Crazing and shear deformation in glass bead-filled glassy polymers

The competition between craze formation and shear band formation at small glass beads embedded in matrices of glassy polymers has been investigated. This has been done by performing constant strain rate tensile tests over a wide range of strain rates and temperatures, and examining the deformation pattern formed at the beads with a light microscope. The glassy polymers under investigation were polystyrene, polycarbonate, and two types of styrene—acrylonitrile copolymer. It was found that besides matrix properties, strain rate and temperature, the degree of interfacial adhesion between the glass beads and the matrix also has a profound effect on the competition between craze and shear band formation: at excellently adhering beads craze formation is favoured, whereas at poorly adhering beads shear band formation is favoured. This effect is caused by the difference in local stress situation, craze formation being favoured under a triaxial stress state and shear band formation under a biaxial stress state. The kinetics of crazing and shear deformation have also been studied, using a simple model and Eyring's rate theory of plastic deformation. The results suggest that chain scission may be the rate-determining step in crazing but not in shear deformation.     

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.     

The effect of cross-linking on crazing in polyethersulphone

Cross-links have been introduced into thin films of PES (polyethersulphone)/1 wt% sulphur by heating them in air at 350 °C. The effect of this is to suppress crazing in favour of shear deformation in high-temperature regimes where disentanglement crazing dominates for uncross-linked films of the same composition. We argue that light cross-linking (one or two cross-links per chain) is sufficient to give rise to a finite gel fraction in the films which, because it effectively forms an infinite network, cannot disentangle. Thus for crazing to occur, chains which form part of the gel fraction must always break rather than disentangle. This has the effect of raising the crazing stress relative to the yield stress in the weakly temperaturedependent regime of crazing at high temperature, where disentanglement is normally considered sufficiently rapid for entanglement loss not to contribute to the crazing stress. Hence as the gel fraction is increased by increasing the heat-treatment time, crazing is suppressed at the highest temperatures with respect to shear deformation, leading to a second transition, this time from crazing back to shear.     

Thin film and bulk deformation behaviour of poly(ether ether ketone)/poly(ether imide) blends

The deformation behaviour of amorphous thin films of poly(ether ether ketone) (PEEK)/poly(ether imide) (PEI) blends was investigated over a wide temperature range by optical and transmission electron microscopy. All the materials showed localized shear deformation at temperatures well below Tg. In pure PEI and in blends with up to 60 wt% PEEK content, a transition from shear deformation to disentanglement crazing occurred as the temperature was raised. However, this transition was absent in PEEK, which deformed by shear over the whole temperature range, and similar behaviour was found for PEI/80 wt% PEEK. It is argued that at high PEEK content disentanglement crazing is suppressed by strain-induced crystallization and some evidence for crystalline order in deformed regions of initially amorphous PEEK thin films was obtained by electron diffraction. The thin film deformation behaviour of the blends was also shown to be consistent with their bulk deformation behaviour, a high temperature ductile–brittle transition being observed at low PEEK content in tensile tests. This revised version was published online in November 2006 with corrections to the Cover Date.     

Similarity between craze morphology and shear-band morphology in polystyrene

The formation of shear bands and crazes in thin films as well as in bulk samples of polystyrene were examined in the electron microscope using a variety of replication techniques. The morphologies of shear bands and crazes are quite similar both depending initially upon the relative shear displacement of 400 to 1000 Å domains. As deformation continues and orientation increases, fibrils varying from 50 to 700 Å are formed within the deformation zone, lateral constraint of the normal Poisson contraction causing voids to form in the crazes but not in the shear bands. Shear-band width was found not to be a unique function of either temperature or strain-rate and both craze and shear-band morphologies were found not to be strong functions of molecular weight. Regardless of molecular weight, fibrils formed within the deformation zone were always on the order of a few hundred Angstroms in diameter. However, for thin films of molecular weight less than 20 000 insufficient numbers of tie molecules between fundamental structural units or domains made it difficult for these fibres to span the craze width.     

Microstructure and origin of cross-tie fibrils in crazes

Crazes were grown in thin films of polystyrene (PS) at various temperatures and the resulting craze fibril microstructures were examined using low-angle electron diffraction (LAED). A quasi-regular array of cross-tie fibrils pull the main fibrils away from the tensile axis by an angle ± /2°. As a result, the LAED patterns from crazes grown at temperatures T<50°C exhibited split diffraction lobes centred about the equatorial axis of the LAED pattern. It was found that decreased with increasing crazing temperature and that the split lobes could no longer be resolved at the highest temperatures. Diffuse meridional diffraction spots due to scattering from the quasi-regular array of cross-tie fibrils were seen in the LAED patterns from crazes grown at low temperatures. The spacing of the cross-tie fibrils, R, determined from these patterns, was found to increase with the crazing temperature. A new model of craze widening was proposed that accounts for the formation of cross-tie fibrils by allowing some of the entangled polymer strands which bridge two fibrils in the active zone to survive fibrillation. Cross-tie fibrils are created when several such strands pile up locally, and the craze/bulk interface bypasses the pile-up.     

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.     

Crazing mechanisms and craze healing in glassy polymers

When held at temperatures above the glass transition temperature, crazes in certain polymers may be made to heal; that is, the crazed material may be made to recover the mechanical properties it had prior to crazing. Using thin films of a variety of polymers, we have investigated whether the mechanism of entanglement loss during crazing influences the heat treatment time necessary for healing. We find that under experimental conditions for which there is evidence of disentanglement during crazing, and if the crazes do not break down, healing occurs after heat treatment times of the order of those necessary to make the craze disappear optically. Similar heat treatments applied to scission mediated crazes however, do not result in healing. We argue that scission crazing results in a high proportion of chain fragments which are unable to contribute to the entanglement network. Since the heat treatments are not sufficiently long to disperse these fragments, and hence to restore the original local entanglement density, healing does not take place. Disentanglement should not result in such damage, so, consistent with our observations, healing times should be relatively short. Hence, these results provide independent evidence for disentanglement during crazing.     

Effect of temperature and strain rate on the strength of a PET/glass fibre composite

Constant strain-rate mechanical testing and surface fractography were used to characterize the failure behaviour of a PET/glass injection-moulding compound and of its unfilled matrix material. Parameters for this investigation were temperature and strain rate. The matrix material exhibited a viscous-brittle transition between room temperature and 60° C. Low temperature failure occurred by craze growth, followed by slow and rapid crack propagation. The composite material likewise behaved as a viscous solid at superambient temperatures. Failure at low temperatures and/or high deformation rates occurred by brittle matrix fracture and fibre pull-out. Under these conditions, mechanical properties improved, relative to those at room temperatures. At intermediate temperatures and/or low strain rates, failure occurred via matrix crazing and crack propagation near the fibre ends. An observed serration of the fracture path at high strain rates is suggested to be due to the need for high shear stresses at the fibre-matrix interface.On leave from the Center for Composite Materials and Department of Chemical Engineering, University of Delaware, Newark, Delaware 19711, USA.     

The structure of solvent crazes in polystyrene

The structure of crazes grown in polystyrene (PS) immersed inn-heptane and methanol at room temperature has been determined using refractive index measurements, transmission electron microscopy, and fractographic analysis of craze fracture surfaces.n-heptane crazes in thin films exhibit a low number density of thick, load-bearing fibrils whereas methanol crazes consist of a highly interconnected network of fibrils not unlike the craze structure found in crazes grown in air. A row of large voids at the centre line of the craze which is typical of the structure observed in air crazes is not found, however, in either methanol orn-heptane crazes, indicating a different growth mechanism for the solvent crazes. The craze structure found in thin films is in agreement with the void contents determined from refractive index measurements and with the results from scanning electron microscopy of fracture surfaces of bulk crazes grown with methanol andn-heptane. Glass transition temperature measurements of equilibrium swollen PS films giveT _g=91° C for methanol andT _g=6° C forn-heptane. The results suggest that the structure of crazes grown with slowly diffusing crazing agents (methanol andn-heptane) is strongly dependent on whether the growth temperature is above or belowT _g, the glass transition temperature of the plasticized region just ahead of, and in, the craze. IfT _g is below the growth temperature, weak crazes are formed with a large void content. During the growth, thin fibrils break by viscous flow leaving only a small number of load-bearing fibrils. The stress in the neighbourhood of the growing craze is strongly relieved favouring propagation of a single craze. IfT _g is above the growth te'mperature, strong crazes are formed with the fibrils strain-hardened during the growth process. There is hardly any change in stress next to the craze and therefore multiple crazing (craze bundles) is favoured over the propagation of a single craze.     

Rubber-toughening of plastics

Mechanisms of deformation in an ABS emulsion polymer were studied quantitatively by a uniaxial tensile creep method. Craze formation was measured in terms of volume strain, which was calculated from simultaneous observations of longitudinal and lateral strains, and shear deformation was measured in terms of lateral strain. The experiments showed that shear deformation predominated during the early stages of creep, but that the rate of shear deformation fell with time. At stresses below 27 MN m⁻², specimens reached extensions of 5% without significant craze formation: at higher stresses, crazing was observed at strains above about 21/2%. Rates of crazing increased with time and with stress, so that the contribution of crazing to creep was greatest during the later stages of the test, and at the higher stresses. The relevance of these results to engineering applications of ABS polymers is discussed.     

Crazing behaviour in oriented poly(ethylene terephthalate)

A study has been made of the two types of crazes formed in oriented sheets of poly(ethylene terephthalate). The crazes have been termed tensile crazes and shear crazes. The tensile crazes formed parallel to the initial draw direction (IDD) whereas the shear crazes formed in a direction close to that of the deformation bands observed when the material yields.The possibility of applying a yield criterion to shear craze formation has been examined and there appears to be fairly good agreement between theory and experiment. Measurements of crazing stress on the tensile crazes indicated that the criterion for tensile craze formation is not purely dependent on the component of stress normal to the extended chains.It is concluded that the two types of crazes are formed by two quite different mechanisms, although the exact nature of these mechanisms is still uncertain.     

Shear and craze development in the fatigue of ductile amorphous polymers

The competitive interplay between shear and craze deformation in the fatigue of ductile amorphous polymers leads to the formation of intricate fatigue crack shapes and stable crack growth. The craze/shear dual deformation mode is expressed in a unique crack tip plastic zone, which has been observed in polycarbonate, polysulfone, a polyarylate block copolymer and a polyestercarbonate copolymer. The fatigue crack growth resistance in these polymers is high, and factors which affect the relative ease of shear flow and crazing are expected to affect their fatigue endurance.     

Ductility of polylactide composites reinforced with poly(butylene succinate) nanofibers

Sustainable “green nanocomposites” of polylactide (PLA) and poly(1,4-butylene succinate) (PBS) were obtained by slit die extrusion at low temperature. Dispersed PBS inclusions were sheared and longitudinally deformed with simultaneous cooling in a slot capillary and PBS nanofibers were formed. Shearing of PBS increases nonisothermal crystallization temperature by 30 °C. Tensile deformation was investigated by in-situ experiments in SEM chamber. Dominant deformation mechanism of PLA is crazing, however, there are dormant shear bands formed during slit die extrusion. Pre-existing shear bands are inactive in tensile deformation but contribute to ductility by blocking, initiating and diffusing typical craze growth. PBS nanofibers are spanning PLA craze surfaces and bridging craze gaps when PLA nanofibrils broke at large strain. Straight crazes become undulated because either dormant or new shear bands become activated between crazes. Due to interaction of crazes and shear bands the ductility increases while high strength and stiffness are retained.     

The mechanical behaviour of polystyrene under pressure

Tensile deformation of polystyrene carried out under pressure up to 4 kbar has shown that the pressure-transmitting fluid (silicon oil) acts as a stress crazing and cracking agent. Unsealed specimens showed a brittle-to-ductile transition at 2.95 kbar, while specimens sealed with Teflon tape and rubber showed the same transition at only 0.35 kbar. Analysis of the stress-strain curves for the sealed specimens indicated that the pressure dependency of the craze initiation stress differs from that of shear band initiation stress. The brittle-to-ductile transition occurs when the initiation stresses of both processes become equal. The principal stress for craze initiation showed almost no pressure dependency, suggesting that crazes initiate when the principal stress level of the tensile specimen reaches a critical value irrespective of the applied hydrostatic pressure. Similarly, no pressure dependency was observed for the principal ductile fracture stress. The pressure dependency of yield stress agreed well with a non-linear pressure dependent von Mises yield criterion.     

Rubber-toughening of plastics

The effects of matrix ductility upon mechanisms of rubber toughening have been studied in a set of materials having identical rubber contents, but differing in matrix composition. The materials were made by solution blending 50% of HIPS (high-impact polystyrene) with polystyrene and PPO^® poly-(2, 6-dimethyl-1, 4-phenylene oxide) in varying proportions. Crazing was studied quantitatively by measuring volume changes during creep. Analysis showed that in blends of HIPS with polystyrene, crazing is the only significant mechanism of tensile creep, whereas in blends containing polyphenylene oxide, shearing mechanisms are also important, and the contribution of crazing to creep deformation can be as low as 30%, depending upon matrix composition. Scanning electron microscopy showed that both crazes and shear bands were present in strained HIPS/PPO blends. Shear band formation appears to be responsible for the increased fracture resistance of blends containing a high proportion of polyphenylene oxide. A theory of toughening is proposed for these blends.     

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.     

Molecular basis of fracture of latex blends of polystyrene and poly(methyl methacrylate)

The molecular basis of fracture of polystyrene and poly(methyl methacrylate) homopolymers and their latex blends was investigated with a custom-built dental burr grinding instrument (DBGI). About a third of the chains were cut several times, the remainder not at all. The number of chain scissions in polystyrene and poly(methyl methacrylate) was quantitatively interpreted by the microscopic parameters of craze fibrils and the energy balance between chain scission and chain disentanglement (chain pullout). The probability that a polymer strand in the craze fibrils is scissioned or disentangled was calculated from the fracture energy balance. In addition, the fracture energy of the latex blends of polystyrene and poly(methyl methacrylate) was studied. The large interface between the polystyrene and the poly(methyl methacrylate) did not lead to a small fracture energy, as initially expected. Rather, the latex blend of the two immiscible polymers primarily absorbs the fracture stress by strong co-continuous bulk phases.     

Plasticization effects on environmental craze microstructure

Environmental crazes were grown in thin films of polystyrene (PS) using the homologous series of alcohols, CH₃OH, C₂H₅OH, n-C₃H₉OH and n-C₄H₉OH. The films were bonded to copper grids, strained to below the minimum crazing strain in air and exposed to the vapour of the various alcohols. The craze microstructure, as measured quantitatively by transmission electron microscopy, varies significantly along the series. The craze fibril volume-fraction, V _f, decreases monotonically from 0.25 for methanol, which depresses the glass transition temperature, T _g, of PS to 91° C, to 0.09 for n-butanol, which depresses T _g of PS to 71° C. All these slowly-growing vapour crazes thicken by drawing more fibrillar material from the craze surfaces rather than by fibril creep. The large decrease in V _f along the series of alcohols cannot be due to a change in the chain-entanglement molecular weight, as a result of swelling by the alcohols, but must result rather from an easier slippage of molecular entanglements in the drawing glassy fibrils. The large decrease in V _f from methanol to butanol crazes must also enhance the nucleation of cracks within these crazes, as evidenced by the ten-fold decrease in the environmental fatigue life of PS along the series from methanol to butanol.