Optical studies on smooth plastic and metal surfaces subjected to high speed impact

Abstract

The deformation of plastics and metals is discussed with, reference to the effect of the rate of strain. A review of the impact of solids is made, including elastic and plastic wave propagation and the effect of the reflection and superposition of the waves on the progress of the impact. An account of the concepts underlying the Hertz theory of impact is given, together with a consideration of its applicability to the present work. The various theories proposed for liquid-solid impact are reviewed and discussed with reference to recent experimental evidence. An air gun and velocity measuring apparatus were constructed for producing impacts and the theory and practice of the optical techniques used for the examination and measurement of the resultant surface distortions is reviewed. The damage sites produced by the impact of solid spheres, deformable pellets and water drops on Perspex and metals arc studied, at striking velocities up to 1250 ft./sec (850 m.p.h.). This enables the complex process of the impact of a deformable solid (or liquid) to be compared with that of a hard solid. The main quantitative results obtained for all the impact phenomena investigated relate to the variation of the diameter and volume of the damaged region with the velocity of impact. In the case of steel ball impact on Perspex the diameter/velocity relation is similar to that calculated, for elastic solid impact using Hertz's theory. For steel ball on Duralumin the volume of the indentation is proportional to the kinetic energy of the impactor. The nature of the damage produced by polythene pellets and water drops is similar, but the rate of size increase with velocity is much greater than in solid-solid impact. This rapid variation is discussed in relation to the dynamic properties of the liquid and the solid. A detailed examination of the surface damage is made including a measurement of the depths of the cracks in Perspex. Single and multiple drop impact are compared by studying a specimen subjected to artificial rain. The changes in the type of depression observed when Perspex is impacted at different temperatures are analysed.