SHADOW MOIRE CONTOURING OF IMPACT CRATERS

The method of shadow moire topography has been used to characterize smooth craters formed on the surface of ductile metals. The craters were made by the impingement of hard spheres (diam 5 multiplied by 10** minus **3m) traveling at speeds of approximately 130 ms** minus **1. A comparison of shadow moire with other methods, in particular the techniques based on holography, shows that the former is simpler to use, less costly, and relatively insensitive to vibrations. A mathematical description of a crater has been developed which describes the surface to a good approximation. It has only two independent parameters: the crater depth and the half-length, and can be integrated to give the volume of the crater. The parameters, which can be found directly from moire topographs, give a measure of the crater volume (10 to 0. 1 multiplied by 10** minus **9m**3) accurate to within 8 to 15%. A more elaborate method for determining the volume, which is also described, optimizes the values of the parameters and has a final accuracy better than 3%.