Glassy polymers strain softening

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

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] ... 

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... 

As with amorphous metals and semiconductors, the unit plastic relaxations in glassy polymers are also thermally assisted shear transformations (STs), which control the temperature dependence of the plastic resistance and encompass other phenomena of strain softening and the pressure dependence of the resistance. Moreover, the incremental processes of molecular-segment alignment, resulting... 

Tg especially wl en deformed under the influence of an overall hydrostatic compressive stress. This behaviour is illustrated in Fig. 5.37 where true stress-strain curves are given for an epoxy resin tested in uniaxial tension and compression at room temperature. The Tg of the resin is 100°C and such cross-linked polymers are found to be brittle when tested in tension at room temperature. In contrast they can show considerable ductility in compression and undergo shear yielding. Another important aspect of the deformation is that glassy polymers tend to show strain softening . The true stress drops after yield, not because of necking which cannot occur in compression, but because there is an inherent softening of the material. 

On the other hand, the removal of strain softening through mechanical rejuvenation also renders PS completely ductile and it can be deformed, even in uniaxial extension, up to strains of 30% (see Figure 20). These experiments clearly indicate the dominant role of strain softening in localization and failure of glassy polymers. 

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