Ductile-to-quasi-brittle transition

Ductile-to-Quasi-Britde Transition. The transition from Type I to Type II and Type III stress-strain behavior was accompanied by a sharp drop in f. The large decrease in fracture strain with a relatively small change in composition has been called a ductile-to-quasi-brittle transition rather than a ductile-to-brittle transition, because even when the fracture strain is low,... 

The ductile-to-quasi-brittle transition occurs because the true fracture stress, as determined by P, decreases with increasing PS more rapidly than the yield stress, determined by P (Figure 9). The critical PS content (Vf>s) at the ductile-to-quasi-brittle transition can be determined from the condition that the true yield stress is equal to the true fracture stress. From equations 8 and 13,... 

Figure 9. Schematic of the effect of PS content on yield stress (atJ) and true fracture stress (ef) to give the ductile-to-quasi-brittle transition.

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

The present study is restricted to a quasi-static formulation and focusses on the beginning of crack propagation. Therefore, our attention has been devoted to exploring possible causes for the onset of the brittle to ductile transition for such loading conditions. Therefore, arguments based on inertia effects are not invoked, nor is crack branching at high crack speeds [15]. 

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