Plane stress 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. 

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... 

Alternatively, where plastie flow is strongly localized into intense dilatant shear bands, as in the cases of cohesionless granular media (Coulomb 1773 Mohr 1900), the S-D effect has been viewed as the effect of a normal stress On acting across the shear plane and thereby increasing or decreasing the shear resistance of such bands through a friction effect on the shearing material. This alternative explanation results in a Coulomb Mohr-type unsymmetrical yield condition similar to the behavior of cohesionless sand or soils, where, however, the linear dependence of the shear resistance is on is continuous around zero in the form of... 

When fatigue failure was studied in more detail all authors agree that the critical crack initiates at the surface and rapidly propagates on a plane perpendicular to the stress direction until final fracture occurs. Frommeyer and Seifert (1981) give further details on this critical point in fatigue life. They observed fine shear band offsets at the... 

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