The impact of a soft metal sphere on a hard metal target II. Deformation of the target

Abstract

Abstract The variation is reported of the depth, diameter and profile of indentations in a quenched-and-tempered steel target due to the normal impact of aluminium, phosphor bronze and brass spheres at velocities up to 280 ms−1.A critical velocity for the onset of plastic flow in the target was determined. A quasi-static analysis modified by the effect of strain rate was unable to predict a contact pressure high enough to cause yield in the target. It is suggested that a state of uniaxial strain compression exists at the centre of the contact zone during the initial stages of impact. The pressure may then be determined as a function of velocity using appropriate one-dimensional strain-rate-independent wave theory. Relevant uniaxial strain compression data were estimated from the elastic constants available in the literature and from uniaxial stress-strain curves determined from quasi-static hardness measurements. When the Vickers hardness of the undeformed sphere is less than about 0·4 times the Vickers hardness H t of the target, plastic flow in the target arises solely from the initial contact pressure; the corresponding critical pressure at the centre of the contact zone causing yield in the target may then be calculated from the critical impact velocity. The critical impact pressure is found to be (0·33 ± 0·03)H t when the target is lubricated with a thin layer of molybdenum disulphide grease and (0·27 ± 0·03)H t when the target surface is unlubricated. The residual impressions in the target are much larger than the contact area at the first arrival of release waves in the sphere at the sphere-target interface.