Brittle polymers under compressive

Ductile Failure of Brittle Polymers under Compressive Shear Stresses... 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

A polymer is more likely to fail by brittle fracture under uniaxial tension than under uniaxial compression. Lesser and Kody [164] showed that the yielding of epoxy-amine networks subjected to multiaxial stress states can be described with the modified van Mises criterion. It was found to be possible to measure a compressive yield stress (Gcy) for all of their networks, while the networks with the smallest Mc values failed by brittle fracture and did not provide measured values for the tensile yield stress (Gty) [23,164-166]. Crawford and Lesser [165] showed that Gcy and Gty at a given temperature and strain rate were related by Equation 11.43. 

Figure 13.18 The stress-strain behavior of a normally brittle polymer, polystyrene, under tension and compression. (From Nielsen, L.E., Mechanical Properties of Polymers and Composites, Mol. 2, Marcel Dekker, New York, 1974. With permission.)...

Figure 12.2 Stress-strain behavior of a normally brittle polymer such as polystyrene under tension and compression.

We have so far in this section on the yield behaviour of polymers only considered tensile deformation. In order to obtain a complete idea of the yield process it is necessary to know under what conditions yield occurs for any general combination of stresses. For example, glassy polymers are usually brittle in tension when the temperature of testing is sufficiently below Tf, whereas when they are deformed in compression at similar temperatures they can undergo considerable plastic deformation. Also a knowledge of yield behaviour under general stress systems is important in engineering structures where components are subjected to a variety of... 

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. 

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