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

Mechanical behaviour of laminated films of PS and PE, especially the altered mechanical properties of PS in the laminated state, were studied as functions of PE volume fraction. It was observed that crazing in PS can be modified by laminating layers of PE to both sides. Elongation at break and hence fracture energy increased conspicuously when PE volume fraction increased. Craze yielding stress in PS layers also increased with increasing PE volume fraction because craze formations in PS layers were suppressed by the reduction of tensile stress concentration effect at craze tips. Craze initiations were always found at the free side edges of the laminated films, which can be correlated with the transverse interlaminar shear stress concentrations existing at the edges of the laminated films caused by the difference in Poisson's ratios between PS and PE.     

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.     

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.     

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.     

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.     

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.     

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.     

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     

Stress relaxation of high impact polystyrene

The stress relaxation behaviour of high impact polystyrene has been correlated with the microstructural changes observed in tensile tests. The inhomogeneity of plastic deformation, manifested as stress whitening, has been measured using microhardness tests. This method has been found to be sensitive to the amount of crazing in the material. The stress relaxation behaviour changed at the onset of crazing, but did not change appreciably as the volume fraction of crazes increased. An analysis of the relaxation in terms of a site population model based on White's approach suggests the macroscopic stress relaxation is related to the crazes in the boundary regions between the stress whitened and unwhitened material.     

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.     

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.     

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.     

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.     

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.     

The effect of film thicknesses on craze microstructure

Transmission electron microscopy of the fibrillar microstructure of air crazes grown in polystyrene (PS) films thicker than 150 nm, shows this microstructure to consist of fine fibrils 4 to 10 nm in diameter with a mean value of 6 nm, in excellent agreement with recent small angle X-ray scattering measurements on crazes in bulk PS. For films 100 nm and thinner, the crazes have a much coarser microstructure, often resembling a perforated sheet more than a set of discrete fibrils. Where fibrils exist they are much larger in diameter (up to 150 nm) than those in thick film crazes. This change in craze micro-structure with decreasing film thickness is attributed to the absence of plastic constraint in the direction of film thickness. Even in much thicker films the absence of plastic constraint at the film surface gives rise to a surface plastic zone producing a surface groove of depth ∼ 25 nm which can extend up to 1000 nm ahead of the craze tip. The absence of lateral stresses in these films increases the wavelength of the meniscus instability (the mechanism of craze tip advance and fibril formation) until for a film thickness less than this wavelength a true fibril structure can no longer form.     

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.     

Correlation between craze formation and mechanical behaviour of amorphous polymers

The formation, growth and fracture of crazes have been studied for several amorphous polymers (PS, PMMA, SAN, NEC, PC). From bulk polymeric materials 0.5 to 5m thick sections were prepared and investigated after uniaxial deformation or duringin situ deformation in a 1000 kV high voltage electron microscope (HVEM). The use of the HVEM also allows one to study irradiation-sensitive polymers (PMMA and PC). The size of crazes (length and thickness), the shape (opening angle at the craze tip, craze thickness profile), the thickness of a pre-craze zone, the structure of the material inside the craze (fibrillar or more homogeneous), and the degree of deformation were measured. Correlations have been found between the type and the size of crazes and their mechanical properties, particularly fracture toughness and elongation at break. There are notable differences between unannealed and annealed samples (SAN and PC) as well as in the craze formation and in the fracture toughness.     

The fracture of notched tensile specimens of a transparent ABS polymer

The fracture of notched samples of a transparent ABS polymer has been studied. The role of crazing in the fracture of these samples has been investigated by examining transverse and longitudinal sections of the specimens before and after fracture, and by following crack growth with a 16 mm movie camera. The fracture surfaces have also been studied by SEM. A three-stage fracture process has been found. Initial crack growth has been observed through a craze in the craze bundle which formed at the notch, producing a highly whitened region on the fracture surface. The crack continually accelerated and, at a later stage in the fracture, was shown to jump between crazes in the craze bundle, leaving islands of whitened material on the fracture surface. When the crack appeared to catch up with the tip of the craze bundle, a third banded region has been observed on the fracture surface.     

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.     

Plastic deformation mechanisms in poly(acrylonitrile-butadiene styrene) [ABS]

Thin films of two poly (acrylonitrile-butadiene-styrene) [ABS] resins have been strained in tension, and the ensuing deformation has been characterized by transmission electron microscopy. To enhance contrast of the rubber particles, some of the specimens were stained with OsO₄. Films containing only solid rubber particles 0.1 m in diameter show little tendency for crazing. Instead, cavitation of the rubber particles occurs, together with localized shear deformation between the particles along a direction nearly normal to the tensile axis. For specimens containing a mixture of the same small particles plus larger (1.5m diameter) particles containing glassy occlusions, some crazing does occur. Crazes tend to nucleate at the larger particles only. When crazes encounter the smaller particles these cavitate without appearing to impede or otherwise affect the craze growth. The occluded particles also show significant cavitation, with voids forming at their centres at sufficiently high levels of strain. These voids do not seem to lead to rapid craze break-down and crack propagation. In commercial ABS, which typically has both large and small rubber particles, both crazing, nucleated by the large particles, and shear deformation, encouraged by the cavitation of small rubber particles, can be expected to make important contributions to the toughness of the polymer.