Shear deformation stress-induced formation

Case c stress-induced formation of shear deformation. The stress concentration of the modifier particles, usually too small, is increased by the formation of cavities inside the particles. By these cavities, and if A is small enough, the originally triaxial stress state between particles is transformed into a more uniaxial stress state. Then the matrix strands between particles can be plastically deformed where the necessary volume increase, which arises from the cavitated particles, appears. 

Fracture initiated in the tensile tested ABS samples, as noted also by Truss and Chadwick from either surface flaws or from internal flaws. Figure 33a shows an SEM picture of the tensile fracture surface of a sample broken at a comparatively high deformation rate of 12.7 cm/min. The fracture surface is unlike that of SAN (Fig. 27 a) or that of rubber modified polystyrene (Fig. 3 a). Fracture, for this specimen, has developed from both a surface source and from an internal source and fine radial flow lines emanate from both sources. The slow growth region adjacent to the source tends to develop a conical shape as has been noted This is probably a result of localized shear formation. In ABS specimens subject to creep deformation at low values of stress, the creep strain is found to be due almost entirely to shear but, at higher stresses, shear is accompanied by crazing Crazes can also be induced... 

The discussion up to this point has focused on the role of free surfaces and internal interfaces, such as grain boundaries, in mass diffusion. Surfaces produced internally in the material as a consequence of permanent deformation and damage induced by stress can also serve, in some cases, as paths along which enhanced atomic diffusion may occur. In amorphous solids undergoing active plastic flow, such increased atomic mobility along shear bands can result in the formation of nanocrystalline particles locally at the bands. An example of such crystallization process is illustrated in this section for the case of a bulk amorphous metallic alloy subjected to quasi-static nanoindentation at room temperature. 

As early as the 1930s Scheil (1932) predicted the formation of martensite above M, by the application of a stress. According to him, the shear stress required to activate the transformation decreases with decreasing temperature (being zero at M,) whereas the shear stress required for austenite slip increases with decreasing temperature. Thus, at temperatures near applied stresses should induce plastic deformation by the martensitic mode rather than by slip. 

---