Three-dimensional interaction of crazes and micro-shearbands in PC-SAN microlayer composites

The three-dimensional interaction of crazes and micro-shearbands in co-extruded microlayer sheets with 49 alternating layers of polycarbonate (PC) and styrene-acrylonitrile copolymer (SAN) was investigated as a function of the relative layer thickness. The deformation processes were observed when microspecimens were deformed under an optical microscope. Deformed specimens were sectioned and examined further in the transmission electron microscope. Two types of craze were observed in the SAN layers: surface crazes initiated at a strain of about 1.8% and gradually lengthened to a maximum of 70 m when they were arrested by micro-shearbands at 4.2% strain, while tunnel crazes appeared above 4.2% strain and rapidly grew through the entire SAN layer. Surface crazes did not prevent yielding and stable neck propagation, while tunnel crazes were responsible for fracture prior to neck formation. The density of surface crazes relative to tunnel crazes increased as the PC-SAN ratio increased or as the strain rate decreased. The surface crazes stimulated micro-shearbanding in both PC and SAN layers. After micro-shearbands initiated in the PC layers where the craze impinged on the PC-SAN interface, they propagated rapidly along the edges of the craze. As they overtook the craze tip, the micro-shearbands penetrated through the PC-SAN interface and continued around the craze tip to entirely engulf the craze. This terminated craze growth, and further strain in the SAN layer was accommodated by shear deformation.     

Deformation behaviour of coextruded multilayer composites with polycarbonate and poly(styrene-acrylonitrile)

The tensile properties of coextruded multilayer composites comprised of predominantly 49 alternating layers of polycarbonate (PC) and polystyrene- acrylonitrile (SAN) were investigated in the bulk and microscopically. The bulk was characterized by three types of behaviour: brittle fracture at low strains, ductile yielding with fracture during neck formation, and formation of a stable neck followed by drawing to high extension. Optical microscopy was utilized to correlate deformation mechanisms within each phase to the observed modes of deformation in the bulk. Optical microscopy showed that in all cases the initial irreversible deformation event was the formation of cracks or crazes in the SAN layers. Good adhesion between the layers resulted in the subsequent initiation of shear bands in the polycarbonate layers at the craze tips. Interaction of crazes and shear bands produced an expanded damage zone ahead of the propagating crack which delocalized the stress and delayed fracture. The ultimate mode of fracture depended on the relative thickness of the SAN and PC layers, as determined by the composition, and the strain rate.     

Craze formation and growth in anisotropic polymers

Craze formation and craze growth in anisotropic polymers has been studied as a function of the degree of anisotropy and the relation between the testing direction and the primary orientation direction. Tests on PMMA and PC indicate that both the morphology and orientation of the crazes are senstivie functions of testing direction. Crazes form in directions which arenot orthogonal to the principal tensile stress, and the data clearly show that craze growth occurs in directions governed by the major principal strain. The fracture process is identical in nature to that in isotropic polymers, i.e. craze formation, crack nucleation within the craze and subsequent crack propagation through the craze. Thus, the angle of fracture coincides with the craze angle rather than occurring perpendicular to the principal tensile stress.     

The effect of specimen size on the mechanical behaviour associated with crazing

Since crazes generally nucleate at the surface it is expected that the size of the specimen, as described by the ratio of surface area to volume, should affect the mechanical behaviour of polymers which deform primarily by crazing. The stress relaxation curves and the stress-strain curves of PS, PMMA, PTFE, and PC were measured in liquid nitrogen for specimens of different size which were machined from the same rod. The predicted size effect was observed in that the smaller (6.4mm diameter) specimens stress-relaxed faster and the stress to produce a given amount of craze deformation was lower than for the larger (12.7 mm diameter) specimens. The range of the tensile strength from 0 to size is also presented based on the stress to nucleate the first craze and on the tensile strength that is observed when no crazing occurs     

A study of polymer fracture surface features and their relationship to toughness

The aim of this study was to research fracture surface features for polymers of different toughness and type. The materials chosen provided for an interesting comparison of fracture surfaces. Two brittle amorphous thermoplastics (SAN & PMMA) of the same toughness had very different fracture surfaces. An amorphous thermoplastic (PC) exhibited similar features as both SAN and PMMA but had a higher toughness. Two semi-crystalline thermoplastics (PE1 & PE2) had similar fracture surface features but one was twice the toughness of the other. A rubber toughened polymer (ABS) showed a very different fracture surface to SAN (the host material) and all the other polymers studied. A particular interest was to use the comparison of the fracture surfaces of the above materials to investigate the toughening effects of rubber particles in ABS.     

Toughening mechanisms in high impact polystyrene

In situ scanning electron microscope crack propagation experiments have been performed on a number of polystyrene and high impact polystyrene blends so that dynamic observations can be made of the mechanisms of failure. Brittle fracture is observed in low rubber phase volume systems, whereas high rubber phase volume systems exhibit a ductile tearing mode of fracture. As the rubber phase volume is increased there is an increased density of crazes, which leads to a reduction in width of material between them. The subsequent failure of the crazes leaves bridging ligaments. Under increasing load these fail in a manner dependent on their thickness such that there is a brittle-ductile transition at a ligament thickness around 3m. We argue that this alteration in mechanism could be caused by either the loss of the triaxial stress state or the reduced probability of extrinsic flaws being found in the smaller ligaments, resulting in inhibition of crazing. The stress required for failure at the crack tip consequently increases from that for craze formation to the yield stress of the dense polymer. This in turn allows a larger crazed deformation zone (already increased due to the stress relief effects of crazing) to form, hence a further toughness increase.     

Synchrotron radiation small-angle X-ray scattering study on fracture process of carbon nanotube/poly(ethylene terephthalate) composite films

Time-resolved small-angle X-ray scattering (SAXS) measurements have been conducted during tensile deformation of carbon nanotube (CNT)/amorphous poly(ethylene terephthalate) (PET) composite films using synchrotron radiation in order to investigate the fracture process. The observed SAXS patterns consisted of the streaks parallel to the loading direction caused by the total reflection at craze/polymer interfaces, the streaks perpendicular to the loading direction caused by the fibril/void structure of crazes and the scattering from CNTs. The formation, widening and fracture processes of the crazes were investigated based on the changes of SAXS patterns during deformation and the fracture toughness of the composite films determined with essential work of fracture method. The influences of CNT addition on the mechanical properties of PET varied depending on the specimen geometries used for the mechanical tests and marked influences were obtained with surface-notched specimens. The CNT addition increased the energy needed to widen the crazes and retarded the growth and fracture of the crazes during deformation. This lead to the increases in the plastic work of fracture and the fracture toughness of PET. The CNT aggregates formed at the CNT fraction beyond 3 wt%, however, caused reduction of the fracture toughness.     

On the generality of discontinuous fatigue crack growth in glassy polymers

Fatigue fracture surface characteristics of five commercially available amorphous polymers [poly(methylmethacrylate) (PMMA), polycarbonate (PC), poly(vinyl chloride) (PVC), polystyrene (PS), and polysulphone (PSF)] as well as bulk-polymerized PMMA prepared over a wide range of molecular weights were studied to determine if common mechanisms of fatigue crack propagation prevail among these glassy polymers. In those polymers with viscosity-average molecular weight ¯M _v2×10⁵, the macroscopic appearance of the fracture surface showed the presence of a highly reflective mirror-like region which formed at low values of stress intensity and high cyclic test frequencies (100 Hz). The microscopic appearance of this region revealed that many parallel bands exist oriented perpendicular to the direction of crack growth and that the bands increase in size with K. In all instances, the crack front advanced discontinuously in increments equal to the band width after remaining stationary for hundreds of fatigue cycles. Electron fractographic studies verified the discontinuous nature of crack extension through a craze which developed continuously with the load fluctuations. By equating the band size to the Dugdale plastic zone dimension ahead of the crack, a relatively constant yield strength was inferred which agreed well with reported craze stress values for each material. At higher stress intensity levels in all polymers and all values of ¯M _v, another series of parallel bands were observed. These were also oriented perpendicular to the direction of crack growth and likewise increased in size with the range in stress intensity factor, K. Each band corresponded to the incremental advance of the crack during one load cycle, indicating these markings to be classical fatigue striations.     

The competition between shear deformation and crazing in glassy polymers

Whereas thin films of some polymers such as polystyrene readily form crazes when strained in tension, thin films of other polymers such as polycarbonate rarely exhibit crazing under the same testing conditions; the polymers that rarely craze tend to form regions of shear deformation instead. Polymers such as polystyrene-acrylonitrile which lie between these two extremes of behaviour may exhibit both modes of deformation. Thin films suitable for optical and transmission electron microscopy (TEM) of six such co-polymers and polymer blends have been prepared. After straining, the nature of the competition between shear deformation and crazing is examined by TEM. It is found that in these polymers many crazes have tips which are blunted by shear deformation. This process leads to stress relaxation at the craze tip, preventing further tip advance. In this way short, but broad, cigar-shaped crazes are formed. Examination of the deformation at crack tips in the same polymers shows more complex structures, the initial high stress levels lead to chain scission and fibrillation but as the stress drops, shear becomes the dominant mechanism of deformation and the stress is relieved further. Finally, at long times under stress, chain disentanglement may become important leading to fibrillation and craze formation again. The nature of the competition is thus seen to be both stress and time dependent. Physical ageing of these polymers, via annealing below T _g, suppresses shear leading to the generation of more simple craze structures.     

Internal structure of rubber particles and craze break-down in high-impact polystyrene (HIPS)

Polystyrene can be substantially toughened by the addition of rubber particles, their role being to act as craze initiators permitting substantial plastic deformation to occur prior to fracture. The internal structure of these particles is variable: typically the smaller (1 m) particles are solid rubber and the larger particles contain sub-inclusions of polystyrene. Thin films of a toughened high-impact polystyrene (HIPS) suitable for optical and transmission electron microscopy (TEM) have been prepared, and the interplay between the internal structure of the particles and the crazes they generate has been examined by TEM. It is found that as crazes form around the solid rubber particles, significant lateral contraction occurs accompanying their elongation in the tensile direction. As this contraction proceeds, decohesion occurs just beneath the particlecraze interface, resulting in the formation of a void. This void will grow under increasing stress, leading to premature failure of the craze. In contrast to this behaviour, occluded particles can accommodate the displacements due to crazing by local fibrillation of the rubber shell which surrounds each sub-inclusion, without the formation of large voids. Consequently, the occluded particles do not act as sites for early craze break-down. These results suggest that the optimum morphology for rubber particles in HIPS will consist of a large number of small PS occlusions, each surrounded by a thin layer of rubber, in which case the size of the inherent flaws introduced during crazing will be minimized.     

Micromechanism of a deformation process before crazing in a polymer during tensile testing

Polymers are popularly used for housing and parts of machines and equipment. However, their mechanical properties, especially the deformation process, have not been clarified. During tensile testing, crazes are thought to be a source of microcracking and fracture, but the relation between the craze formation process and the deformation process before crazing is not understood. In the present work, scanning acoustic microscopy and X-ray diffraction were used to investigate the micromechanism before craze formation in polymethylmethacrylate (PMMA) and polycarbonate (PC). The velocity change of the surface acoustic wave and X-ray diffraction intensity indicated that molecular orientation occurred in a very small area from early stages of plastic deformation. From the results it was thought that texture was heterogeneous and anisotropic in a very small area, the shape of the area was spheroidal with a longer radius in the direction perpendicular to the applied stress, and the molecular chain in the area was oriented parallel to the stress axis. The area is thought to increase with increasing plastic strain.     

Mechanical behaviour of poly(methyl methacrylate)

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using a commercial poly(methyl methacrylate) (PMMA). Tensile properties and fracture toughness data were obtained for the various conditions. In general, both tensile strength and fracture toughness increase with increasing loading rate and decreasing temperatur E. However, when the temperature reaches the glass transition region, the relationships between fracture toughness, loading rate, and temperature become very complex. This behaviour is due to the simultaneous interaction of viscoelasticity and localized plastic deformation. In the glass transition region, the fracture mechanism changes from a brittle to a ductile mode of failure. A failure envelope constructed from tensile tests suggests that the maximum elongation that the glassy PMMA can withstand without failure is about 130%. The calculated apparent activation energies suggest that the failure process of thermoplastic polymers (at least PMMA) follows a viscoelastic process, either glass or transition. The former is the case if crack initiation is required.Deceased.     

Fracture of crazes by the propagation of interfacial stress waves

Fracture of crazes in glassy polymers can occur by a quasi-brittle separation at the interface between the craze and the adjacent bulk. In some grades of polystyrene this type of fracture can take the form of a very regular pattern, the so-called mackerel pattern, of parallel or concentric craze strips as fracture alternates from one side of the craze layer to the other. The alternating pattern of fracture is determined by the coupling between stress waves propagating along the craze—bulk boundaries.     

Mixed-mode effects on the toughness of polymer interfaces

Normal, symmetric fracture toughness tests can give high values for the toughness of the joint between the immiscible polymers polystyrene and polymethylmethacrylate. These high values, which are caused by crazes growing away from the interface into the polymer with lower craze resistance, are not a fair characterization of the toughness of the joint. Much lower, and more realistic, toughness values are obtained by the use of an asymmetric test that tends to drive the crack and crazes more along the interface.     

Thickness effects in microlayer composites of polycarbonate and poly(styrene-acrylonitrile)

Coextruded microlayer sheet consisting of alternating layers of polycarbonate (PC) and styrene-acrylonitrile copolymer (SAN) exhibits improved properties such as toughness and ductility as the number of layers is increased. In this study, the composition was kept essentially constant, as was the sheet thickness at 1.2 mm, and the layer thickness was changed by varying the total number of layers from 49 to 776. All the compositions exhibited macroscopic yielding in uniaxial tension but the fracture strain, which represents neck propagation, increased with the number of layers. The increased ductility was attributed to a transition in the microdeformation behaviour observed when microspecimens were stretched in the optical microscope. When the layers were thicker, individual layers exhibited behaviour characteristic of the bulk, that is SAN crazed or cracked while shear bands initiated in PC from the craze tips. As the layer thickness decreased, crazing or cracking of the SAN was suppressed and shear bands that extended through several layers produced shear yielding of both PC and SAN. Calculations showed that when the layer thickness is sufficiently small, impingement of a PC shear band on the interface creates a local shear stress concentration. As a result the shear band continues to grow through the SAN layer and subsequently, at the point of instability, shear yielding can occur in both PC and SAN layers.     

Micromechanics of solvent crazes in polystyrene

Double exposure holographic interferometry (DEHI) is used to determine the strain energy release rate, craze opening displacement profile, and craze stress profile ofn-heptane and methanol crazes growing from cracks in polystyrene.n-heptane crazes have strain energy release rates (SERRs) close to those of cracks and their stress profile is almost crack-like in that the tensile stress across the craze falls almost to zero. On the other hand, the SERRs of methanol crazes are only 30 to 55% the SERR of a crack depending on stress intensity factorK _I of the precrack from which they are grown. The stress profile of the methanol craze shows it to be strongly load-bearing away from the craze tip, apparently as a result of the strain hardening of the craze fibrils. The stress concentration in front of the methanol craze tip is only 40% of that in front of then-heptane craze tip. The opening displacements of the methanol craze are almost as large as those of a crack very near its tip but are much less than those of a crack at large distances behind the tip. The Dugdale model of a strip yielding zone provides a poor representation of the craze opening displacements of the growing methanol craze. Dry (static) methanol crazes have larger opening displacements in response to an incremental tensile strain at moderate prestrains than at either low or high prestrains, suggesting that the craze fibrils undergo a yielding/strain-hardening process as the strain is increased similar to that observed in polycarbonate crazes by Kopp and Kambour. Dryn-heptane crazes do not show this response but rather open linearly with increasing prestrain. The opening displacement for long (dry)n-heptane crazes is almost crack-like whereas the largest opening of a dry methanol craze is only 20% of that of a crack. Dry methanol crazes break at aK _IC that is 40% of theK _IC of precracked but uncrazed specimens. The strongest (shortest) dryn-heptane crazes fail at only 7% ofK _IC of uncrazed specimens and theK _IC of the dryn-heptane crazes decreases markedly with increasing craze length.     

Non-destructive evaluation of the micromechanism of the deformation process during tensile tests on polymers by the elastic-wave transfer function method

The elastic-wave transfer function method (ETFuM) was applied to make clear the micromechanism of the deformation process during dynamic tensile testing of polymethylmethacrylate (PMMA) and polycarbonate (PC). In PC, the transfer function began to change at a high frequency. After that, it decreased abruptly in the low-frequency region. The variation of the transfer function at high frequency was caused by the nucleation and growth of microdefects such as crazes and microcracks. The variation at low frequency was caused by plastic deformation such as inclined necking and microdefects due to shear stress. On the other hand, in PMMA the transfer function changed homogeneously with elongation at high frequencies and did not change at low frequencies. The variation of the transfer function during tensile testing related to the micromechanism of elastic and plastic deformation processes in both PC and PMMA. The results suggested that the ETFuM is a useful and powerful method for evaluating the micromechanism of deformation processes in polymers in a non-destructive and dynamic way.     

Craze yielding and fracture mechanism in PE/PS/PE laminated films

The change in polystyrene (PS) layer thickness, which has been simultaneously determined during post-yield deformation, shows that crazing is the basic mechanism of toughening in all laminated films, and that shear deformation supplements the contribution of crazing especially for samples with high polyethylene (PE) volume fractions. Crazes formed in PS layers in the laminated films are slender and regular compared with the short and lenticular crazes formed in bulk PS film. When PE volume fraction increased, craze advance speed decreased because of the reduction of the stress concentration effect at craze tips. The life-time of the first mature craze to be formed at a given strain rate increased with PE volume fraction because the PE supporting the mature crazes could effectively inhibit craze rupture and blunt out the propagating crack by absorbing the stored elastic energy in the PS layer that would have been dissipated as fracture surface energy.     

Craze intersections in biaxially-stressed polystyrene

Crazes are produced on two orthogonal planes in both thin film and macroscopic samples of polystyrene by sequentially applying two orthogonal tensile strains ₁ and ₂. Although many crazes produced by the second strain ₂ (secondary crazes) are stopped when they meet a primary craze, some intersections occur. The fraction of craze meetings resulting in intersection increases from 20% at ₁= ₂=3% to 55% at ₁= ₂=5%; intersections also occur preferentially in thin regions of primary crazes. The craze fibril structure in the intersection has a much lower fibril volume fraction, v _f, than either of the two crazes from which it formed. The fibril volume fraction in the intersection is approximately given by the product of the fibril volume fractions of the two crazes, in agreement with a prediction based on the surface drawing mechanism of craze thickening. At higher strain levels the v _fs of the intersections are lower, leading to higher fibril stresses and enhanced fibril fracture; an increasing fraction of intersections breaks down to form large voids at these higher strain levels. Fractography of macroscopic samples containing intersecting crazes demonstrates that voids formed at the intersections can act as nuclei for cracks causing premature fracture of the material.