Craze mechanism brittle separation

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). 

The force requirement for separation, in a mode in which rigid facing surfaces remain exactly parallel, would be enormous (25). Brittle separation at an interface, in the absence of a dissipation mechanism resembling the mechanism that controls bulk fracture, would occur at a much lower level of force. So it will be necessary to postulate a dissipation mechanism, such as an interfacial craze, for those cases in which a polymer that has a high Tg does not separate easily from a hard solid. 

Next, we may make an estimate with respect to brittle separation by a craze mechanism. We can assume a fibril radius of the order of... 

Amorphous polymers exhibit two mechanisms of localized plasticity crazing and shear yielding. These are generally thought of separately, with crazing corresponding to a brittle response while shear yielding is associated with ductile behavior and the development of noticeable plastic deformation prior... 

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