The Slow Mode of Crack Growth

Griffith s anafysis predicts a stabfe crack for stresses befow the criticaf limit of failure and an uncontrolled accelerated growth above the limit, due to 

The growth rate dependence of the work suspended in plastic flows is conceivable if we recall, for example, the strain rate dependence of the force applied for the shear yielding during cold-drawing. In the range of subcritical growth one finds a power law for the relation between Kj and c. 

The cusp-like prohle of the craze zone in front of the crack may be deduced from another experiment on poly (methylmethacrylate), presented in the lower part of Fig. 10.45. The displayed fringe pattern is observed in a microscope, when the region around the crack tip is illuminated by a light beam from above. As a result of the interference of two light beams, one reflected at the upper and the other at the lower surface of the craze with crack, an interference pattern emerges. Analysis enables the prohle of the craze in front of the crack to be determined. The prohles thus measured are cusp-like, as in the schematic drawing. 

With the knowledge of the structure of the plastic zone in front of the crack tip one can write down a simple equation for the surface parameter 2w, known also as material resistance. The total expended work equals that consumed on drawing the fibrils in the craze up to their maximum length at the point of break. As was discussed in the previous section, Hbrils elongate 

Fractions of PS, subject to uniaxial stress. Molar mass dependencies of the minimum stress for the observation of crazes and the stress at failme. Data from FeUers and Kee [143] 

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