Craze front initiation length

With the above considerations on initiation and inactivation of craze fronts we now develop relations for q, the steady state active craze front length per unit volume. Here it is necessary to distinguish two separate cases sinface initiation limited crazing and volume initiation limited crazing. The first category applies to the cases of most homopolymers and to many block copolymers with phase dimensions too small to result in stress concentrations in the requisite volume element that was mentioned above. In these instances crazes are observed to initiate from sources at... 

Volume initiation limited crazing. Consider a large volume V of a polymer with internal heterogeneities of approximately spherical shape with a radius R and an initial density of Hq/V (m ). The rate of increase of craze front length under a distant stress of in this case is... 

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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