High-speed photography of low-velocity impact cracking of solid spheres

Results from a high-speed photographic study of low-velocity impact cracking of optically transparent acrylic resin and poly(methyl methacrylate) spheres of 12.7 mm diameter are described. It is shown that, in a sphere undergoing an impact, cracking occurs after the sphere has been plastically deformed. The point of initiation of cracking is shown to be located at the circle of contact between the sphere and the impacting platen. The cracking then develops throughout the sphere, with cracks attaining velocities of 600-800 m s(-1) It has been suggested that crack initiation at the contact circle is caused by the tensile circumferential stress, which is generated owing to the plastic flow of the sphere.