Shear yielding glassy polymers

Strain rate, test temperature and the thermal history of the specimen all affect the appearance of shear bands in a particular glassy polymer [119]. The differences in morphology of shear bands was proposed to be due to different rates of strain softening and the rate sensitivity of the yield stress. Microshear bands tend to develop in polymers with a small deformation rate sensitivity of Oy and when relatively large inhomogeneities exist in the specimen before loading. This is sometimes characterized by a factor e j, introduced by Bowden in the form [119] ... 

At low temperatures the theoretical maximum shear yield stress in most glassy polymers is... 

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. 

Glassy polymers with highercohesiveness, like polycarbonate and cross-linked epoxies, preferentially exhibit shear yielding [7], and some materials, such as rubber-modified polypropylene, can either craze or shear yield, depending on the deformation conditions [8]. Application of a stress imparts energy to a body which... 

In general, the activation of shear yielding in a glassy polymer reduces its plastic resistance to further deformation. When strain softening occurs the deformation becomes unstable to small perturbations of the stress field. This instability results in the formation and growth of defonnation zones, the shape of which are controlled by the strain softening and strain hardening characteristics of the material... 

The principal mode of deformation in fatigue cycling of glassy polymers is crazing. In some polymers, like PC and PSF, and in some rubber-modified polymers, like ABS, crazing may be accompanied by localized shear yielding. 

When a solid undergoes shear yielding, the local packing of its constituent units—atoms, molecules, or ions—changes to a new configuration that is stable in the absence of stresses. In glassy and semicrystalline polymers the plastic deformation takes place by means of local shear strains, without any appreciable changes in volume or density. 

For macroscopically isotropic polymers, the Tresca and von Mises yield criteria take very simple analytical forms when expressed in terms of the principal stresses cji, form surfaces in the principal stress space. The shear yield surface for the pressure-dependent von Mises criterion [Eqs (14.10) and (14.12)] is a tapering cylinder centered on the applied pressure increases. The shear yield surface of the pressure-dependent Tresca criterion [Eqs (14.8) and (14.12)] is a hexagonal pyramid. To determine which of the two criteria is the most appropriate for a particular polymer it is necessary to determine the yield behavior of the polymer under different states of stress. This is done by working in plane stress (ct3 = 0) and obtaining yield stresses for simple uniaxial tension and compression, pure shear (di = —CT2), and biaxial tension (cti, 0-2 > 0). Figure 14.9 shows the experimental results for glassy polystyrene (13), where the... 

---