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.

Acknowledgements

This work was funded by EPSRC under ROPA grant GR/R42344. We are grateful to Solomon Aluko [30], an LSBU project student, for demonstrating the possibility of crack re-direction in PMMA with the help of small thermal sources, and to G. Barenblatt, L. Botvina, E. Terentjev, J. Willis and D. Zakharov for illuminating discussions. Particular thanks are due to G. Mishuris, who helped us to clarify many of the arguments and conducted preliminary mathematical modelling.