Jump in crack speed

The transition from slow to rapid crack growth, however, is relatively sharp for many polymers. This is exemplified by PMMA. At the critical value of stress intensity factor, Kj, a jump in crack speed is observed at the end of the slow growth region around 0.1 ms to much higher speeds which may be in the range of 10 to 100 ms , depending on testing conditions (e.g. With this jump is also associated a... 

On this basis the jump in crack speed may be explained as follows The decrease in molecular mobility due to freezing of the P-relaxation also decelerates the fibrillation... 

The crack frequently initiates from the breakdown of a craze that formed at an internal defect, as a void or impurity particle. Then, as shown by various investigators, as crack speed increases, the crack jumps rapidly from one craze bulk interface to another and from one craze to another. This can lead to a so-called mackerel type pattern on the fracture surface or to a craze island type structure see also Chapter 1 and 3. As crack length increases and local stress rises, numerous secondary fractures, as shown in Fig. 2 b, are generated ahead of the crack front. 

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