where Gf is shearing stiffness, ν is Poisson's ratio which is assumed to be 0.3 and Ef is the elastic modulus of the oxide films. The fracture volume is considered to be connected not with all of impact energy but rather with the part of the impact energy of the projectile (see Fig. 10). The force F which applies to the films to be fractured is assumed to be equal to the product of average contact pressure [10] and [11] and contact area A of the films with the projectile, as shown in Eq. (5). The average contact pressure is the impact energy IEp divided by the indentation volume V.