Crazing and Shear Yielding

We present a finite element study which includes both shear yielding and crazing within a finite strain description. This provides a way of putting together all aspects of glassy polymer fracture crazing and shear yielding but also thermal effects. 

As a first incursion into the thermomechanical analysis of the problem, we present recent results [57] in which only thermoplastic effects are accounted for. The related temperature variations appear larger than those from thermoelastic effects, and are expected to be of major importance in the competition between shear yielding and crazing. The influence of thermoelastic effects will be briefly discussed at the end of this section. 

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. 

In the discussion of the shear yielding and crazing behavior of PC (in Sect. 3.2 and 4.1), the existence of characteristic extension ratios has become apparent (1) The extension ratio, 7. , after shear yielding referred to as natural draw ratio, (2) the extension ratio, V, of craze I fibrils and (3) the extension ratio, at craze II initiation. 

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. 

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. 

Localized shear yielding and crazing are competing mechanisms of brittle fracture under tensile loading. Two principal interactions can occur between craze and shear band in an early mechanism proposed by Bucknall et al. (171), Newman and Wolock (169), and Jacoby and Cramer (170). Later, however, Donald et al. (172,173) showed that interaction A cannot effectively lead to a stabilization of craze and only interaction B will stabilize craze growth and contribute to toughening of the polymeric material. 

Fig. 14. Deformation structures of PP impact-modified with EPDM at different test temperatures (a) RT — cavitated particles as well as adjacent matrix strands are strongly plastically deformed by shear yielding (b) —40°C—coexistence of shear yielding and crazing (c) —40°C — fibrillated crazes between elongated particles (thin sections, deformed and investigated by hvem (a) and tern (b and c)). From Reference 1.

Another approach to the fracture of ductile polymers stems from the recognition that for such materials the crack tip deformation zone has two components, as shown in Figure 12.21. There is an inner zone where the fracture process occurs -which could involve a combination of shear yielding and crazing - and an outer zone where extensive yielding and plastic deformation occur. This approach was originally proposed by Broberg [70], and has been developed by Mai and Cottrell [71], Hashemi and Williams [72], Mai [73] and others. 

In an excellent review Bucknall [124] explains that rubber toughening involves three principal deformation mechanisms shear yielding, crazing and rubber particle cavitation. The rubber particles, with a much lower stiffness than the matrix polymer, give rise to stress concentrations for the initiation of shear yielding and crazing. 

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