Rain erosion behaviour of polymethylmethacrylate

Abstract

Mass loss, optical transmittance and the microscopic erosion sequence have been monitored for polymethylmethacrylate exposed to a 2.54 cm h⁻¹ rainfall of 1.8 mm diameter water drops at an impact velocity of 222 m sec⁻¹. Initial drop impacts produced well-defined fracture patterns consisting of a circular area free of damage surrounded by an annulus containing a dense array of fine, short cracks together with a sparse distribution of deeper fractures initiated along surface scratches. Continued exposure to the rainfield produced crack grown and crack intersections at sites of fracture annuli overlap followed by crevice growth as these fracture systems were enlarged by a hydraulic penetration mechanism. An extensive network of subsurface fractures continued to be produced within the expanding cavities with longer exposures. The transient stress distributions generated during a water drop impact on polymethylmethacrylate are considered in terms of their potential for producing circumferential fractures.