Re-directing slow cracks in PMMA

Abstract

Crack re-direction is a little explored area in the management of structural integrity. We have examined this subject by performing experiments with PMMA (polymethylmethacrylate), working mainly with quasi-static (slow) cracks, whose mean crack speed varies between 0.1 and 0.4 mm s−1. We observed that in this regime, secondary thermal and acoustic sources of relatively low power (of the order of 2 and 10 W, respectively) could be used to achieve significant crack re-direction. We argue that this effect is due to an interplay between macro- and micro-scale phenomena. We show that micro-photographs of quasi-static cracks present a fishbone structure and that the side lobes may be micro-cracks inside the shear bands. The respective fracture surfaces appear rippled (hackled). We also show that, in our experiments, the angle of re-direction decreases with the mean crack speed, and above a mean crack speed of 0.4 mm s−1—when the fracture surface becomes mirror smooth—no crack re-direction is observed. It is conceivable that the micro-cracks act as notches and alleviate crack re-direction. Therefore, one of our conclusions is that it might be possible to reproduce the effect in an entirely different speed regime, that is use relatively weak secondary sources to re-direct fast cracks that propagate at super-critical speed. This hypothesis is advanced because fast cracks are known to possess a fishbone structure and a hackled fracture surface.