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Rate-Effect on Fracture Behavior of Core-Shell-Rubber (CSR)-Modified Epoxies.

LIN YE [*]

A bisphenol A diglycidylether (DGEBA) epoxy was modified with up to 20 wt% of a core-shell rubber (CSR), and mechanical properties were investigated at different crosshead rates (from 0.5 mm/min to 500 mm/mm). The yield strength and Young's modulus decreased almost linearly with increasing rubber content or decreasing crosshead rate. Fracture toughness. [K.sub.IC] was significantly improved by the addition of CSR to the pure epoxy. The optimum rubber content was between 15% and 20% at the lowest crosshead rate (i.e. 0.5 mm/mm) and shifted to higher rubber content at higher crosshead rates. [K.sub.IC] decreased slightly as the crosshead rate was increased from 0.5 mm/min and dropped significantly when the crosshead rate approached 500 mm/min. Rubber cavitation-induced local plastic deformation was identified as the principal energy absorption mechanism, and it was independent of rubber content and crosshead rate. The variation of [K.sub.IC] with rubber content and crosshead rate correlated well with the lengt h of the stress-whitened zone.

1 INTRODUCTION

The advantages of standard epoxies have made them attractive materials in structural application. However, the poor fracture toughness of epoxies is a major disadvantage limiting their applications. The most common approach used to enhance the toughness of epoxies is to add a softer second phase to them [1]. The dispersed rubber phase plays an important role in the toughness improvement of the materials. Rubber particles, by acting as stress concentrators, allow the matrix between the particles to undergo shear yielding, relieving the plane strain constraint, because the matrix resin between the cavities is in a plane stress condition [2].

However, concerning the size of rubber particles, results reported by different researchers are sometimes contradictory. Sultan and McGarry [3] found that toughening mechanisms are affected by rubber particle sizes. A small rubber particle size (a few hundred Angstrom in diameter) favors shear deformation, whereas crazing is activated by large rubber particles (1.5-5 [micro]m). The fracture energy of large particle systems is five times as much as that of small particle systems. Yee [4] first concluded that the rubber particle size is not important for the toughening, but found in a subsequent study that large particles provided a modest increase in fracture toughness by particle bridging/crack deflection mechanisms, while small particles provided a significant increase in toughness by cavitation-induced shear banding [5]. Other researchers [6, 7] concluded that the optimum toughening effect can be obtained when both large and small rubber particles exist in bimodal distribution, which can activate both shea r and craze deformation mechanisms. The other studies showed that rubber particle interaction also has an important influence on the toughness of materials, due to the overlap of the stress concentration field when inter-particle distance is smaller than a critical value [8-12]. It is the overlapping stress fields between neighboring particles that promote an extensive shear deformation of matrix resin and result in an improvement of fracture toughness, addressed by Yee [4].

The toughening mechanisms in rubber-modified epoxy systems have been reviewed by Garg and Mal [13]. Massive crazing and shear flow were proposed by Bucknall and co-workers [14-16] as two major toughening mechanisms, which were supported by their analysis on volume strain data for rubber-modified epoxies. However, some other results showed that volume dilatation of materials can not be unambiguously and exclusively attributed to crazing [17-19]. It is not clear that whether massive crazing exists in thermoset epoxies, although crazing was observed as a major energy absorption mechanism in some thermoplastic polymers. A simple quantitative model was proposed by Kunz-Douglass et al. [20] to describe rubber stretching and tearing mechanisms. This model can account for an increase of a factor of two in fracture toughness. However, for the rubber-modified low crosslink density epoxies, many experimental results [7, 14, 15, 21] showed an increase of an order in fracture toughness. Hence, it seems that the energy di ssipated by matrix deformation cannot be ignored, and matrix shear deformation may be the major sink for the energy absorption. Kinloch et al. [22] reported that rubber cavitation and plastic shear yielding in epoxy matrices are the microdeformation mechanisms occurring at the crack tip, which dissipate energy and produce the toughening effect. A similar conclusion was also reached by Bascom et al. [21, 23-25] for rubber-modified epoxy adhesives, confirmed by Yee and Pearson [4, 26] assuming a deformation process occurs during crack propagation. The sharp starter crack is first blunted by a plastic zone that grows as the stress increases. Because of the constraint at the center of this zone, a triaxial tensile stress is generated, inducing eventually cohesive failure of rubber particles along a plane perpendicular to the major principal tensile stress. Thus the initial plastic zone that blunts the start crack consists of a voided rubber phases. But as soon as the shear bands form, they become the dominant mod el of deformation, causing the plastic zone to grow further.

Core-shell rubber (CSR) is a special kind of toughener used in toughening brittle epoxies, consisting of a core and a shell in structure. The toughening mechanisms in CSR-modified epoxies are very similar to those in liquid rubber-modified epoxies, i.e. rubber particle cavitation and shear deformation [27, 28]. A craze-like damage form, being a collection of line arrays of cavitated rubber particles, was observed by Sue et al. [29] for a diglycidyl ether of bisphenol A (DGEBA) epoxy. However, the formation condition of the craze-like damage in relation to properties of rubber particles is still not clear. Pearson and Yee [5] found no craze-like damage in the same epoxy but with different core-shell rubber.

In this work, core-shell rubber (CSR) was used to toughen a standard epoxy with the rubber particle size being almost constant. The effect of rubber particle space on fracture toughness was investigated. The toughening mechanisms were examined, and variations of fracture toughness with rubber content and crosshead rate were discussed.

2 EXPERIMENTAL PROCEDURE

2.1 Materials

The base material used in this study is a standard epoxy, Araldite F, which is a diglycidyl ether of bisphenol A (DGEBA) epoxy resin, supplied by Ciba-Geigy, Australia. The curing agent was piperidine (Crown Scientific Pty Ltd.), in a ratio of 5:100 by weight mixed with the pure epoxy. Rubber-toughened epoxies were produced by adding core-shell rubber in a powder form, supplied by Rohm and Haas Co., USA, under the trade name Paraloid (product code KM-330). The average particle size is about 0.2-0.4 [micro]m in diameter. The core-shell rubber (CSR) was first dried in an oven at 80[degrees]C for two hours, and then mixed with the pure epoxy using an electrical mixer to achieve a uniform blend. The blend was then degassed in a vacuum oven (-100 kPa) at 100[degrees]C for about two hours. Then the vacuum was removed and piperidine was added to the mixture while stirring slowly, which was then cast into a preheated mold and cured at 120[degrees]C for 16 hours. The mold was removed from the oven and allowed to cool gradually to room temperature. The obtained plate has a thickness of 5.5 mm for tensile test specimens or 12 mm for compact tension (CT) specimens.

2.2 Tensile Test

The test specimens were of the dogbone variety and machined from the cured resin plates. The specimen described in ASTM D638M-91a with a thickness of 4.5 mm was used. Tensile properties were determined using an Instron 5567 universal testing machine. The strain was measured using a clip gauge of a 50 mm gauge length connected to a computer data acquisition system. At least five specimens were tested for each crosshead rate or rubber content. All specimens were polished using 1200 grade emery paper before testing.

2.3 Compact Tension Test

The CT specimen geometry is based on ASTM D 5045-93. The specimen width W is 50 mm, and thickness 12 mm. The precrack of specimens was introduced by first machining a 45[degrees] notch and then inserting a fresh razor blade by tapping [30]. The sharp precrack induced by tapping a razor blade varies from 2 mm to 5 mm in length. At least three specimens were tested for each crosshead rate or rubber content. All tests were performed on the Instron 5567 testing machine. The data for the load and extension were collected at a time interval of 2 ms for the crosshead rate of 500 mm/min. The fracture toughness was evaluated according to ASTM D5045-93. The critical stress intensity factor in plane strain, [K.sub.IC], was calculated using [31]

[K.sub.IC] = [P.sub.Q]f(a/W) / [BW.sup.1/2] (1)

where [P.sub.Q] is the critical fracture load, which corresponds to the maximum load in the present study, B the specimen thickness, W the width, a the crack length, x the ratio of a to W, and f(x) a non-dimensional shape factor given by

f(a/W) = (2 + a/W)[0.886 + 4.64(a/W) - 13.32[(a/W).sup.2] + 14.72[(a/W).sup.3] - 5.6[(a/W).sup.4]] / [(1 - a/W).sup.3/2] (2)

The validity of [K.sub.IC] as the plane strain fracture toughness was evaluated using (31)

B[greater than] 2.5[([K.sub.IC]/[[sigma].sub.y]).sup.2] (3)

where [[sigma].sub.y] is the yield stress. It was found that a specimen thickness of 12 mm used in this study was large enough to meet the size criteria of the plane strain constraint. The fracture energy ([G.sub.IC], strain energy release rate) was calculated using

[G.sub.IC] = (1 - [v.sup.2])[[K.sup.2].sub.IC]/E (4)

where E and v are Young's modulus and Poisson's ratio, respectively. In the calculation, v is set to be 0.35 for all materials.

2.4 Scanning Electron Microscopy

Fracture surfaces of compact tension specimens were examined using a C505 scanning electron microscope (SEM) with a accelerating voltage of 20 kV. The surfaces were coated with a thin layer of gold to reduce charge built-up on the surfaces.

3 RESULTS AND DISCUSSION

3.1 Mechanical Properties

The typical stress-strain curves of the CSR-modified epoxies at different crosshead rates are shown in Fig. 1. All systems clearly illustrate a non-linear stress-strain response before final failure. As expected, the yield stress increases with increasing of the crosshead rate. However, the addition of the rubber to the pure epoxy does not improve the failure elongation. This phenomenon is most obvious when the tensile curves are compared between the pure and 20% CSR-toughened epoxies. For the former, specimens keep extending to a quite extent after the stress reaches its yield point (maximum stress), and looks quite ductile; while for the latter, the curves does not explicitly have a yield point. This is consistent with the observation during the test that no obvious sign of necking was found for both pure epoxy and CSR-modified epoxies. SEM micrographs of fracture surfaces (Fig. 2 and Fig. 3) reveals that the fracture process of the specimen originates from a small area in the cross section, which locates at the corner edge for the pure epoxy but somewhere near the center for CSR-modified epoxies. Although a necking process was not observed, clear plastic deformation throughout the fracture surface occurred for the pure epoxy. But large plastic deformation was found to occur only at the nucleation area for the CSR-modified epoxies, and beyond this area, the fracture surface looks quite flat. Elongation of specimens after yielding point is a result of total plastic deformation of materials, and failure mechanisms of CSR-modified epoxies under the tensile loading are responsible for the reduction in elongation at failure. Rubber cavitation is clearly seen at the nucleation area, and its distribution is fairly homogenous (Fig. 3b). It was found that rubber cavities are not spherical, expressing that a triaxial stress state exists and the matrix around rubber particles is highly shear deformed. The spherical cavities are found at those areas far from the nucleation region, and the density of rubber cavitation sign ificantly decreased, and most of rubber particles did not cavitate. These results indicate that the ability of rubber particles to cavitate is largely dependent on the stress state, and a triaxial stress favors rubber particle cavitation. The fracture behavior of specimens observed here is quite different from the mechanism reported by Sue et al. (29), i.e. yielding of both pure and CSR-modified epoxies, followed by the necking of the tensile specimens.

Figure 4 depicts the yield stress and Young's modulus as a function of rubber content and crosshead rate, respectively. The maximum stress was adopted for the 20% CSR-modified epoxy. The addition of 20% rubber to the pure epoxy reduces yield stress by about 50%. While the effect of crosshead rate on the yield stress almost obeys the well-known Eyring's law [14], i.e. yield stress linearly increases with the logarithm of crosshead rate. Young's modulus for the CSR-modified epoxies decreases almost linearly with increasing rubber content and the result is similar to the work by Verchere et al. [32], in which a linear decrease was found between Young's modulus and the volume fraction of dispersed rubber phase. The crosshead rate has almost a linear effect on the Young's modulus, E. but the influence is not significant, i.e. Young's modulus only slightly increases with increasing the crosshead rate.

3.2 Crack Growth Behaviors

The typical load-extension curves for compact tension specimens at different crosshead rates are presented in Fig. 5. One of the most essential characteristics is that the crack growth behavior changed remarkably with the variation of rubber content and crosshead rate. For the pure epoxy specimens, the linear elastic relationship between load and extension is well satisfied. It seems that the crack initiates its growth in a unstable manner and then proceeds in a stable manner at crosshead rates of 0.5 mm/min and 5 mm/min. When the crosshead rate was increased to 50 mm/min and 500 mm/min, only stable crack growth was observed. For the 10% CSR-toughened specimens the relationship between load and extension is also almost linear. Crack shows a brittle unstable growth manner in a range of crosshead rate from 0.5 mm/min to 50 mm/min and a stable growth manner at the crosshead rate of 500 mm/min. For the 20% CSR-toughened specimens, the load-extension curve shows a clear non-linearity in the load range close to th e peak value, and a stable crack growth, followed by a unstable growth, is observed at the crosshead rates not exceeding 50 mm/min. Again, stable crack growth occurs at the crosshead rate of 500 mm/min.

Generally speaking, the increase of rubber content favors stable crack growth. The effect of crosshead rate on the crack growth manner seems to be not significant in the range from 0.5 mm/min. to 50 mm/min. However, at the crosshead rate of 500 mm/min, all load-extension curves for the pure epoxy and rubber-toughened specimens (5%, 10%, 15%, 20%) are very similar to each other, and the crack grows at a stable manner. This observation seems to be in good agreement with that reported by Kinloch et al. (22), and it was found that the increase of strain rate favored stable crack growth in rubber-modified epoxies. It is usually believed that the initial stable crack growth, corresponding to the non-linear region on the load-extension curves near the peak, leaves a stresswhitened zone on the fracture surface of specimens. For the 10% CSR-modified epoxy at the crosshead rates up to 50 mm/min, a brittle unstable crack growth with absence of non-linear regions on the load-extension curves near the peak can be clearly identified in Fig. 5, but even so, a stress-whitened zone on the fracture surfaces still exists. It should be mentioned that owing to variation of the precrack length with the specimens, the slope of the curve does not always scale with modulus.

3.3 Fracture Toughness

The fracture toughness of specimens with different rubber content is presented in Fig. 6. [K.sub.IC] increases from 0.75 [MPa.m.sup.1/2] to 2.75 [MPa.m.sup.1/2] when the rubber content is increased from 0% to 15%, and it slighfly decreases to 2.67 [MPa.m.1/2] with the further increase of rubber content to 20% at the crosshead rate of 0.5 mm/min. An optimum rubber content for the maximum KIC seems to exist in the range between 15% and 20% at the crosshead rates of 0.5 mm/min and 5 mm/min. This is in good agreement with those reported elsewhere [4, 13, 23]. With further increase of the crosshead rate to 50 and 500 mm/min, the [K.sub.IC] values for the 20% CSR-modified specimen becomes higher than that for the 15% CSR-modified specimen. It can be summarized that the optimum rubber content is clearly dependent on the crosshead rate and it increases when the crosshead rate is increased. This result is in contradiction with that reported by Pascom et al. (7), in which the rating of fracture toughness does not alter with the strain rate. Because the rubber particles in this study are almost of the same size, the variation of fracture toughness with rubber content shows the average inter-distance between rubber particles plays a very important role on improving fracture toughness of brittle epoxies. It can be deduced that a critical inter-distance between rubber particles exists, at which the maximum improvement of fracture toughness can be obtained, and this critical inter-distance becomes smaller at a high crosshead rate.

The effect of crosshead rate on the fracture toughness, [K.sub.IC], is depicted in Fig. 7. The [K.sub.IC] value generally decreases with increasing crosshead rate. However, the variation of [K.sub.IC] with crosshead rate is not linear. A slight reduction of [K.sub.IC] is observed for all rubber-toughened specimens when the crosshead rate is increased from 0.5 mm/min to 50 mm/min, and a remarkable drop is followed with further increase of the crosshead rate to 500 mm/min. This drop corresponds to a reduction of 47%, 35%, 32% and 25% for the 5%, 10%, 15% and 20% CSR-toughened specimens, respectively, compared to the values at the crosshead rate of 50 mm/min.

Figure 8 represents the normalized ratio of fracture energy, [G.sub.IC] in relation to the rubber content or crosshead rate. It is seen that the relative improvement of fracture energy can be increased by as much as 20 times. It seems that the relative improvement of fracture energy is proportional to the rubber concentration but almost independent of the crosshead rate when the rubber content is less than 20%. While, for the 20% CSR-modified epoxy, the toughening ability increases with shifting the crosshead rate from 0.5 mm/min to 500 mm/min.

Although the addition of the rubber to the pure epoxy does not improve the failure elongation (or ductility), Fig. 1, it improves the fracture toughness significantly. This fact shows that capability of the global shear deformation does not play a major role in toughening epoxy. The remarkable improvement in the fracture toughness is related to local shear deformation of matrix around rubber particles in the processing zone ahead of the crack tip.

3.4 Morphology Observations

The relationship between the plastic zone and fracture toughness has been widely investigated. Bascom et al. [16, 23-25] reported that the plastic zone size in rubber-modified epoxy adhesives is directly related to the toughness. Other studies [22, 33] reached similar conclusions. The typical fracture surfaces of 15% CSR-modified epoxy specimens are displayed in Fig. 9. Two distinct regions with characteristics of stress-whitening and smooth surfaces can be clearly observed. The length of the stress-whitened zone decreases at a high crosshead rate or for a low rubber content. The lengths of averaged stress-whitened zone are summarized in Table 1 for the CSR-modified epoxies and plotted against fracture toughness in Fig. 10. The variation of stress-whitened zone length with the crosshead rate or rubber content is very well correlated with that of [K.sub.IC] The length of stress whitened zone increases with fracture toughness of materials. A rapid change of the length of stress whitened zone is found to occur when fracture toughness goes to about 2.5 [MPa.m.sup.1/2]. The results unambiguously support that the micromechanism of stress-whitened zone is the principal toughening mechanism and responsible for the enhancement of toughness.

The details of stress-whitened zones for the toughened epoxies with different rubber contents at a cross-head rate of 0.5 mm/min were revealed by SEM images at high magnification, shown in Fig. 11. The rubber cavitation was clearly observed in all specimens. It is apparent that all cavities are approximately spherical in shape and uniform in size. This indicates that the cavitation induced by rubber particle was well developed and but the effect of shear deformation on distortion is not very significant. It can be seen that two characteristic areas existed on the stress-whitened zone. One was the relatively flat area, where there is little rubber cavitation, and the other was the highly rubber cavitated area. For the 5% CSR-toughened epoxy, the fracture surface in the stress-whitened zone was dominated by the flat zone. For the epoxies with high rubber contents (15% and 20%), the density of rubber particles significantly increased and the fracture surfaces were dominated by highly rubber cavitated areas. Thi s indicates that rubber cavitation plays an important role at a high rubber content during the process of energy absorption. The characteristics of stress-whitened zones at 500 mm/min were very similar to those at 0.5 mm/min, i.e. flat area and highly rubber cavitated area. These results indicated that the toughening mechanisms remain almost unchanged with crosshead rate. The morphology of smooth zone in the fracture surface for the epoxies toughened by different rubber contents at a crosshead rate of 0.5 mm/min is shown in Fig. 12. The details in the smooth zone are quite different from those in the stress-whitened zone. The fracture surface in the smooth zone was dominated by flat area and there were barely voids left by rubber particle cavitation. Furthermore, the rubber particle cavitation was premature and not well developed, and cavities were shallow and the edge was not clear. No significant morphological difference was identified with variation of rubber contents. The TEM images, taken from the same p iece of a thin section, are presented in Fig. 13 for the 10% CSR-toughened specimen at the crosshead rate of 0.5 mm/min. The depth directly under the fracture surface is increased from Fig. 13a to Fig. 13c. Rubber particle cavitation was clearly more apparent in Fig. 13a, being closer to the fracture surface. The large holes in Fig. 13a may be due to peeling off of the section surrounded by large rubber cavities, because of coalescence of large cavities. It is interesting to see this phenomenon eventually disappears when it is far away from the fracture surface shown in Fig. 13c.

4. CONCLUSION

The mechanical properties for CSR-toughened epoxies were found to be strongly dependent on rubber concentration and crosshead rate. The variation of mechanical properties with crosshead rate or rubber content is consistent with those reported for the pure [34] and reactive rubber-modified epoxies [4,35].

The crack growth behavior was found to be dependent on rubber content and crosshead rate. No apparent relation between stable crack growth and the stress-whitened zone on fracture surface was obtained. The effect of crosshead rate on the fracture toughness was not apparent at low crosshead rates (from 0.5 mm/min to 50 mm/min), but became significant when the crosshead rate was increased to 500 mm/min.

The major toughening mechanisms were identified as rubber particle cavitation and its locally induced plastic deformation. Rubber cavitation plays an important role at a high rubber content during the process of energy absorption.

ACKNOWLEDGMENT

The authors thank the Electron Microscope Unit of Sydney University for the access to its facilities. Keqin Xiao is supported by an Overseas Postgraduate Research Scholarship (OPRS), a University Postgraduate Research Award (UPRA) and a supplementary Scholarship from the Department of Mechanical and Mechatronic Engineering, Sydney University.

(*.) To whom correspondence should be addressed.

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 The Average Length of Stress-Whitened Zone as
 a Function of CSR Content and Crosshead Rate.
Croashead Length of Stress-
 Rate Whitened Zone [mm]
[mm/min] 5% 10% 15% 20%
 0.5 0.2 1.1 20.3 14.6
 5 0.14 0.7 4.0 4.2
 50 0.1 0.4 1.0 2.2
 500 0.0 0.1 0.4 0.9
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Author:XIAO, KEQIN; YE, LIN
Publication:Polymer Engineering and Science
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Geographic Code:1USA
Date:Jan 1, 2000
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