structures [21].
Perhaps the single most critical parameter in strengthening reinforced concrete
with externally bonded FRP/steel plate is the bond achieved between the
reinforcing FRP/steel and concrete, which is responsible for the mechanism of
stress transfer and therefore the composite behavior. Debonding of the
FRP/steel plate from the concrete surface can significantly limit the potential
flexural and shear strength enhancement provided by the FRP/steel to the
repaired structure[81-84]. A brief review of this topic was presented by Nehdi
et al. [81] and De Lorenzis et al. [84]. Figure 3-11 summarizes some of the
experimental techniques used in the literature for testing the bond of FRP plates
to concrete. These tests often involve direct tension or bending and therefore
require special handling.
39
Figure 3-11. Various techniques for testing bond of FRP plates to concrete.
3.7 Fusion Bonding for Joining Thermoplastic Matrix Composites
An ideal structure would be designed without joints, since joints are potentially
sources of weakness and additional weight. In practice however, upper limit to
component size is generally determined by the manufacturing processes.
Further requirements for inspection, accessibility, repair and transportation or
assembly mean that load-bearing joints will be part of an engineering structure.
This is particularly so in the manufacturing of thermoplastic composites (TPC)
for which high melt resin viscosity and constraints imposed by the continuous
reinforcement limit the production to relatively simple geometry components
which must be joined together to produce large, complex structures. The
extensive experience available from the thermoplastic polymer (TP) industry
showed that to make large or complex parts, the most cost-effective method
often involves molding two or more parts and joining them together.
Thermosetting (TS) adhesive bonding is inherently preferable to mechanical
40
fastening because of the continuous connection avoiding large stress
concentrations induced at each discrete fastener hole. However, there are a
wide range of contaminants present on substrate surfaces [85]. These need to
be eliminated prior to bonding by using a surface preparation treatment and
increase the surface roughness (improving mechanical interlocking and
increasing bonding surface area). Extensive surface preparation and long curing
times make adhesive bonding labor intensive. Typical surface treatments used
for adhesive bonding are generally hard to control and affect directly the
strength and durability of bonded joints.
Fusion bonding, or welding, is a long established technology in the
thermoplastic industry where the efficiency of the welded joint can approach
the bulk properties of the adherends [85]. Although welding may induce
residual stresses if performed without adequate control, it eliminates the stress
concentrations created by holes required for mechanical fasteners and so does
TS adhesive bonding. In addition, welding reduces processing times and
surface preparation requirements. However, the high content in carbon fiber
(CF) reinforcement in TPCs, resulting in high thermal and electrical
conductivity, imposes difficulties such as uneven heating, delamination and
distortion of the laminates. These problems become more difficult when
bonding large components. In addition, as fiber volume fraction increases, the
amount of resin available to melt and reconsolidate into a fused joint is reduced
and this can affect the welding quality.
Fusion bonding techniques have often been classified according to the
technology used for introducing heat. In this review article, fusion bonding
techniques were classified into four classes, as shown in Fig. 3-12, namely bulk
heating (co-consolidation, hot melt adhesives, dual resin bonding), frictional
heating (spin welding, vibration welding, ultrasonic welding), electromagnetic
heating (induction welding, microwave heating, dielectric heating, resistance
welding) and two-stage techniques (hot plate welding, hot gas welding, radiant
41
welding). Bulk heating techniques such as autoclaving, compression molding
or diaphragm forming are available for performing co-consolidation.
Co-consolidation is an ideal joining method as no weight is added to the final
structure, no foreign material is introduced at the bondline, essentially no
surface preparation is required and the bond strength is potentially equal to that
of the parent laminate. However the entire part is brought to the melt
temperature, and this generally implies the need for complex tooling to
maintain pressure on the entire part and to prevent de-consolidation. Hot melt
thermoplastic adhesive films may be inserted at the bondline to improve filling
of parts mismatch. Inserting of an amorphous polymer interlayer proved to
reduce the scatter of strength, which widens the processing window.
The dual resin bonding, or amorphous bonding, involves co-molding an
amorphous TP film to a semi-crystalline TP matrix laminate prior to bonding as
in the Thermabond® process, in which a PEI film is coated onto APC-2
laminates. During the joining step, the amorphous PEI film can be fused at a
temperature above its glass transition temperature below the melting
temperature of the semi-crystalline PEEK polymer avoiding any deterioration
of the bonded structure.
In two-stage techniques the heating device needs to be removed from between
the substrate surfaces between the stages of heating and forging. This aspect
involves limitations on size of the component since the whole welding surface
must be heated in a single step. Heating times are normally long as they rely on
the low thermal conduction of heat through the polymer. Between the heating
and forging steps, surface temperature drops and the region experiencing the
maximum temperature is located below the skin of the laminate. The high
pressure required to consolidate the bondline may cause warpage/flow in the
higher temperature inner region.
42
Spin welding and vibration welding have been extensively used in the plastics
industry but are less appropriate to joining TPCs as the motion of the substrates
relative to one another may cause deterioration of the microstructure, such as
fiber breaking. The process was however investigated for joining APC-2 and
GF/PP systems.
Microwave and dielectric welding are available for joining thermoplastics but
the fact that heating occurs volumetrically and that multi-layer composites are
excellent shields in the microwave range make these techniques poorly suitable
to welding of TPCs particularly when they are reinforced by CFs. The three
most promising fusion bonding techniques are ultrasonic welding, induction
welding and resistance welding. In these techniques, only the welding interface
is brought to the melt temperature, minimizing the impact on the rest of the
structure. Welding times are very short. Large-scale welding may be performed
through sequential or scanning approaches, and on-line monitoring of the
consolidation is possible.
Bulk Heating
Co-consolidation Hot-melt Adhesives Dual Resin Bonding
Frictional Heating Electromagnetic Heating
Hot Plate Welding Hot Gas Welding Radiant Welding
Infrared Welding Focused Infrared Welding Laser Welding Solar Energy
Two-stage Techniques
Spin Welding Vibration Welding Ultrasonic Welding
Induction Welding Microwave Heating Dielectric Heating Resistance Welding
Fusion Bonding
Figure 3-12. Fusion bonding techniques.
43
4. SNAP JOINT IN COMPOSITE STRUCTURES
In the majority of the composite structural components, both bolted and/or
adhesive bonded joint was used. Most of the details are similar to those for
metal joints. It was shown from extensive testing on bolted composite joints
that failure always occurs in a catastrophic manner due to high stress
concentration developed at the bolt locations. Due to the inherent low bearing
and interlaminar shear strengths of composites, these stress concentrations
threaten the downfall of every piece of the composite structure [86-87]. The
optimum composite joint design is the one capable of distributing stresses over
a wide area rather than to concentrate them at a point. Adhesively bonded joints
can satisfy these requirements, however, most of the adhesives are brittle, and
brittle failure is unavoidable. This was the motivation of developing what is
called the SNAP joint, Fig. 4-1.
Snap Joint Concept Hardware for fasternerless snap joint
B A
Figure 4-1. Snap joint.
The snap joint technology developed by W. Brandt Goldworthy & Associates,
Inc. [86]. The concept is based on similar joining technology used for
connecting wooden parts. Also, this technique is very similar to techniques
which were used a decade or so again for plastic. Figure 4-1 shows a pultruded
structural composite member (A) with one end shaped as a fir-tree, and
therefore has a large load bearing area. In this figure, part (A) has been snapped
into another structural shape (B). From Fig. 4-1 one can see that, the later shape
has been designed to combine its structural shape with functionality that allows
44
for the engagement of the load-bearing surface of member (A). It is possible to
"snap" joint both parts together since part (A) has been cut for a short distance
along it length to provide enough lateral flexibility to move out of the way
when entering part (B). In order to make this joining concept successful, the
fiber architecture of part (A) must be designed in such a way that the load
bearing surfaces have higher interlaminar shear strength capacity. Also, it can
be noticed from the figure, that a circular hole was introduced at the end of the
horizontal slot of part (A) to inhibit the crack propagation along the length of
the pultruded member. The applications of this technology in composite
structures will have benefits as follow:
• The structures are easy to assembly.
• Installation of structure members become faster.
• Installation needs smaller number of labor and equipment.
• Since it use composite materials, its weight is less than traditional
structures.
The first prototype or "Demonstration project" using this joining method was in
designing and construction of three Transmission Tower Structures, Fig.4-2(a),
near Los Angeles by W. Brandt Goldworthy & Associates, Inc. and Ebert
Composites Corporation [86]. Through California Department of
Transportation, they have proposed the design and the construction of a truss
structure to carry highway singes, Fig. 4-2(b).
The snap joining technique is considered to be one of the optimum techniques
to join composite structural members. However, it has a major limitation, and
can only be used in specific applications. That is, this method can only be used
to transmit axial loads, which make it ideal for truss-type structures. However,
in my opinion, this method should NOT be recommended when out-of-plane
loads or any shear loads are introduced since the connection is not design to
carry any major bending moments. Under flexural loads, it is expected that the
joint will be very flexible, and the artificial cracks introduced to members will
45
propagate and a complete failure will occur even under moderate service
flexural and/or shear loading.
(a) Composite Transmission Tower (b) Truss Joint
Figure 4-1. Snap joint projects.
5. HYBRID JOINTS
In an attempt to improve the joint strength of composite materials, a hybrid of
adhesive and bolted joints has also been explored [88], Fig. 5-1. Hybrid joints
failed at a higher load than the bolted joints and with the proper clamping
torque reached the same failure load as the adhesive joints. Furthermore, unlike
the adhesive joints, hybrid joints failed in two steps, first by initiation of fiber
tear (akin to delamination in laminated continuous fiber composites) at one of
the lap ends and then by tensile failure across the bolt hole. This led to a
slightly higher overall elongation at failure for specimens with the hybrid
joints. Failure in fatigue also started by fiber tear and when the fiber tear
progressed to the bolted area, a combination of half-net-tension failure and
splitting (cleavage failure) occurred. In both static as well fatigue, failure was
initiated by fiber tear and the round washers with their edges located slightly
away from the lap ends were not effective in preventing fiber tear. Fu and
Mallick [88] found that, hybrid joints give better static as well as fatigue
46
performance than adhesive joints in structural reaction injection molded
composites when fiber tear, the primary failure mode in adhesive joints, is
either prevented or delayed by the presence of clamping. Their finite element
analyses proved that the presence of the lateral clamping pressure can
significantly decrease the maximum peel stress at the interface, which helps in
achieving improved joint performance.
Figure 5-1. Sketch of an adhesive/bolted (hybrid) joint.
5.1. Hybrid Joints in Plated Beams
The main drawback of the RC member strengthened by externally bonded
plates is the peeling failure [89,90], Fig. 5-2. Several researchers suggested
analytical models to predict the critical tensile and shear stresses at the ends of
glued plate at the bottom face of the RC beams. This means that, the peeling
crack may grow under mode I, opening mode, or mixed mode, opening and
sliding modes. The anchorage applied at the ends of externally bonded plate
may take several forms, including bolted anchorage systems, bonded angle
sections to provide anchorage of the reinforcing plate to both sides of the beam,
and trapping the plate under the beam supports, Fig. 5-3.
Figure 5-2. Cover separation failure mode.
47
Adhesive-filled bore
CFRP plate Mild steel bolt
Anchorage block
a: Plate ends under supports
CFRP plate
Support roller
Bearing pad
b: Plate ends bolted to beam
Figure 5-3. Plate end anchorage systems adopted by
Garden and Hollaway [see Ref. 90]
Saba [89] suggested two different techniques to preclude the peeling failure at
the ends of steel plates glued to the soffits of reinforced concrete, RC, beams.
In the first technique, concrete cover was replaced by grout to enhance the
resistance of substrate to crack initiation and propagation. In the second one,
permanent compressive forces at the ends of bonded plate were inserted using
different plate end anchorage systems, i.e. end anchorage by bolts or by
clamps, Fig. 5-4.
Grout 25 cm
2 m
5 cm35 cm
70 cm 80 cm80 cm
P/2 P/2
P/2 P/2
(a)
(b)
15 mm St. 6 Studs 14 mm
2 m
70 cm 80 cm80 cm
P/2 P/2
P/2 P/2
25 cm
Figure 5-4. Plated beam with (a) partial replacement of concrete cover, and (b) plate end anchorage by steel clamps
48
Schnerch et al. [91] used a steel angle to clamp the fiber to the steel base plate
before the pot life of the epoxy had been reached as shown in Fig. 5-5.
Figure 5-5. Steel Angles used to mechanically anchor CFRP plys.
Figure 5-6 shows the upgrading techniques, which used hybrid joint concepet,
for reinforced concrete beam–column joints proposed by El-Amoury and
Ghobarah [92].
FRP sheets
Steel angle
U-shaped steel plates Steel plates
Steel plate
FRP sheets
Steel rods
Figure 5-6. Retrofitting schemes
49
6. SUMMARY 6.1 Mechanical Joints (Bolts or Rivets)
Mechanical joints are used when repeated disassembly and reassembly is required or
when surface preparation is not practical. Unfortunately, well-established joining
technologies for metallic structures are not directly applicable to composites for several
reasons, e.g.:
• Stress concentration created by the presence of holes and cut-outs, which is worsened by the lack of plasticity limiting stress redistribution.
• Delamination originating from the localized wear occurring during drilling. • Differential thermal expansion of fasteners relative to composite. • Water intrusion between fasteners and composite. •Possible galvanic corrosion at fastened joints. •Additional weight of fastening system.
Evolution of Mechanical Joints In mechanical joining, there seems to be a distinct shift toward integral attachment using
features either designed into or formed into the parts to be joined:
Snap-fit integral features are
beginning to appear in composites:
The use of so-called “hook-and-loop” attachments (likeDuPont VelcroTMand 3M Dual-LocksTM) will find greater and more-
diverse application:
Tog-L-Loc®
A generic hook-and-loop concept representing DuPon's VelcroTM
Punch side
Cross Section
Die side "Button"
6.2 Adhesive Bonding
This joining technique is inherently preferable to mechanical fastening as the continuous
connection avoids large stress concentration induced at each discrete fastener hole. The
50
presence of a wide range of contaminants on substrate surfaces (release agents and
bagging materials, fluorocarbon release sprays and films, machining oils, fingerprints,
etc.) makes however necessary a preliminary surface preparation treatment, which may
also serve to:
• Improve wetting of low energy surfaces. • Chemically modify the surfaces (introducing polar groups or coupling agents). • Increase the surface roughness.
Adhesive Bonding Benefits
• It is still possible to make joints not only between two composite laminates, but also between a composite laminate and a metal, or a concrete, or a wood structure.
• Loads are distributed over a much wider area than in the case of mechanical joints. • Since no holes are required, the risk of local delamination is practically eliminated. • Significantly reduced weight of the joint. • Possibility to make a wide range of configurations (e.g. stepped or scarfed joints).
Adhesive Bonding Drawbacks
• Adhesively bonded joints cannot be disassembled without destroying the substrate. • Some adhesives are susceptible to degradation by temperature and humidity. • Poor inspectability(critical joints may require ultrasonic inspection over their entire
area). • Corrosion can be a problem due to galvanic action. • Typical surface treatments are generally hard to control in an industrial
environment and directly affect the strength and durability of bonded joints.
6.3 Hybrid joints
Hybrid joints give better static as well as fatigue performance than adhesive joints in
composites when fiber tear, the primary failure mode in adhesive joints, is either
prevented or delayed by the presence of clamping. The presence of the lateral clamping
pressure can significantly decrease the maximum peel stress at the interface, which helps
in achieving improved joint performance. The square washer representing full clamping
to the edges of the overlap area, gives a better performance compared to round washer,
representing partial clamping.
51
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