Fracture Studies on a Polyacetal- New Thermoplastic Polyurethane Blend*

REJI JOHN, N. R. NEELAKANTAN, and N. SUBRAMANIAN

Department of Chemical Engineering Indian Institute of Technology

Madras-600 036, India

and

E. M. S. NAIR

Department of Ship Technology Cochin University of Science and Technology

Cochin-682 022, India

Studies based on fracture mechanics have been made on polyacetal toughened with a synthesized thermoplastic polyurethane. The effects of elongation rate and defects like holes and notches on tensile properties have been investigated. Using three-point bending specimens, fracture mechanics parameters such as strain energy release rate, G, Rice’s contour integral, J , and fracture toughness, K have also been determined under plain stress conditions. The mechanical loss from the hysteresis curves, and ductile, brittle fractography using scanning electron microscopy have also been studied.

INTRODUCTION materials under loads at various conditions. In con-

olyacetal (polyoxymethylene, POM) is one of the P major engineering thermoplastics in use today. It is somewhat brittle in nature, especially when defects are present, and this is at times the case in practice. Hence, it is necessary to develop a tough- ened POM to widen its scope of utility as an engi- neering material. Patent reports (1 -4) regarding the impact toughening of POM indicate that the impact resistance of POM can be improved by blending a rubbery polymeric material having a sub-zero Tg. Since these are patents, details are not known. F. C. Chang, et al. (5). have reported that thermoplastic polyurethane (TPU) is a better toughening agent for POM because of the significant physical interactions between these two in blends. TPU is being exten- sively used to fabricate components for various engi- neering applications, because of its many desir- able properties. Hence, a judicial combination of POM and TPU is an excellent engineering material Which can even replace metals in certain areas of applications.

Design of engineering components requires sound understanding about the fracture behavior of the

ventional design, the material is assumed to be ho- mogeneous and defect-free, and design practices do not consider prevention of failure initiation at the defects, or imperfections that will be inherently present or caused in all materials, either during fabrication or in service. So the nature of the origi- nal flaw and its subsequent behavior under load are of importance. If the flaw is small, it is possible that most of the life may be spent in an initiation phase or the flaw may not grow at all and in certain cases this could be controlled by yielding and crazing mechanisms. So if flaw size and its behavior are known, a safe working stress may be computed.

A component with a defect may not fail immedi- ately on loading, but may experience a stable crack growth before final failure. If the load is kept below a certain value, the crack may not grow at all. Hence, it may be of immense importance of one can predict the load at which instability sets in when defects are present in structures. Fracture mechanics can give answers to some of these problems. Fracture studies of this newly developed polyurethane blend were undertaken with the intention of gathering design information and also to see how this TPU behaves compared to the commercial TPU (Estane) when blended with a POM (Celcone). Chang, et al. (5). and Flexman (6) have studied some aspects of fracture mechanics of the blend of commercial TPU and POM,

~ _ _ _ _ _ ~ . ~~- ~ _ _ *Part of this paper has been prepared for presentation at the national seminar on Advances in Polymer Technology, held at the Dept. of Poly-

Technology. Cochin. India, on Feb. 5-7, 1991. mer Science and Rubber Technology, Cochin University of Science and

20 POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, VOI. 32, NO. 1

Fracture Studies on a Polyacetal-TPU Blend

based on its impact fracture properties. Here we are presenting an approach based on the flexural and tensile failure properties.

FRACTURE MECHANICS APPROACH

Energy Release Rate

Linear elastic fracture mechanics concepts are ap- plied to evaluate fracture parameters for the plane stress case. The strain energy release rate, G, known as the crack driving force for fracture (7), comes froin the energy which is released when a crack grows resulting in lower final energy state and is computed using the following equation.

P 2 dc

2 B da G=--

P IS the load ( N ) at which instability sets in the specimen and u is the displacement (mm) at the corresponding point. B is the thickness of the speci- men (mm), c is compliance-u/P, and a is the crack length (mm). Using a number of three point bend specimens with various crack lengths, the compli- ance curve is generated and dc lda is obtained. Then the calculation of strain energy release rate, G, is straightforward.

J Integral

Jr is a path independent integral and is used to characterize fracture criteria for non linear elastic materials. Based on the proposal by Begley and Landes (8) , the J integral can be computed as,

2 u J =

B(W-U)

where W is the width of the sample (mm) and U is the area under the load-displacement curve ( N . mm). The load-displacement diagrams show that the plas- ticity effects are negligible (when defects are pres- ent) and linear elastic fracture mechanics concepts appear valid. It is well established that G and J are equal for the linear elastic case. For small specimens utilized in this study the above computations agree quite well.

Fracture Toughness

For brittle materials, linear elastic fracture me- chanics can be conveniently applied if one can de- termine the fracture toughness K or stress intensity factor. The stress intensity factor can be expressed as (91,

K = aJ.lra~( (3)

where u is the applied stress (N/mm2). For three point bending,

where A4 is the bending moment ( N mm), L is the span distance (mm), F (01) is the geometry factor taken from (9) as,

F ( a ) = 1.107 - 2.12001+ 7.7101'

- 1 3 . 5 5 ~ ~ ~ + 14.2501~ 01 = a / w

In this study attempts are made to relate K as a function of crack length for plane stress cases. It may be noted that in testing it is desirable to employ the most severe conditions to explore the worst case as defined by ASTM (10) for the bending test.

EXPERIMENTAL

Materials

A new thermoplastic polyurethane (TPU) has been made out of a castor oil based polyol (the polyol has been developed by the Indian Space Research Or- ganization, a patented product and used for solid rocket propellant binder), toluenediisocyanate and 1, 4 butanediol (single shot isothermal bulk poly- merization, block ratio 1:7.2:6). It is a low molecule weight TPU ( aw = 26,000, = 17,000, measured by gel permeation chromatography). It was blended with a POM (Celcon M-140, Hoechst Celanese, USA, unmodified copolymer of trioxane (the cyclic trimer of formaldehyde) and cyclic ether, hereafter this blend is referred to as type 1). The blend ratio by weight was P0M:TPU = 70:30. We have selected this composition because this yields better mechanical properties compared to other compositions. During mechanical testing of this blend, brittle failure oc-

curred when flaws were present. This prompted us to study its behavior based on fracture mechanics concepts. It may be noted that the plane strain behavior requires thicker specimens. But as it is difficult to injection mold very thick specimens, test were conducted with injection molded specimens with smaller thickness. The blending was melt blending with a single screw extruder at various zone temperatures ranging from 180-200°C using a mixing head.

To compare the tensile and flexural properties, a blend of the same POM with a commercial TPU (Estane-BF Goodrich, Belgium) of the same composi- tion has been prepared in a similar manner at the same temperatures (hereafter this blend is referred to as type 2). The test specimens of the dimensions 150 x 14 x 1.5 mm and 120 x 14 x 5 mm were injec- tion molded at temperature ranges of 190-210°C in the various zones of the injection molding machine.

Procedure

The tensile and flexural tests were conducted us- ing a Zwick universal testing machine at 32.5"C and relative humidity 65 %. The normal cross-head speed for all tests was 2Omm/min. unless otherwise speci- fied. For fracture studies, the notch parameters used are: (1) 2r/w-0.128, 0.143, 0.157, and 0.171 and (2) a/w-0.0714. 0.143, 0.214, and 0.286, where r

POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, VOI. 32, NO. 1 21

Reji J o h n , N. R. Nee lakan tan , N. S u b r a m a n i a n , a n d E. M . S . Nair

is the radius of the hole which is drilled at the middle of the specimens, w is the width and a is the notch length of the razor cut specimens. To study the effects of loading rate on tensile properties, other cross-head speeds of 2 and 10 mm/min were also employed.

For the three-point bending tests, notched and unnotched samples were used. For notched sam- ples, notches of 1 to 4 mm length were employed. The notches were carefully made with a sharp razor to avoid crazing and crack branching. The distance between the two spans was 80 mm and the loading rate was 5 mm/min. A fractographic study was made using a scanning electron microscope.

RESULTS AND DISCUSSION

The stress-strain behavior of the type 1 blend at various cross-head speeds were studied and shown in Fig. 1 . It is observed that the behavior is depen- dent on the speed at which the specimens were elongated. Increasing strain rate decreased the de- formation capability (ductility) of the blend and re- sulted in a brittle type of failure. This is because at high strain rates, the crack initiates through the relatively smaller precrack plastic zone and the competition of ductile and brittle modes of fracture takes place during the early stage of crack propaga- tion. Because of a high deformation rate, the critical size of plastic zone is not well established, the prop- agating crack front will quickly pass through the relatively smaller plastic zone and brittle failure oc- curs. For example, the type 1 blend, at a cross-head speed of 2 mm/min., was capable of deforming to a stress level of 20%. while at 20 mm/min. this was reduced to about 3% or below. Type 2 also exhibited a similar type of behavior and is also shown in Fig. 1. The tensile modulii of elastic resilience were com-

60

A 3

0 1 I I I I

*I. STRAIN

Fig. 1 . Effect of cross-head speed on tensile properties. A,. Estane blend, B, . synthesized TPU blend at 2 mmlmin., A, and B, that at 10 mmlmin., A, and B, that at 20 rnm min.

parable for both types through the type 2 blend exhibited a slight increase in the ultimate strength. Tensile modulii for both were, however, found to be almost the same and were 2647.8 and 2670.5 N/mm2, respectively.

It is well known that stress concentration due to defects can adversely alter the strength of the mate- rials. To evaluate the influence of defects present in the type 1 blend, holes of various notch parameters were created in the test specimens and load deflec- tion curves were obtained €or these various cases and are shown in Fig. 2. It is observed that the stress concentration produced by the holes reduced the strength and ductility. Similar effects were ex- hibited by side notched specimens in tensile strength and the results are presented in Fig. 3. Breaking strength based on nominal area ( P / w t , where P is the load at break) and net section area ( P / ( u, - 2 r ) t ) are given in Table 1 . The corresponding strengths €or side notched specimens are given in Table 2. The strength reduction due to a notch is found to be much larger than that due to a hole of same

/-

0 5 I0 15

ELONGATION (mm)

Fig. 2. Effect of holes on tensile strength. Holes diame- ters in mm are shown on the corresponding curves.

ELONGATION (mm)

Fig. 3. Effect of notches on tensile strength. The notch lengths in mm are shown on the corresponding curves.

22 POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, Vol. 32, NO. 1

Fracture S tud ie s on a Polyacetal-TPU Blend

0.9

0 . 7 -

- d

0

. 0.5-

<

8 0.4- A

0.2

diameter as crack length. This is due to the high stress intensity at the notch tip. For example, at - 2 mrn notch, the strength reduced to 52% of that of unnotched specimen; whereas for 2 mm hole diame- ter, the strength reduced to 74% of the unnotched specimen. Deformation capacity also reduced to 58 % and 80%, respectively, of the unnotched specimens.

The above behavior of the specimens in the pres- ence of a hole or notch can be represented in a nondimensional form which will be more useful to designers. The breaking strength based on the nom- inal area can be found for both notched and un- notched specimens. Their ratios are calculated and presented as a function of notch parameters 2 r / w or a / w as the case may be. The results are pre- sented in Fig. 4. Results again indicate that a hole of

tbls 1-2

-

7 - Stress whitrning __ 0

envelope

-

I

Table 1. Effect of Hole on Tensile Strength of the Blend.

Net Nominal Strength Strength

2rlw (N I mm ') (Nlmm')

0 55.38 55.38 0.1 28 54.76 48.28 0.143 48.49 41.03 0.157 45.70 37.95 0.171 40.25 32.82

Table 2. Effect of Side Notch on Tensile Strength of the Specimen.

Net Nominal Strength Strength

alw Nlmm' Nlmm'

0 55.38 55.38 0.071 51.17 47.18 0.143 33.94 28.72 0.21 4 29.33 22.56 0.286 23.70 16.40

0.16 0.00 0.08 0.16 0.24 0.32

a / W OR 2r /W

Fig. 4. Relative strength based on crack parameters for notch / hole parameters.

particular diameter present in a material is less criti- cal than a notch of the same dimension. When holes are present up to a notch parameter of about 0.15. the strength is almost equal to that of unnotched specimen. A further increase in hole diameter re- sults in drastic reduction of strength. Similar but enhanced reduction in strength is observed with notches. At small notches, say, up to a notch para- meter 0.08, the reduction in strength is not that sig- nificant. This type of information is very valuable while designing components with geometrical discontinuities.

Stress whitening is the opening up of small voids throughout the whole volume under load. Knowl- edge of this intrinsic crazing mechanism or stress whitening is essential to understand the fracture mechanics of polymers under various types of load- ing. The initiation of stress whitening is preceded by some strain hardening and followed by strain soften- ing mode, resulting in a flattening of the stress-strain curve. With this view, we have given a relationship in Fig. 5 between stress whitening and load, for type 1 samples. A number of specimens having a 1 mm precrack were subjected to different stress lev- els and stress whitening was noted. The relationship is represented in terms of nondimensional parame- ters P / P u and ( a + Aa)/ W, where P is load at ob- served stress whitening, P u is the maximum failure load of unnotched specimen, a is initial crack length, Aa is the length of stress whitening observed per- pendicular to the axis of tension and normal to the precrack (measured using a compound microscope). The results indicate that the variation follows al- most a straight line, which can be conveniently used for design purposes. Once the damage (stress whitening) is known, the permissible load should be within the damage envelope as shown in Fig. 5.

The scanning electron microscope photographs taken for type 1 samples which have failed in ductile

1.1

0

0.01 I I I I I 0 0.18 0.36 0.54 0.12 0.90

CRACK PARAMETER (a t I W 1

Fig. 5. Relative strength based on crack parameters for stress whitening.

POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, YO/. 32, NO. 1 23

Reji J o h n , N. R. Nee lakan tan , N. S u b r a m a n i a n , a n d E. M. S . Nair

and brittle manner under tension are shown in Figs. 6 and 7. To cause samples to fail in a ductile man- ner we have applied a cross-head speed of 0.1 mm/min, and for brittle failure 20 mm/min. The role of TPU in toughening is by the tensile strength contribution from its rubbery nature, that increases the force required to rupture the specimen and/or prevents a craze from developing into a crack. The brittle fracture surface shows a flake-like structure with large and isolated crazes (Fig . 6). Considerable crazing occurs under faster deformation rates and the crazes are arrested in the ductile TPU rich phase by by interception of rubber particles, and thus iso- lated crazes are seen like flakes. The ductile fracture surface shows lots of fiber-like drawings. Under very low deformation rates TPU particles are preferen- tially drawn to form fiber like textures by cold flow (Fig. 7).

Figure 8 shows the load displacement records of three point bend tests of notched samples of type 1. The diagram indicates that the behavior is brittle when notches are present. The nominal bending properties (for the unnotched samples) such as elas- tic strength, stiffness (modulus of elasticity), modu-

Fig. 6. SEM photograph of brittle f a i lu re surfal'e.

lus of elastic resilience and toughness, of the type 1 and type 2 materials are compared. We have calcu- lated these properties according to the formulas given in Ref. 1 1 and these values are 65.51, 1075.05, 0.2263, and 0.6843 N/mm2 for type 1 and 66.35, 1088.83, 0.2269, and 0.7024 N/mm2, respectively, for type 2. Elastic strength, stiffness and toughness values are smaller for type 1 samples, compared to those of the type 2. It might be due to the low molecular weight of our TPU. The load-displace- ment diagrams obtained for type 1 specimens (Fig. 9) clearly establish the significant decrease in strain energy absorbed as the material is cracked. For un- notched specimens, the load-displacement diagram showed an energy content of about 4725 N mm, while that for a notched specimen, indicated a sub- stantial decrease. The strain energy absorbed by the notched specimens were found to be 640, 250, 120,

'"r I

150 -

125 -

I I 0 2 4 6 8 10 I2

1 14

ELONGATDM (mm)

Fig. 8. Three point bending test using various pre- cracked samples . The crack lengths i n mm are s h o w n o n the corresponding curves.

Fig. 7. SEM photograph of ductile f a i lu re surface.

24

0 I 2 3 4 5 a ( m m )

Fig. 9. Compliance (displacement/ load) a n d i ts Jrst derivative w i t h respect to crack length, us. crack length.

POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, VOl. 32, NO. 1

Fracture Studies on a Polyacetal-TPU Blend

and 80 N mm for crack lengths 1, 2, 3, and 4 mm, respectively; for other crack lengths, intermediate values resulted. When a crack grows to 4 times the original crack length, the energy decrement was a factor of 8 times, and this may be considered as a measure of the energy release while the crack grows. Based on this concept, the energy release rate for unit crack surface area can be viewed as a material property for components with defects. Based on the above test results the compliance, C and dC/da are calculated and plotted in Fig. 9. Figure 9 indicates that dC/da varies linearly with crack length. Know- ing the load at fracture, the strain energy release rates are computed and are presented in Fig. 10. Using the same load-displacement record, the J in- tegral is also calculated and plotted as a function of displacement for various crack lengths as shown in Fig. 1 1 . From the known displacement at failure the critical value of J can be found, knowing the failure load for a particular notch case and this is also shown in Fig. 10 and this can be used as design information. The fracture toughness K as a function of crack length is also shown in Fig. 10. G and J exhibited the same trend, as crack length increases, these decrease whereas K remains almost constant. At larger crack lengths G and J approach a nearly constant value, so that they can be used as a mate- rial property. It can be observed that both G and J are almost equal when computations are based on E q s 2 and 3, respectively while using three point bend specimens.

In linear elastic fracture mechanics the hysteresis (loss) energy is very important, because the stored strain energy available for crack initiation is the input energy minus the hysteresis energy. The hys- teresis curves obtained for the type 1 blend are shown in Fig. 12 (results from several samples). A constant cross-head speed of 2.0 mm/min and dif- ferent loading levels of 200, 100, 75 Newtons were

160 I

120 c OoK 1

40

I I I I -I 5.0

01 0.0 1.0 2.0 1.0 L.0

Fig. 10. Fracture parame ter s based on crack length. CRACK LENGTH (rnrn)

21.44

M.76

16.

114

D. R

8.OL

5 .%

2.68

0 0.0 0.8 1.6 2.4 3.2 L.0 6.6 5.6

DISPLACEMENT, u (rnrn)

Fig. 1 1 . J integral as a func t ion of displacement at vari- ous crack length.

a 10 0 L5 0.90 1 3 5 180 225

DISPLACEMENT I rnrn )

Fig. 12. Hysteresis curues at different loading leuels.

applied. The percentage ratios of the loop areas ob- served for different cases were 62.23, 15.16, 5.43; the displacements were 1.82, 1.13, 0.90 mm; and loss energies were 10.26, 2.32, 0.25 Joules, respec- tively. These results show that at higher load levels, higher percentage of hysteresis ratio, deformation strain and energy occur.

CONCLUSION

1. The blend fracture is brittle in mode under higher deformation rates such as 20 mm/min.

2. At a low deformation rate the unnotched sample fractures in ductile manner, whereas that with a notch or a hole fractures in brittle mode.

3. The strength reduction due to a notch is found

POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, YO/. 32, NO. 1 25

Reji John, N. R. Neelakantan, N . Subrarnanian, and E. M. S . Nair

4.

5.

6.

7.

8.

to be much larger than that due to a hole of the same diameter as crack length. The stress whitening, otherwise called intrinsic craze, limits the permissible load a material can bear. It depends on the applied stress. Its occur- rence results in flattening of the stress-strain curve. Fractographs show crazing throughout the de- formed region and the particles of rubbery TPU isolate the large crazes, that would otherwise promote a catastrophic failure. The toughening mechanism of our TPU under low deformation rate is essentially by cold flow. The plane stress fracture toughness of the type 1 blend is found to be independent of notch size. The strain energy release rate, G and J integral (which is also a measure of fracture toughness) are found to be constant for notch length 2 2 mm. Three point bend tests can be used to evaluate

9.

1. 2. 3. 4. 5.

6. 7.

8.

9.

10.

11.

the fracture parameters such as strain energy release rate. G or J integral for plane stress. The hysteresis is found to be considerable par- ticularly at higher load levels for type 1 blend.

REFERENCES

U S . Patent 2,768,994 (1956). British Patent 770,717 (1957). U S . Patent 3,850,837. U.S. Patent 3,749,755. F. C. Chang and M. Y. Yang, Polym. Eng. Sci., 30, 543 (1990). E. A. Flexman, Jr., Mod. P l a t . , February 1985, p. 72. Encyclopedia of Polymer Engineering and Science., Vol. 7, p. 346, John Wiley and Sons, New York (1987). J. D. Landes and J . A. Begley, ASTM STP 560, p. 170 (1974). Y. Murakami, Stress Intensity Factor Handbook, Pergamon Press, New York (1987). ASTM standards 3 1 (1969) and ASTM STP 463 (1970). ASTM, Philadelphia. J. V. Schimtz, ed., Testing of Polymers, Vol. 1, p. 127, Interscience Publishers (1965).

26 POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, Yo/. 32, No. 1