The erosion of metals by the normal impingement of hard solid spheres

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Abstract
Results of experiments are presented which show that single hardened steel spheres (8 mm in diameter) travelling at velocities in the range 50-400 m s-1 can remove material when they strike mild steel targets at normal impingement. Impacts were photographed using a high-speed camera at microsecond framing rates. The results are interesting since some erosion theories predict zero material removal at normal angles of impingement, whilst others require cooperative effects between impacts at normal impingement which result in incubation times. It is shown that there is an initial regime where the contact circle expands faster than any wave speed (elastic or plastic) in the target. Material loss eventually occurs by the rapid radial expansion (jetting) of material from under the sphere when the velocity of the plastic wave exceeds that of the expanding contact circle.