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.     

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.     

The effect of temperature on crazing mechanisms in polystyrene

At room temperature scission is the dominant mechanism for the modification of the entanglement network required for craze formation in polystyrene, but as the temperature is increased towardsT _g, there is the possibility that disentanglement processes may contribute. These will be most important for short chains. If disentanglement can occur, a molecular weight dependence of the crazing stress as a function of temperature will result. This prediction is tested by straining thin films of a range of monodisperse samples of polystyrene at temperatures between 40 and 90° C. The nature of the ensuing deformation has been characterized by transmission electron microscopy. It is observed that whereas only crazing occurs over the entire temperature range for the lowest molecular weight sample, shear processes become important for higher molecular weight materials. For the longest chains, crazing is almost entirely suppressed at 80° C, with the preferential formation of shear deformation zones occurring. These observations are consistent with the idea that disentanglement is playing a significant role in craze formation at sufficiently high temperatures.     

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.     

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.     

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.     

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.     

Modes of deformation in rubber-modified thermoplastics during tensile impact

Real-time small-angle X-ray scattering (RTSAXS) studies were performed on a series of rubber-modified thermoplastics. Scattering patterns were measured at successive time intervals as short as 1.8 ms and were analysed to determine the plastic strain due to crazing. Simultaneous measurements of the absorption of the primary beam by the sample allowed the total plastic strain to be computed. The plastic strain due to other deformation mechanisms, e.g. particle cavitation and macroscopic shear deformation was determined by the difference. Samples of commercial thicknesses can be studied at high rates of deformation without the inherent limitations of microscopy and its requirement of thin samples (i.e., plane strain constraint is maintained on sample morphology). Contrary to the conclusions drawn from many previous dilatation-based studies, it has been demonstrated that the strain due to non-crazing mechanisms, such as rubber particle cavitation, and deformation of the glassy ligaments between rubber particles, occurs before that due to crazing mechanisms. Crazing accounts for at most only half of the total plastic strain in HIPS (high impact polystyrene) and ABS (rubber-modified styrene-acrylonitrile copolymer) materials. The proportion of strain attributable to crazing can be much less than half the total in thermoplastic systems with considerable shear yield during plastic deformation. The predominant deformation mechanism in polycarbonate-ABS blends is shear in the PC (polycarbonate) with associated rubber gel particle cavitation in the ABS. This cavitation means that there appears to be a direct relationship between gel particle rubber content in the ABS and toughness of the blend. The mechanism is the same whether the tensile stress is in the direction parallel or perpendicular to the injection-moulded orientation, with simply less total strain being reached before fracture in the weaker perpendicular direction. Crazing, although the precursor to final fracture, occurs after the predominant mechanism and contributes only a few per cent to the total plastic deformation.     

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.     

On criteria for craze initiation in glassy polymers

The previous published data on biaxial critical stresses for craze initiation on the surface of cylindrical specimen of glassy polymers have been reexamined. A new interpretation is presented for the trend of crazing stress near pure shear state in the second quadrant of principal stress space which was observed in air at elevated temperatures. That is, an increase in the tensile stress for crazing with an increase in the magnitude of the compressive stress is interpreted not to be followed by a decrease in the dilatational stress but by development of shear yielding. On the basis of this interpretation, a new empirical criterion for craze initiation is proposed by considering the stress concentration due to surface scratch. The theoretical crazing locus accorded with the previous experimental results, except for the data near the shear yield locus in the second quadrant, which were considered to be affected by shear yielding. It was also indicated in the calculation that the shape and direction of surface scratch exert a considerable influence on the trend of biaxial crazing stress.     

Toughening mechanisms in elastomer-modified epoxies

Some brittle epoxies can be toughened significantly by the addition of an elastomeric phase. A great deal of controversy still exists on the nature of the toughening mechanisms. In this work tensile dilatometry at constant displacement rates was used to determine whether voiding, crazing or shear banding are the deformation mechanisms. Diglycidyl ether-bisphenol A epoxies toughened by various levels of several types of carboxyl-terminated butadiene nitrile liquid rubber were studied. The results indicate that at low strain rates the rubber particles simply enhance shear deformation. At sufficiently high strain rates the rubber particles cavitate and subsequently promote further shear deformation. No indication of crazing as an important toughening mechanism is found. No significant effect of rubber particle size or type can be ascertained.     

The failure processes,morphology and mechanical properties of a co-polyimide glass based on benzophenone tetracarboxylic acid dianhydride

The phyiscal structure, failure processes and mechanical properties of solution-soluble copolyimide films based on benzophenone tetracarboxylic acid dianhydride are reported as a function of sample preparation. The failure processes and mechanical response are modified by the presence of residual solvent and microvoids, which are produced by the elimination of solvent clusters from the glass. The polyimide is amorphous, with the exception of a few isolated clusters of poorly formed spherulites and networks of 50 to 500 nm wide lamellae. The deformation modes observed when thin films were strained directly in the electron microscope were crazing, shear-band deformation and an edge-yielding phenomena. Edge-yielding, which has characteristics of both crazing and shear-banding, occurred in 1 m wide bands which were 20 to 30° to the tensile stress direction. Shear-band deformation occurred in fine -100 nm wide bands, which exhibited a sharp boundary between themselves and their surroundings. TEM indicated that the shear strain was uniform within these bands. Microvoids, 1.5 to 15 nm diameter, were found to initiate shear bands some of which were 1 nm wide. These bands increased in width by tearing at the microvoid initiation sites.     

A mathematical relation between volume strain,elongational strain and stress in homogeneous deformation

A model is presented for the volume strain of a two-phase blend which elongates homogeneously in a tensile test apparatus. In the case when only elastic deformation and crazing take place the volume strain against elongation curve can be constructed and calculated from the data of the stress-strain curve alone. When, as well as crazing and elastic deformation, shearing takes place, the data of the stress against elongation curve and the volume strain against elongation curve can be used to calculate the separate contributions of the three deformation mechanisms at any elongation. In principle, the model can be also used for any homogeneous system which deforms without necking and where one or more deformation mechanism is present.     

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.     

PMMA蠕变银纹的分形维数演化

通过面积转换,修正了有限变形条件下的分形维数,并证明在应变小于10%的条件下,面积变化对分形维数的影响很小。通过蠕变试验,采集了聚甲基丙烯酸甲酯(PMMA)在不同应力和不同时间下的蠕变银纹图像,并用盒维法计算了其分形维数。试验证明,蠕变银纹引发的应力阈值随时间的增长呈指数关系降低,并最终趋于常数;蠕变银纹的分形维数与应力呈线性关系,与时间呈指数关系;在达到应力阈值的瞬间,能产生大量银纹,导致银纹的分形维数产生阶跃。根据实验结果,建立了蠕变银纹的分形维数与应力、时间的关系式。     

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.     

The effect of temperature on the deformation and fracture of polystyrene

Tensile and compression tests have been carried out over a range of temperatures between 78 and 360 K to determine the micromechanisms of deformation and the mode of fracture. In compression, deformation occurred by shear band propagation whereas, in tension, deformation was restricted to crazing followed by fracture. Anomalously low crazing stresses were observed for tests on specimens immersed in liquid nitrogen. Analysis of the mechanism of fracture confirmed previous work relating crazing and fracture and the fracture model has been modified slightly to take account of additional features observed in failure of crazes at high temperatures.     

Rubber-toughening of plastics

A new and quantitative method for studying rubber-toughening is presented: the extent of crazing in creep specimens is determined from time-dependent volume changes, which are compared with longitudinal strains in order to determine the contribution of other mechanisms. The results show that creep deformation in HIPS under high tensile stress consists essentially of elastic deformation followed by crazing. A simple model for the kinetics of crazing is developed, from which rate constants for initiation (k _I) and propagation (k _P) of crazes are obtained. Bothk _I, andk _P exhibit an Eyring-type dependence upon stress, with the same apparent activation volume of 5000 ų.     

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