Fig. 9 shows the relations between a crater diameter a and an equivalent diameter b which is defined as a square root of the whole detached area S including the indentation area, divided by π/4, under impact velocities of 50 and 70 ms−1, film thickness of 20, 35, 55 μm and particle size of 2, 3, 5 mm. b roughly related to a in each environment independent of the different size of the craters with the different impact velocity, particle diameter and film thickness. The slope of the linear curve in wet air was higher than those in the other environments. The relation of a and b was nearly same irrespective of surface roughness of the mild steel in the case of this study