Craze yielding, mechanism

From measures of band width and knowledge of the prevailing stress intensity factor, K, one can compute the craze yield stress, S, from the fracture mechanics relation... 

Crazing requires the presence of dUatational component in the stress tensor and may be inhibited by hydrostatic pressure. On the other hand, it is enhanced by the presence of triaxial tensile stress (Kinloch and Young 1983). Unfortunately, such a stress state exists ahead of large flaws or notches in relative thick specimens (plane-strain conditions). Therefore, the presence of sharp cracks, notches, or defects in thick specimens wUl favor craze initiation leading to brittle fracture, which is opposite to a bulk shear yielding mechanism that leads usually to ductile behavior. 

SPIF parts made from PET, PA and PC experience no variation in density, allowing us to conclude that cold plastic deformation is incompressible (Figure 8.11). In relation to PVC, the variation in density and colour (from dark to light grey) is not due to plastic deformation but to the accumulation of crazes because, as recently mentioned by Bucknall (2007), crazing should be treated as a frustrated fracture process rather than a yield mechanism. 

On the one hand, it is possible to increase the toughness of polymers by incorporating rubber or polyurethane (HD-TPU) particles. Compliant particles, if well dispersed, may influence the deformation behavior towards a higher energy absorption by activating localized cavitational craze-like and shear yielding mechanisms. Their lower stiffness, however, has a disadvantageous effect on the composite s modulus. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

There are three major mechanisms of energy absorption shear yielding, crazing, and cracking. The latter two are often dealt with together and called normal stress yielding. 

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