The stress-whitened damage zone of PVC blends

The stress-whitened damage zone that formed ahead of a semicircular notch during slow tensile loading has been measured from optical micrographs of translucent blends of poly(vinyl chloride) (PVC) with experimental chlorinated polyethylene (CPE) resins. When the zone was small, the plane strain condition applied and from the elastic stress distribution a constant mean stress condition was found at the boundary of the crescent-shaped zone. The critical mean stress did not depend on the chlorine content or the chlorine distribution of the experimental CPE resin used in the blend. While the critical mean stress decreased as the amount of CPE in the blend was increased, the critical volume strain, calculated from the bulk modulus, was independent of composition and was thought to be the controlling parameter for stress-whitening. When the zone was larger, the shape was qualitatively described by concepts of stress redistribution in the presence of a plastic zone ahead of the notch. Macroscopic flow and necking were only detected near the maximum in the stress-displacement curve.     

Development of a risk profile for intentions to use alcohol among fifth and sixth graders

OBJECTIVE: The relationship between intentions to use alcohol and risk factors was examined among fifth and sixth graders. METHOD: Subjects completed a questionnaire that assessed intentions to use alcohol and eight risk factors. Risk factors included peer and parental use and attitudes toward use, sensation seeking, tolerance of deviance, rejection of parental authority, and family cohesion. RESULTS: Multivariate analyses showed that the risk factors were able to discriminate between those who intended to use alcohol and those who did not. Family factors showed stronger relationships to intentions among fifth graders, and peer factors were more strongly related to intentions among sixth graders. CONCLUSIONS: Risk factors can be used to target preadolescents who may be at risk for early alcohol use, and programs that attempt to prevent early initiation of alcohol use among adolescents can be designed.     

Deformation and fracture of urethane–methacrylate resins

The tensile properties of three urethane–methacrylate resins that varied in the soft segment content of the urethane were characterized. The strain birefringence at a circular hole was observed during loading–unloading cycles to progressivley higher displacements. The shear strain distribution at the hole was calculated from the isochromatic fringe contours and compared with results from linear elastic analysis. When the onset of nonlinearity, and the subsequent appearance of residual strain at the root of the hole, were correlated with features of the macroscopic stress-displacement curves, three regions of prefracture deformation were defined. A region of linear elastic behavior was observed at the lowest strains. The maximum shear strain at the linear limit was the same in all the resins, and appeared to correlate with the yield condition at the hole. When the shear strain at the hole exceeded about 2.8%, the fringe patterns started to deviate from the elastic prediction. However, strain was fully recoverable in this region as indicated by the absence of residual birefringence at the hole after unloading. This region of nonlinear, recoverable deformation extended to progressively higher strains as the amount of urethane soft segment increased. This feature was attributed to the network structure of the urethane–methacrylate resins. A region characterized by nonrecoverable deformation at the hole followed at higher strains; the urethane soft segment content had a major effect on the amount of permanent deformation sustained before fracture. The fracture surfaces exhibited features typical of brittle fracture without crazing. © 1995 John Wiley & Sons, Inc.     

The mechanical behaviour of PVC short-fibre composites

Competitive deformation processes between interfacial debonding and matrix cracking at the fibre ends is shown for the short-fibre reinforced composites of polyvinyl chloride (PVC). The increase of interfacial shear strength by chemical coupling prevents early failure at the interface, thus increasing the tensile failure stress of short-fibre composites. The previously proposed general yield criterion for PVC and its short-fibre composites is also examined. No significant effect due to improved fibre-matrix adhesion on the upper shear yielding of short-fibre composites is observed. The matrix flow in the post-yield region is less restricted when debonding occurs.     

Kinetics of fatigue and creep crack propagation in PVC pipe

The kinetics of creep and fatigue crack growth in PVC pipe were studied in order to develop a methodology for predicting long‐term creep fracture from short‐term fatigue tests. Fatigue and creep crack propagation followed the conventional Paris law formulations with the same power 2.7: da/dt = A_fΔK^2.7_I and da/dt = BK^2.7_I, respectively. The activation energy for creep crack propagation, obtained from the temperature dependence of the Paris law prefactor, allowed extrapolation of high temperature creep fracture to low temperature creep crack growth rates. The activation energy for fatigue crack propagation was much lower than that for creep. Therefore, fatigue and creep could not be directly correlated by using the prefactor in the covenentional Paris law formulations. Furthermore, a unique value of the Paris law prefactor did not describe frequency and R‐ratio (amplitude) effects in fatigue crack propagation. Nevertheless, conformity of crack growth rates measured under all conditions to the same Paris law power suggested that correlation should be sought in alternative formulations of the crack growth rate.     

Improving oxygen barrier properties of poly(ethylene terephthalate) by incorporating isophthalate. I. Effect of orientation

The present study examined poly(ethylene terephthalate) (PET) and a series of statistical and blocky copolymers in which up to 30% of the terephthalate was replaced with isophthalate by copolymerization and melt blending, respectively. Some level of transesterification during processing of melt blends resulted in blocky copolymers, as confirmed by NMR. Random and blocky copolymers exhibited similar properties in the glassy state, including a single glass transition, due to miscibility of the blocks. However, random copolymerization effectively retarded cold‐crystallization from the glass whereas blocky copolymers readily cold‐crystallized to a crystallinity level close to that of PET. The polymers were oriented at four temperatures in the vicinity of the T_g and characterized by oxygen transport, wide‐angle X‐ray diffraction, positron annihilation lifetime spectroscopy, and infrared spectroscopy. Orientation of all the copolymers resulted in property changes consistent with strain‐induced crystallization. However, blocky copolymers oriented more easily than random copolymers of the same composition and after orientation exhibited slightly lower oxygen permeability, higher density, and higher fraction trans conformers. Analysis of oxygen solubility based on free volume concepts led to a two‐phase model from which the amount of crystallinity and the amorphous phase density were extracted. Dedensification of the amorphous phase correlated with the draw temperature. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1615–1628, 2005     

Microstructure and its relationship to deformation processes in amorphous polymer glasses

The microdeformation morphology of a number of vinyl polymers with bulky side chains (type I) and arylene polymers with flexible oxygen linkages (type II) was studied by electron microscopy. The polyarylenes crazed only near the glass transition while the polyvinyls exhibited a crazing regime that extended to liquid nitrogen temperatures. In addition significantly less plastic strain was localized in type II glass crazes relative to those in type I glasses. In compatible blends of polystyrene (PS) and 2,6-dimethyl poly(phenylene oxide) (2MPPO), ca. 30% 2MPPO was sufficient to induce a transition from type I to type II crazing behavior. Small amounts of PS suppressed the low-temperature 2MPPO β relaxation but enhanced the intermediate transition of 2MPPO at higher temperatures. Blending increased the conformational energy of the 2MPPO chain and improved interchain packing. The propensity for the polymer glass to form sharp shear bands at the expense of diffuse bands was increased by a decrease in the conformation energy of the polymer chain and an improvement in the glassy state packing.     

Damage zone development in PVC under a multiaxial tensile stress state

The irreversible deformation mechanisms of poly(vinyl chloride) with a semicircular notch under slow tensile loading have been studied as a function of sheet thickness. Initially, core yielding was observed in the optical microscope as two families of slip lines growing from the notch surface in the centre of the specimen. The size and shape of the core yielding zone could be described by plasticity analysis. A stress-whitened zone subsequently initiated near the tip of the slip line zone. The stress whitening was caused by 1 m voids that were visible in the scanning electron microscope. The mean stress for stress whitening was calculated to be 43.0±1.5 MPa by a plastic stress analysis of a pressure-dependent yield material. By assuming a constant mean stress along the boundary of the stress-whitened zone, the one-dimensional shift of the elastic stress distribution was obtained. At higher stresses, hinge shear and intersecting shear were observed for thick and thin sheet, respectively.