Crazing and stress whitening

Low temperature. The rubber is unable to relax at any stage of fracture. There is no craze formation and brittle fracture occurs. 

High temperature. The rubber is able to relax even in the rapidly forming stress field ahead of the travelling crack. Stress whitening occurs over the whole of the fracture surface. Bucknall and Smith [128] report similar results for other rubber-modified impact polymers. 

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Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

Bucknall and Smith [128] remarked on the connection between crazing and stress whitening. It was observed that the fracture of high-impact polystyrene, which incorporates rubber particles into the polystyrene, is usually preceded by opaque whitening of the stress area. Figure 12.11 shows a stress-whitened... 

Glassy polymer-diluent mixtures deformed in a temperature range close to are susceptible to exhibit a cavitational mode of plasticity at hi stresses and strains. Activation of this mechanism in mixtures of polycarbonate with esters of the phthalic acid results in extensive fibrillation and stress whitening of the material. There is strong evidence that the diluent plays an important role in enhancing chain slippage, which is required for the formation of craze fibrils. One of the most fundamental problems which is still unsolved is the elucidation of the molecular mechanism by which diluents become active. 

Figure 3.25. Crazes in stress-whitened high-impact polystyrene. (Bucknall and Smith, 1965.)...

Overall behavior. It is the time-dependent strain at which crazing, stress whitening, and rupture decreases with a... 

Good stress-crack and craze resistance, brittle Good processibility, no stress whitening Higher-heat styrenic, brittle... 

Failure Mechanisms. BPF polycarbonate develops crazes at ascending stresses and fractures in a pseudo-brittle manner similar to polystyrene or PMMA. At room temperature the block polymers develop few separate crazes. As the yield is approached, shear bands grow from the edges. Fracture initiates at an edge from a point where the two shear bands initiated. When a neck forms, the plastic strain in the neck is ca. 80% however fracture occurs shortly after the neck is formed so that the ultimate elongation of the specimen is only 10 or 12%. The shear bands and necks show some stress whitening (Figure 9). 

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