Plane strain shear bands

Fig. 8. Schematic view of the epsilon CTPZ region. The plane stress shear zones occur near the surface on either side of the crack opening (AA). The plane strain shear bands are internal, forming at the crack tip...

The above rationalization explains why epsilon CTPZ are usually observed in 3-D surface cracks with a clamshell shai . Once the first set of plane strain shear bands are formed, it provides the stress relief that maintains the lar stress difference and high shear tendency in the next plastic zone. This process continues with each successive zone formation. Thus, the epsilon nature of the CTPZ can actually continue deep into the surface of the sample, well beyond the short crack stage. Alternatively, one may consider this in terms of a shielding effect of the prior... 

In PEI the DCG process, as in any polymer, is active. The epsilon CTPZ, however, was not observed. No plane strain shear bands have yet been observed. Some form of localized crack tip shear process can be activated, however, as evidenced by the inversion transition that occured at higher stresses (at the higher temperatures). The fracture surface did not show fracture to occur on a slanted 45 degree plane. The fracture plane was still normal to the leading direction. The fracture surface, however, was not smooth, as seen with craze fracture, but has a definite roughened texture which is associated with active localized shearing. This texture is often described as honeycomb or tufted. 

In PEI (Fig. 37), the shear fracture process never fully develops (under the tested conditions). A mixed mode develops, with some elements of shear fracture, but without the epsilon plane strain shear bands, a true 45° shear fracture process cannot develop. The lifetime enhancement is not as substantial, as seen in PC and PEC. 

Fig. 30. Plane strain, shear fatigue fracture surface in PC. The fracture surface is oriented approximately 4S to the load direction. Fracture occurs through coalescence of voided areas within a shear band. Repnnled courtesy of MatsumotC ...

Huang et al. (1993b) proposed a two-dimensional plane strain model, which was successfully used to identify the stress field around the rubbery particles and to simulate the initiation and growth of shear bands between rubbery particles. A model was proposed to quantify the different mechanisms. GIc of the rubber-modified network was written as... 

Figure 19.1 Thin sections cut from specimens deformed just beyond the yield point in plane-strain compression viewed through crossed polarizers in the optical microscope, (a) Microshear bands formed in polystyrene, (b) Diffuse shear bands formed in poIy(methyl methacrylate). (Reproduced with permission from ref. 2.)...

Fig. 5.38 Shear bands in polystyrene deformed in plane strain compression (courtesy Dr. R.J. Oxborough).

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