The influence of physical aging on the creep rupture behavior of polystyrene

The effects of postannealing aging time on the brittle fracture behavior of polystyrene were studied. A combination of mechanical properties, including creep and creep rupture under constant load and the behavior under constant extension rate deformation were examined for polystyrene samples of different prior aging times (from 1h to 2 months). The specimens and fracture surfaces were examined by optical microscopy and SEM to observe any change in the fracture behavior. It was found that longer aging times caused not only a change in the time-dependent modulus of the material but also a significant decrease in the creep rupture life and a decrease in strain to failure. It was found that the reasons for this are that although aging delays craze formation, craze breakdown and ultimate failure are accelerated by aging. The importance of these findings are discussed, particularly in relation to failure criteria involving the use of critical strains. © 1993 John Wiley & Sons, Inc.     

用透射电镜研究PS薄膜银纹的微观结构、变形特性和破坏过程

本文介绍了用透射电镜观察PS薄膜银纹的微观结构、变形特性和破坏过程。实验观察到PS银纹质完整的网络结构和取向银纹质断裂以后的松弛特性。实验发现PS银纹质断裂转变成微裂纹的过程类似有机玻璃慢裂纹区的“撕布”模式,裂端银纹的外形符合Dugdale模型。     

Crazing in two polystyrene/polybutadiene block copolymers

Crazing was investigated in two commercial polystyrene/polybutadiene block copolymers made by the Phillips Petroleum Co. and marketed under the trade names of KRO-1 and KRO-3 resins. The two block copolymers each with 23% polybutadiene (PB), have radically different microstructure and radically different crazing behavior, leading to strains to fracture of 0.1 and 1.0, respectively. Of these, the KRO-1 Resin has a phase microstructure that consists of randomly wavy and often interconnected rods of PB of 20 nm diameter surrounded by polystyrene (PS). The microstructure of KRO-3 Resin consists of lamellae of PB with 20 nm thickness and large aspect ratio which range in packing from regular aligned lamellar domains with randomly varying misorientation in the annealed material, to randomly corrugated and wavy sheets in the as-received material. Crazes in KRO-1 Resin have well delineated planar shapes with a conventional, tufty craze matter structure which suggests growth by the now well-established meniscus instability mechanism proposed by one of us. In KRO-3 Resin, on the other hand, crazing involves profuse cavitation if the PB lamellae, giving rise to less well delineated zones of cavitational growth dispersed over the volume and suggests a mechanism of craze growth by stable, interfacial cavitational degradation in a process zone ahead of the craze tip. The measured stress and temperature dependences of craze velocities in these two polymers is in partial support of the suggested mechanisms which are also developed in outline.     

A scaling analysis of the fracture mechanisms in glassy polymers

Recent experimental work on craze growth in polymers has shown that the reptation model for polymer motion may be successfully applied, in certain cases, to the behavior of glassy polymers in the solid phase. In this paper, a simple scaling analysis is presented to show how the primitive path model for reptation may be applied to the behavior of glassy polymers before and during slow crack growth. Different dynamic regimes are identified and related to the relaxation times of the polymer chain. The dependence on polymer chain length for each mechanism is described. Estimates for the fracture energy of each process are presented.     

Polytetrafluoroethylene: Effects of γ-radiation on fine structure

The structure of granular polytetrafluoroethylene has been studied by electron microscopy. On the basis of the texture of surfaces resulting from fracture a model of the structure is proposed which suggests that PTFE consists of extended chain crystals with both inter- and intra-lamellar noncrystalline regions. The effects of γ-radiation on the structure have been investigated by examining the texture of irradiated fracture surfaces and also the texture produced by post-irradiation fracture. The irradiations have been performed in vacuo and in oxygen. In both atmospheres PTFE undergoes degradation with a concurrent increase in crystallinity. However, the texture of the surfaces of high crystallinity PTFE, prepared by radiation, differs markedly to the texture of fracture surfaces of high crystallinity PTFE prepared by thermal annealing. It is proposed that radiation causes rupture of bonds in the interlamellar (chain fold) and intralamellar regions, resulting in the production of chain ends and interlamellar links. Due to scavenging of the free radicals, interlamellar linking is pobably a minor process with irradiation in oxygen. These chemical changes cause modifications to the extended chain lamellar crystals and consequently alterations to the physical properties of the polymer.     

Bond rupture in highly oriented crystalline polymers

The strength-limiting process in the fracture of semicrystalline fibers and highly oriented films is the rupture of tie molecules connecting the folded chain lamellae in the machine direction. This view is supported by the data on stress and temperature dependence of lifetime of fibers under load and on radical formation during the fracture experiment. The observed tensile strength, however, is about 10 times smaller and the number of fractured chains between 100 and 1000 times larger than expected on the basis of the known number of tie molecules in the fracture plane. This discrepancy is a consequence of the inhomogeneity of the micromorphology of fiber structure, which causes a much larger stress concentration on the most unfavorably located tie molecules than the average value one would expect in the case of perfectly uniform stress distribution on identical tie molecules. The fluctuation of amorphous layer thickness, of number and length of tie molecules, produces such a high stress concentration on some tie molecules throughout the sample that they rupture long before the average stress concentration is sufficient for chain fracture. By accumulation of damage caused by gradual chain rupture the weakening of the sample locally proceeds so far that at the maximum damage concentration, microcracks start to form, and the fiber breaks.     

Chain scission in polystyrene by impact fracture

The number of chain scissions n_s per unit fracture area by impact in high-molecular weight polystyrene is determined to be approximately 3.3 × 10¹⁴/cm² at room temperature. This is almost 20 times larger than would be expected if chain scissions took place only at, or very close to, fracture surfaces. This result was obtained by measuring the molecular weight decrease and the total fracture area of the impact fragments by using size exclusion chromatography and statistical particle size measurements, respectively. The large n_s strongly indicates that significant chain breakage occurs during crazing before the propagation of cracks. An average craze thickness before breakdown under impact is estimated from n_s to be around 2 μm. In a diluted polymer, n_s is found to be significantly lower than the extrapolated value, assuming a linear dilution of entangled chain crossings at the fracture surface. This low chain scission density, however, can be explained by taking into account the reduction of craze breakdown strain in the diluted polymers. Finally, the broken chain ends of polystyrene appear to be stable under ambient conditions. © 1992 John Wiley & Sons, Inc.     

Dependence on strain rate of the nucleation of cracks in polystyrene at 293°K

The processes associated with the deformation and fracture of polystyrene tested in uniaxial tension have been studied over a range of strain rates from 1.4 × 10^?2 to 4.3 × 10^?7 sec^?1 and at constant stresses between 4.1 and 2.9 kg/mm². The effect of strain rate on the surface craze distribution prior to fracture, the fracture stress, the mechanism of nucleation of cracks, and the nature of fracture surfaces associated with slow and fast crack propagation have been determined. The changes in fracture surface appearance have been studied using optical and stereoscan microscopy. The observations are consistent with the model presented in a previous paper. Fracture is preceded by craze formation, cavitation in the craze, coalescence of cavities to form large planar cavities which propagate slowly until a critical stage is reached at which fast crack propagation occurs. The effect of changes of strain rate and material variables on these processes is discussed.     

Craze growth and healing in polystyrene

The kinetics of craze growth and craze healing were studied by dark-field optical microscopy in monodisperse molecular weight polystyrene (PS) that varied in molecular weight from 88,000 to 1,334,000. The following observations were made. (1) G₁ the virgin growth rate, decreased rapidly with increasing molecular weight until M_n ～ 200,000 and then remained constant. (2) G₁ decreased with increasing craze density. (3) The growth rates of approaching craze tips decreased when the craze tips overlapped, and the effect was less for crazes whose parallel growth paths were greater than 40 μm apart. (4) Complete craze healing was observed by comparison of the nucleation times, τ₂, and growth rates, G₂, of healed individual crazes with the craze kinetics of the virgin sample. (5) The extent of healing was characterized using four cases in which τ and G were measured as a function of healing time, temperature, constant stress, and molecular weight. (6) Craze healing times were found to increase with molecular weight and were analyzed in terms of the modified molecular weight of the craze zone. (7) Significant bond rupture was determined to occur during crazing by comparison of healing times with stress relaxation and diffusion data. (8) Craze healing studies provide insight into both crack healing and fracture of glassy polymers.     

Fracture surface of polystyrene: Mackerel pattern

Characteristic markings of concentric bands are formed on the fracture surface of tensile specimens under certain loading conditions. The marking form in the fast crack growth region of the mirror area of fracture. Optical and electron optical microscope techniques have been used to study the morphology of the markings. It is shown that in this region the crack propagates along the interface between the craze, in which the crack nucleated, and the bulk material. The mackerel pattern is caused by the crack jumping from one craze–matrix interface to the other.     

Fatigue crack propagation mechanisms in poly(methyl methacrylate) by in situ observation with a scanning laser microscope

Crack propagation tests were performed on an amorphous polymer, poly(methyl methacrylate), to investigate fatigue crack propagation mechanisms. A scanning laser microscope with a newly developed tensile testing machine was used to observe in situ crack propagation in compact‐type specimens. A crack usually propagated within the craze located at the crack tip under both static and cyclic loading conditions. When a crack stably propagated into the craze under static loading conditions, bright bands composed of the broken craze were observed at the edges along the crack wakes. However, there were successive ridges and valleys in place of bright bands along the crack wakes under cyclic loading conditions. When stable fatigue cracks were propagated at the loading half‐cycle in each cycle, new craze fragments appeared that were similar to the bright bands under static loading. However, the thickness of these fragments decreased in the following loading cycle, and a new valley was formed. This suggested that the valleys were formed by the contact between the fracture surfaces near the crack tip during unloading. Fatigue crack propagation is thought to be due to fibrils weakened by crack closure between fracture surfaces. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3103–3113, 2001     

Mechanisms of fracture in an oriented polystyrene

Fracture surfaces of an oriented polystyrene (by stretching 150% at 280°F) show many of the same features observed in the fracture of ordinary polystyrene: craze formation followed by quasi-viscous separation of the craze layer or by quasi-brittle fracture along the craze boundary. In cleavage fracture along the direction of orientation, advance fractures are initiated in flat areas covered, if at all, by an extremely thin layer of craze: the initiation of the “crazeles” fractures appears to arise from the orientation of the molecules rather than the presence of impurity particles.     

Quantitative approaches to particle cavitation,shear yielding,and crazing in rubber-toughened polymers

Models for rubber particle cavitation, shear yielding, and crazing are reviewed, and their ability to predict the large-strain deformation behavior of toughened polymers is discussed. An existing model for void initiation and expansion in rubber particles correctly predicts the observed trends: cavitation resistance increases when either the shear modulus or the surface energy of the rubber is increased, or the particle size is reduced. However, further work is needed to improve quantitative modeling of the thermally- and stress-activated void nucleation step. Shear yielding, which is also a rate process, is much better understood; here, the main problems in modeling relate to the formation and evolution of porous shear bands. Craze growth and failure are also reasonably well understood, but previous attempts at modeling have been hampered by uncertainties about craze initiation. To overcome these difficulties, a new theory of crazing is proposed, which treats initiation as a fracture process, and defines a new materials property, G_nasc, the energy required to form unit area of nascent craze. Because nascent crazes are ∼20 nm thick, G_nasc is low: calculations give values <0.5 J m⁻² for polystyrene. A new criterion incorporating a plasticity factor fits the data of Sternstein and coworkers on crazing under biaxial loading. In combination with theories of particle cavitation and shear yielding, the fracture mechanics model explains why the balance between crazing and shear yielding is governed by particle size, for example in ABS. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1399–1409, 2007     

Properties of fracture surfaces of glassy polymers: Chain scission versus chain pullout

Fresh fracture surfaces formed by tensile failure of craze in molded polystyrene (PS) bars have been compared with the molded surfaces of the same bars, using an atomic force microscope with a thermal probe and operated in local thermal analysis. The results indicate that molecular weight is much higher in the interior of the sample than at the surface. No evidence was found for degradation of the PS chains via chain scission during crazing. Alternative explanations for the low‐molecular weights at the molded surface are discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Nonequilibrium statistical theory of damage and fracture for glassy polymers, 1. The statistical distribution and evolution of microcracks in glassy polymers

The purpose of this paper is to construct a unified theoretical framework to link micro to macro-mechanical properties of glassy polymers. Starting from a model of microcrack propagation in craze on a mesoscale, the kinetic process of microcrack propagation resulting from fibril breakdown in the crack tip zone is mathematically formulated by a combination of fracture mechanics and fracture kinetics. A microcrack evolution equation involving both the geometric structure parameters of craze and the meso-mechanical quantities is obtained. After solving this evolution equation, a statistical distribution function of microcrack size which evolves with time and the moment generating function of microcrack size are derived. Any-order averaged damage functions can be therefore deduced. Specifically, the analytical expressions of the first-order averaged damage function and its damage rate are presented, which correspond to a similar definition of damage mechanics.     

Rapid healing of crazes in ruptured rubber-toughened plastics

High-speed photographic studies in connection with photodensitometry were performed to study craze healing in ABS/MMA sheets occurring within 5 microseconds to 20 milliseconds after rupture. Observations on various parts of the deformed specimens, particularly at a plastic zone in the vicinity of a crack, show that there are two stages of healing; first, a relatively elastic stage, and second, a relatively viscoelastic stage. Results of the observation also indicate that the rate of healing is influenced by the magnitude of plastic strain to which the specimens were subjected. The healing mechanism near an advancing crack is discussed briefly.     

Micromechanics of fatigue crack propagation in glassy thermoplastics

By the aid of the optical interference method the size of the craze zone at the crack tip has been measured during fatigue crack propagation (FCP) in two glassy thermoplastics thus giving a basis to re-examine proposed models. In contrast to previous assumptions it has been found, that in PMMA of high molecular weight crack propagation occurs only during a short interval of the loading cycle when the fibrils are stretched most severely and it is not limited by crack tip blunting; between the dimensions of the craze zone and the crack advance per cycle which is also reflected by markings on the fracture surface no simple correlation has been found. In PVC first the craze grows continuously during many loading cycles up to its final size and then the crack propagates by a jump separating the craze zone only partly. Thus at all stress intensity levels investigated the length of the final craze zone has been found to be distinctly larger than the jump spacing on the fracture surface. By aid of SEM-photography it is shown that in PVC during FCP cracking occurs by separation of fibrils instead of void coalescence.     

聚合物的银纹研究进展

综述了国内外有关聚合物银纹研究的进展,内容包括银纹的定义、分类、引发、生长、断裂及结晶聚合物中银纹的情况。     

Mechanical properties of methanol crazes in poly(methyl methacrylate)

Methanol crazes are grown from sharp cracks in poly(methyl methacrylate) (PMMA). The craze thickness profile is measured using a replica technique after the craze opening displacement profile of the growing craze has been measured with holographic interferometry. The craze strain profile is then computed from these data. The craze surface stress profile is determined by two methods: (1) from the uniaxial strain profile of regions adjacent to the craze as measured from the fringe spacing on the reconstructed hologram and (2) from the craze opening displacement profile using the Fourier transform method of Sneddon. From the surface stress and craze-strain profiles a true stress-strain curve for the craze fibrils has been constructed. The extrapolated fibril yield stress is in good agreement with the yield stress of bulk PMMA plasticized with methanol indicating that surface tension effects do not contribute importantly to craze fibril mechanical properties at room temperature. The craze strain increases from 0.4 near the craze tip to 1.4 near the craze base implying that methanol crazes in PMMA thicken by further straining of the existing craze fibrils and not by drawing new material into the craze from the craze surfaces. The primordial craze thickness, i.e., the original thickness of polymer which fibrillates to form the craze fibrils, is approximately 1 μm and is constant over most of the craze length. This thickness may be determined by diffusion of methanol normal to the craze surfaces in a process zone just behind the craze tip.