Microshear bands

Strain rate, test temperature and the thermal history of the specimen all affect the appearance of shear bands in a particular glassy polymer [119]. The differences in morphology of shear bands was proposed to be due to different rates of strain softening and the rate sensitivity of the yield stress. Microshear bands tend to develop in polymers with a small deformation rate sensitivity of Oy and when relatively large inhomogeneities exist in the specimen before loading. This is sometimes characterized by a factor e j, introduced by Bowden in the form [119] ... 

Interestingly, the contribution of diffuse shear bands to the total deformation of the specimen is large, despite relatively low deformation existing in them. On the other hand, large plastic deformation in microshear bands does not contribute so substantially to the total deformation of the specimen. It appears that a small deformation over a large volume has a much larger effect on crack resistance than a large deformation within a small volume. 

Regarding the effects of shear band structure on the fracture mode in glassy polymers, Wu and Li [170] concluded that when microshear bands propagate in a specimen cross section, a shear failure is produced with a very small overall de-... 

Figure 19.1 Thin sections cut from specimens deformed just beyond the yield point in plane-strain compression viewed through crossed polarizers in the optical microscope, (a) Microshear bands formed in polystyrene, (b) Diffuse shear bands formed in poIy(methyl methacrylate). (Reproduced with permission from ref. 2.)...

In rubber-toughened ABS, shear yielding is dominant. Optical microscopy examination by Newman and Strella [126] showed that plastic deformation had occurred in the matrix around the rubber particles. Later studies, notably by Kramer and co-workers, suggested that the rubber particles initiate microshear bands. Donald and Kramer [127] showed that cavitation in the rubber particles initiates shear yielding of the matrix and that shear deformation occurs when the particles are small, and crazing when the particles are large. 

Shear bands can be either localized or diffuse. In the case of localized shear bands, they develop at about 45° to the tensile direction, which corresponds to the maximum shear stress direction (see Fig. 1.48 in Part II). Thicker shear bands are usually constituted of a set of microshear bands. The interface between the bulk and the shear bands is diffuse and up to some 100 nm wide. The molecular micromechanism is stretching and gliding. The patterns of crossed shear bands as well as the broad shear bands are visible macroscopically in the tensile bars as short necking zones or longer cold-drawing zones. 

Figure 14 Shear yielding in glassy polymers. The micrographs are from polished sections of polymers deformed just past the yield point and viewed in polarized light (a) polystyrene showing microshear bands (b) PMMA showing diffuse shear...

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