Perturbing Paths of Slow Cracks in PMMA by Local Heating

Real-time control of the direction of a propagating crack is little explored area of management of structural integrity. We have examined it with experiments on quasi-static (slow) cracks in PMMA (Polymethyl-Methacrylate). We have observed that in PMMA when the mean crack speeds are low (between 0.1 and 0.4 mm/s) secondary thermal sources of relatively low power (of the order of 2 W) can be used to achieve a significant crack redirection. In this interval the micro-photographs of cracks present a fishbone structure, and we argue that the side lobes arc: micro-cracks which appear inside shear bands. The respective fracture surfaces look rippled (hackled). We have observed a clear reverse correlation between the crack speed and the angle of redirection. When the mean crack speed exceeds 0.4 mm/s) and the fracture surface is mirror smooth no trajectory redirection is achieved. It appears the micro-cracks act as notches and alleviate crack redirection. Therefore, one of our conclusions is that it might be possible to use relatively weak sources to redirect fast cracks that propagate with a super-critical speed. This hypothesis is advanced because fast cracks are also known to possess a fishbone structure and hackled fracture surface.