A NEW PROCEDURE FOR MEASURING THE DECOHESION ENERGY FOR THIN DUCTILE FILMS ON SUBSTRATES - COMMENTS

A recently proposed method for measuring decohesion energy of ductile films on substrates is discussed. The loading mechanism that causes th e decohesion of the ductile film is that of gradually depositing an ad ditional layer, with large residual tensile stresses, on top of the fi lm. Hence, the method involves the decohesion of a bilayer film on a s ubstrate. The suggested method assumes that the unloading of the film is controlled entirely by elasticity. This assumption is a prerequisit e for the suggested linear elastic analysis, from which the interfacia l debond energy is derived in closed form. However, as is shown in the present comment, large scale yielding can occur in the wake of the cr ack tip and is prohibitive to the suggested linear elastic analysis. A sufficient condition for the occurrence of said large scale yielding is outlined in the present comment. Indeed it is shown that large scal e plasticity must have occurred in the experiments described by Bagchi et al.(1)