Yielding mechanisms crazing

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

Evidence for differences in activation volume and enthalpy between shear yielding and crazing has already been presented. In discussing kinetics, it is convenient to treat the two mechanisms as independent, and to calculate activation parameters for each process accordingly. It must be noted, however, that interactions do occur between crazes and shear bands under certain conditions, so that the kinetics cannot be regarded as completely independent. 

Fig. 2.3. Schematic representation of failure mechanisms in rubber toughened adhesives, (a) Shear yielding, b) Crazing (not necessarily applicable to epoxies).

The presence of liquids or vapors in the enviromnent of a polymeric component affects the response to external mechanical forces. Thus, for instance polyarylate (Par) under uniaxial extension exhibits exclusively shear yielding without crazing. However, exposure to organic vapor (methy-lethyl ketone) results in crystallization, embrittlement, and conversion of the response to deformation from shear yielding to crazing [42]. 

Both principal fracture mechanisms, shear yielding and crazing, are influenced by the particle size. In PPBC matrix, where spherical elastomeric particles are chemically bonded, the energy absorption takes place mainly by deformation of the matrix. In such systems, a large amount of shear yielding is to be expected. The shear yielding becomes more prominent upon increasing the concentration of EPDM as well as reduction of their particle size. The micro-shear bands in the fracture surface (Pig. 10.23e) clearly support these expectations. 

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. 

---