Brittleness of glassy homo-polymers and alleviating it through craze plasticity

2 Brittleness of glassy homo-polymers and alleviating It through craze plasticity 

Homo-PS is known to be brittle in tension at room temperature in unmodified form, as Fig. 13.3 demonstrates. It has a compressive yield strength of around 103 MPa that, with a substantial strength-differential effect, translates into a tensile yield strength of 73 MPa, and undergoes plastic flow if its brittleness can be suppressed. Experimental evidence, such as that in Fig. 13.3, shows that homo-PS undergoes brittle behavior initiated from surface flaws, and that elimination of these is impractical, primarily because, even if that could be achieved, crazes could still be initiated at free surfaces, as is discussed in Chapter 11, and craze matter breaks down from either extrinsic or intrinsic imperfections in craze matter at stress levels of around 40 MPa at 293 K. 

To achieve craze plasticity with substantial potential to alleviate brittleness, well under the stress levels given above, a definite strategy needs to be followed, which is not possible through surface-initiated growth of crazes. 

Consider a cylindrical bar of homo-PS of diameter D that is to undergo plastic flow through crazing, subjected to an applied strain rate that is initially producing a purely elastic response in the absence of crazing, i.e.. 

Considering only growth of sporadically initiated surface crazes, as discussed in Chapter 11, at a prevailing stress a, the craze-mediated plastic-strain rate would be 

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