Micromechanism of a deformation process before crazing in a polymer during tensile testing

Polymers are popularly used for housing and parts of machines and equipment. However, their mechanical properties, especially the deformation process, have not been clarified. During tensile testing, crazes are thought to be a source of microcracking and fracture, but the relation between the craze formation process and the deformation process before crazing is not understood. In the present work, scanning acoustic microscopy and X-ray diffraction were used to investigate the micromechanism before craze formation in polymethylmethacrylate (PMMA) and polycarbonate (PC). The velocity change of the surface acoustic wave and X-ray diffraction intensity indicated that molecular orientation occurred in a very small area from early stages of plastic deformation. From the results it was thought that texture was heterogeneous and anisotropic in a very small area, the shape of the area was spheroidal with a longer radius in the direction perpendicular to the applied stress, and the molecular chain in the area was oriented parallel to the stress axis. The area is thought to increase with increasing plastic strain.     

The environmental stress crazing of polycarbonate

The environmental crazing from a central hole in extruded polycarbonate sheets, using ethanol as a crazing liquid, was studied. A stress analysis was made to analyse the criteria for craze initiation and growth. The craze length was found to change linearly with the square root of time and the rate of crazing was found to vary exponentially with the applied stress (above a certain stress level). It was concluded that the principal strain is the controlling parameter for environmental craze initiation and growth.     

Plastic deformation by crazing in polycarbonate

On the basis of the data of craze behaviour under static tension, the deformation curves followed by continuous initiation and growth of crazes are plotted for a variety of testing conditions, such as constant stress, constant strain-rate and constant strain tensions applying the Johnston-Gilman theory for dislocations. Experimentally determined values of the density and growth rate of crazes, which are regulated in accordance with a simple rate theory, are used for the calculations. Comparison of the theory with the experimental results is favourable except for the results of high strain-rate tension and stress relaxation. The application of the dislocation analogue approach to the craze deformation kinetics was found to be valuable.     

Micromechanical and thermodynamical aspects of environmental crazing

The present study aims 1) to investigate theoretically the relation between the craze microstructure and the basic materials parameters such as the yield stress and the surface energy and 2) to provide a detailed thermodynamic treatment of a single isolated craze in glassy polymer tested in an aggressive liquid environment. Based on the assumption that the craze tip is somewhat blunted by small scale yielding and on the Taylor meniscus instability as the mechanism responsible for the propagation of the leading edge of the craze, the detailed micromechanical analysis is used to provide estimates of the critical opening displacement of the craze for growth initiation, mean fibril spacing, mean fibril diameter and fibril volume fraction at the craze tip. The influence of aggressive liquid environments on the yield stress and the surface energy is discussed together with predicted changes in the craze microstructure. The thermodynamic analysis starts with the recognition that induced high negative pressures around the craze tip can increase the solubility of a liquid at this site by several orders of magnitude. As a consequence the local density of thermodynamic potential drops significantly. This unbalanced fall in thermodynamic potential provides an additional driving force for the craze advance. It is shown that a corresponding release of external load is required to preserve the overall balance of the specimen with craze.     

Hypothetical mechanism of crazing in glassy plastics

Crazing in glassy plastics is attributed to a stress-activated devitrification of a small amount of material at the tip of a chance nick or flaw, to a softer rubbery state. Subsequent cavitation of the softened material is then assumed to take place under the action of the same dilatant stress responsible for its formation. A transition to ductile yielding is proposed to occur when the material in the tip region undergoes large deformations before softening.The proposed mechanism of crazing is shown to provide quantitative predictions for the magnitude of tensile stress at which crazing occurs, the increase in crazing stress with hydrostatic pressure, the transition at high pressures to a yielding process without crazing, the reduction in crazing stress in the presence of certain liquids and vapours and, to some extent, for the effects of temperature and pre-orientation. These theoretical predictions are found to be in reasonably satisfactory agreement with experiment. In view of the limited number of adjustable parameters in the theory (the principal one being the stress-magnification factor associated with a typical nick or flaw), this general agreement over a wide range of experimental conditions and variables suggests that the proposed mechanism of stress-crazing is basically correct.On leave of absence from the University of Akron, Ohio, for the year 1969–70.     

Methanol crazing of coarse shear bands in polystyrene

A specimen with coarse shear bands produced at stress concentrations by compression was immersed in methanol to observe craze formation. Thin crazes were initiated at shear bands and joined together while propagating to form thick crazes. Crazes were formed only on the tension side of shear bands with the craze planes perpendicular to the shear bands. When a craze propagated through a shear band, each displaced the other at intersections. Some secondary shear bands were transformed partly into crazes resulting in about a factor of ten increase in thickness. This transformation was achieved by a tensile deformation of fibrous sheets in the shear band with simultaneous production of fine fibres.     

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.     

Deformation kinetics by crazing in glassy polymers

In order to confirm the proposed theory of the plastic deformation caused by crazing, the theoretical deformation curves derived from the dislocation analogue were calculated using the data of craze behaviour measured under static tension and compared with the experimental results of creep and constant strain-rate tension in PMMA and PVC plates wetted by kerosene as a crazing agent. Favourable agreement between the theoretical and experimental results may provide evidence that the proposed method is valuable for estimating the amount of the plastic strain caused by crazing.     

Statistical aspects in stress crazing of polymethyl methacrylate

The statistical nature of the stress crazing phenomena in polymethyl methacrylate (PMMA) has been investigated. The experiments suggest that the time required for the formation of crazes is an inherent, but statisticaL characteristic of the material itself incorporating a stress depending rate process reflecting to some extent the initial microscopic flaw distribution.

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Zusammenfassung Es wurde der statistische Charakter der Polyfissuration in Polymethyl-Methacrylat untersucht. Die durchgeführten Versuche deuten darauf hin, daß die fur die Bildung von Poly-Rissen erforderliche Zeit eine dem Material eigene, wenn auch statistische, Eigenschaft ist. Diese Eigenschaft begreift einen spannungsabhängigen Geschwindigkeitsprozeß welcher in einem gewissen Masse die Verteilung der ursprünglichen mikroskopischen Fehlstellen reflektiert.

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Résumé On a étudié le caractère statistique des phénomènes de polyfissuration dans le metacrylate de polyméthyle.Les essais exécutés laissant entendre que le temps nécessaire à la formation de criques multiples est une caracteristique inhérente, encore que statistique, du matériau lui-même. Cette caractéristique comprend un processus évolutif fonction de la contrainte appliquée, qui traduit, dans une certaine mesure, la distribution des défauts microscopiques présents à l'origine.

     

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.     

A nonlocal model of fracture by crazing in polymers

We derive and numerically verify scaling laws for the macroscopic fracture energy of polymers undergoing crazing from a micromechanical model of damage. The model posits a local energy density that generalizes the classical network theory of polymers so as to account for chain failure and a nonlocal regularization based on strain-gradient elasticity. We specifically consider periodic deformations of a slab subject to prescribed opening displacements on its surfaces. Based on the growth properties of the energy densities, scaling relations for the local and nonlocal energies and for the specific fracture energy are derived. We present finite-element calculations that bear out the heuristic scaling relations.     

A dangerous entanglement

Entrapment of guidewires by inferior vena cava filters can occur during the blind insertion of a jugular or a subclavian central venous catheter. Recently, few case reports have been published in the radiology literature. In addition, others have described endovascular techniques aimed at removing entrapped guidewires, avoiding the possibility of vena cava rupture. Given that a temporary hemodialysis venous catheter is frequently used as a first access, the possibility of entrapping the dialysis catheter guidewire should not be neglected.