Effect of molecular weight on craze shape and fracture toughness in polycarbonate

The shape of the craze at the tip of a crack has been studied using optical microscopy on polycarbonates of various molecular weights at ?30°C. For all molecular weights studied the craze shape was well approximated by the Dugdale plastic zone model and this model was used to calculate the craze stress and the release rate in plane strain. It was found that the craze dimensions, the craze stress and the strain energy release rate in plane strain all increased with increasing molecular weight. Fracture of macroscopic specimens showed a mixed mode fracture in all molecular weights. By studying the effect of thickness the strain energy release rate in plane strain was calculated for various molecular weights. Agreement was found between these values and those determined from the craze shape measurements. The overall strain energy release rate, the strain energy release rate in plane strain and the contributions from the plane stress mode increased with increasing molecular weight.     

A study of the fracture behaviour of polyethersulphone

The fracture behaviour of polyethersulphone has been studied by combining fracture toughness measurements with optical examination of the craze and shear lips at the tip of the crack. It is shown that the behaviour can be very well described by a mixed mode fracture model in which the total strain energy release rate contains a plane strain term from the craze and a plane stress term from the shear lips. In unannealed samples the craze shape was well approximated by the Dugdale plastic zone model, but this was not so for annealed samples. The effect of annealing on the fracture behaviour is discussed in qualitative terms.     

Fracture toughness and crack instability in tough polymers under plane strain conditions

The plane strain fracture toughness and fracture mechanisms of several tough engineering plastics have been studied and compared with poly(methyl methacrylate) (PMMA), a relatively brittle polymer. The tough polymers all are observed to form a multiple craze zone at the crack tip, which is shown to be the primary source of plane strain fracture toughness in these materials. The multiple craze zone is retained during slow crack growth but is metastable, and at a critical stress intensity and associated crack velocity, the system passes through a transition to a greatly accelerated single craze mode of unstable propagation.     

Craze shape and fracture in poly(methyl methacrylate)

The shape of the primary craze at the tip of a crack has been studied by optical microscopy for two grades of poly(methyl methacrylate). The craze shapes are compared with the predictions of the Dugdale model for the plastic zone at a crack tip, and used to obtain a quantitative estimate of the craze stress and the effective stress intensity factor. The values of the stress intensity factor are then compared with those obtained directly from fracture toughness measurements.     

Temperature dependence of craze shape and fracture in poly(methyl methacrylate)

The shape of the craze at the tip of a slowly moving crack has been determined by optical microscopy for poly(methyl methacrylate) over the range of temperatures ?30° to +45°C. In all cases the craze shape can be described by the Dugdale model for the plastic zone at a crack tip. It was of particular interest that the crack opening displacement was found to be constant over the whole temperature range. Fracture toughness values deduced from the craze shape were in good agreement with those obtained directly. Quantitative estimates of the craze stress were obtained and are discussed.     

Crack–Tip Toughness of a Soft Lead Zirconate Titanate

Crack–opening displacement (COD) measurements were performed on a commercial lead zirconate titanate (PZT). The intrinsic fracture toughness (or crack–tip toughness) of this material was determined using a new evaluation procedure, which takes into account the near–tip CODs and complete crack profile CODs. The crack–tip toughness K _I0 was determined from an extrapolation of COD data obtained at various loading stages, thus avoiding the complications caused by subcritical crack growth in PZT. Results for plane strain and plane stress condition are presented.     

The effects of stress cracking on the fracture toughness of polycarbonate

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in k_IC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.     

Stress analysis around a three-dimensional craze in glassy polymers

Stress analysis around a three-dimensional craze and a three-dimensional craze containing a penny-shaped crack has been made. The craze is treated as a transversely isotropic, oblate spheroid embedded in an isotropic glassy polymer. The craze is assumed to consist of primary fibrils and cross-tie fibrils, such that a penny-shaped crack may form at the central regime of the craze. The craze surface stress, the stress field near the craze tip outside the craze region, and the stress intensity factor in the crack tip are determined by using Eshelby's equivalent inclusion method. Numerical values of the fracture toughness and the stress needed to sever a craze fibril at the crack tip are calculated and the results appear to be in good agreement with the data given in the literature.     

Craze growth and crack growth in poly(vinyl chloride) under monotonic and fatigue loading

Cracks in poly(vinyl chloride) sheet were loaded to known stress intensity factors and the craze length measured. These measurements, and the craze thickness profile, were compared with the Dugdale model of crack tip yielding. The fracture toughness of poly(vinyl chloride) was measured, and an analysis made of circular ‘advance fractures’ that occur on the fracture surface. Fatigue crack growth studies confirmed that crack growth occurs discontinuously once every few hundred cycles, and showed that the craze fracture mechanism is quite different from the monotonic loading failure mechanism.     

Plane strain fracture toughness of polyethylene pipe materials

The plane strain fracture toughness of medium density polyethylene pipe materials has been investigated over a range of test temperatures and rates. Conditions are defined under which valid fracture toughness values can be obtained; at higher temperatures the material is notch-insensitive. Fracture surface morphology is described, and features are compared with predictions from the Dugdale model. The toughness derives from a band of fibrillar, drawn morphology associated with crack initiation or slow growth. The plane strain fracture toughness correlates with percent crystallinity according to the same relationship whether the crystallinity is varied by thermal treatment, comonomer content, or molecular weight.     

Molecular weight dependence of the fracture toughness of glassy polymers arising from crack propagation through a craze

We investigate criteria for craze failure at a crack tip and the dependence of craze failure on the molecular weight of the polymer. Our micromechanics model is based on the presence of cross-tie fibrils in the craze microstructure. These cross-tie fibrils give the craze some small lateral load bearing capacity so that they can transfer stress between the main fibrils. This load transfer mechanism allows the normal stress on the fibrils directly ahead of the crack tip in the center of the craze to reach the breaking stress of the polymer chains. We solve for stress field near the crack trip and use it to relate craze failure to the external loading and microstructural quantities such as the craze widening (drawing) stress, the fibril spacing, the molecular weight, and the force to break a single polymer chain. The relationship between energy flow to the crack tip due to external loading and the work of local fracture by fibril breakdown is also obtained. Our analysis shows that the normal stress acting on the fibrils at the crack tip increases linearly as the square root of the craze thickness, assuming that the normal stress distribution is uniform and is equal to the drawing stress acting on the craze-bulk interface. The critical crack opening displacement, and hence the fracture toghness is shown to be proportional to [1–(M_e/qM_n)]², where M_e is the entanglement molecular weight, M_n is the number average molecular weight of polymer before crazing, and q is the fraction of entangled strands that do not undergo chain scission in forming the craze.     

Fracture toughness of phenolphthalein polyether ketone

The static and impact fracture toughness of phenolphthalein polyether ketone (PEK-C) were studied at different temperatures. The static fracture toughness of PEK-C was evaluated via the linear elastic fracture mechanics (LEFM) and the J-integral analysis. Impact fracture toughness was also analyzed using the LEFM approach. Temperature and strain rate effects on the fracture toughness were also studied. The enhancement in static fracture toughness at 70°C was thought to be caused by plastic crack tip blunting. The increase in impact fracture toughness with temperature was attributed two different mechanisms, namely, the relaxation process in a relatively low temperature and thermal blunting of the crack tip at higher temperature. The temperature-dependent fracture toughness data obtained in static tests could be horizontally shifted to match roughly the data for impact tests, indicating the existence of a time–temperature equivalence relationship. © 1995 John Wiley & Sons, Inc.     

Multiple crazing at crack tips in PVC under plane strain conditions

The nature of the yield zone at the crack tip of poly(vinyl chloride) (PVC) pipe materials has been investigated. Microscopy studies employing a plasma etching technique reveal the presence of multiple crazes ahead of the crack tip in the interior of specimens of pure PVC, CaCO₃ filled PVC, and PVC pipe compound. The craze zone and the fracture toughness of blade-notched specimens are compared with those of fatigue pre-cracked specimens. Both types of specimens have similar fracture toughness values and form multiple crazes upon loading, suggesting that multiple crazing Is an intrinsic property of the material. The kinetics of craze initiation and the development of the multiple craze zones have also been explored.     

Size and mechanical properties of craze zones at propagating crack tips in poly(methyl methacrylate) during fatigue loading

In specimens of high molecular weight poly(methyl methacrylate) the microregion at the crack tip has been investigated during fatigue crack propagation by applying the optical interference method in a specially constructed experimental set-up. Thus, in the frequency range of 0.4 to 50 Hz the size and contour of the craze zone were directly measured at upper and lower load of the cycles. In contrast to previous assumptions it is established that the maximum craze width at the crack tip and hence the maximum length of stretched fibrils increases strongly with crack propagation rate. The directly measured craze data and also the material parameters indirectly derived by the aid of the Dugdale model are related to those data which have been measured during continuous crack propagation under quasi-static tensile load. The spacings of the fatigue striations on the fracture surfaces are compared with the lengths of the craze zones.     

Stress-strain behavior of the craze

The constitution and properties of crazes in glassy polymers and their relation to crack propagation are reviewed. New evidence is discussed which shows the craze to be much softer than the parent polymer but capable of sustaining large stresses and strains up to the point of failure. Craze failure is much more dependent on polymer molecular weight than is craze formation, and this difference is reflected in changes in both fracture surface morphology and crack toughness with molecular weight. Finally craze mechanical properties are thought to be integrally related to the mechanical behavior of high impact plastics.     

Craze morphology and molecular orientation in the slow crack growth failure of polyethylene

An optical microscopy study and a micro‐Raman spectroscopy study were carried out on polyethylene samples subjected to an environmental stress crack resistance (ESCR) test. The aim was to elucidate the molecular deformation mechanisms associated with the failure process. It has been shown that in the early stages of the ESCR test, in a regime of low local stress, failure in the craze occurs via a brittle process with limited ductility and with molecular orientation being detected. As the experiment progresses, however, extensive fibrillation takes place. The molecular orientation in these fibrils was found to be comparable to that measured in cold‐drawn samples. Moreover, the fibril molecular orientation decreased from the crack to craze tip and was found to be higher in the midrib part of the fibril (fibril failure point). As a consequence, fibril creep is the most likely mechanism of failure in the craze. Microscopy and Raman measurements showed that the extent of the brittle process is molecular weight‐dependent, that is, the brittle process seems to operate longer at higher molecular weights. These observations are in agreement with a previous work which showed that the molecular stress per macroscopic strain/stress decreases with increasing molecular weight, therefore holding the high molecular weight craze in a regime of low local stress for longer testing times. Fibrils spanning the craze are envisaged as the anchor points that hold the structure during the process of failure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 283–296, 2000     

聚碳酸酯／弹性体合金的断裂过程形貌研究

采用扫描电子显微镜 (SEM)和长距离显微镜 (LDM)对PC/弹性体共混体系试件中的裂纹在平面应变和平面应力下发展的形貌进行观察。结果表明 :弹性体的加入影响了材料断裂的各个过程。在平面应变下 ,裂纹尖端出现超钝化现象 ;平面应力下 ,第一次观察到裂纹后岸两侧出现副裂纹的现象。采用透射电子显微镜 (TEM )观察慢速加载时裂纹前沿各区的形貌 ,从微观上解释弹性体增韧PC的原因     

Fracture behavior of polyetherimide (PEI) and interlaminar fracture of CF/PEI laminates at elevated temperatures

To investigate the effects of environmental temperature on fracture behavior of a polyetherimide (PEI) thermoplastic polymer and its carbon fiber (CF/PEI) composite, experimental and numerical studies were performed on compact tension (CT) and double cantilever beam (DCB) specimens under mode‐I loading. The numerical analyses were based on 2‐D large deformation finite element analyses (FEA). Elevated temperatures greatly released the crack tip triaxiality (constraint) and promoted matrix deformation due to low yield strength and enhanced ductility of the PEI matrix, which resulted in the greater plane‐strain fracture toughness of the bulk PEI polymer and the interlaminar fracture toughness of its composite during delamination propagation with increasing temperature. Furthermore, the high triaxiality was developed around the delamination front tip in the DCB specimen, which accounted for the poor translation of matrix toughness to the interlaminar fracture toughness by suppressing the matrix deformation and reducing the plastic energy dissipated in the plastic zone. Especially, at delamination initiation, the weakened fiber/matrix adhesion at elevated temperatures led to premature failure of fiber/matrix interface, suppressing matrix deformation and preventing the full utilization of matrix toughness. Consequently, low interlaminar fracture toughness was obtained at elevated temperatures. POLYM. COMPOS., 26:20–28, 2005. © 2004 Society of Plastics Engineers.     

The stress and displacement fields at the tip of crazes in glassy polymers

This analysis models a craze region in glassy polymers as an elastic transversely isotropic homogeneous inclusion of thin elliptical shape with different elastic properties from the bulk polymer. The plane elasticity problem for an applied uniform stress field is solved and the results dimensionalized with respect to the craze tip radius. Stress and strain enhancements of several times far field values are found to occur at the craze tip and are independent of craze tip radius. These results are consistent with experimentally observed characteristics of craze growth and should be important in assessing the relative merits of different criteria that have been proposed for craze growth in glassy polymers.     

Crack propagation in biaxially oriented polypropylene at low temperature

The crack growth behavior of polypropylene biaxially oriented by cross-rolling was studied at low temperature. Single edge notch testing produced a stable tearing type of crack growth in both 50% and 80% biaxially oriented polypropylene at ?40°C, in contrast to the brittle fracture of unoriented polypropylene. The crack growth in the two oriented materials began slowly and accelerated to a constant rate that was higher in the 80% oriented material than in the 50% oriented material. The main difference between the crack growth behavior of the two was the longer period of initial slow growth in the case of 80% orientation. This period of slow growth corresponded to crack growth through the notch tip damage zone. Residual strength diagrams were used to present the crack growth data obtained when the stress state was intermediate between plane stress and plane strain. Fractography revealed large differences among the fracture surfaces of the three materials with the unoriented polypropylene showing a grainy appearance from the brittle fracture. The two oriented materials showed considerable ductility. The 50% oriented material showed many voids in the fracture surface, indicating that voiding during the fracture process contributed significantly to the toughness improvement. The 80% oriented polypropylene showed delamination crazing on the fracture surface with layered material and fibrils bridging the crazes.