Abstract: Based on Gathers work,general equations for traditional measurement of the spall strength are given for the planar spall experiment with a low impedance buffer placed behind the target.In the case of no buffer,the equations become the expression given by Novikov within the acoustic approach.It is indicated that the spall strength equations determined by the traditional methodology are valid only for the numerically simulated ‘pullback' by a local instantaneous critical stress criterion.Numerical simulations show that the propagation of wave profile in the spall scab is affected seriously by the stress relaxation due to the damage evolution on the spall plane.Therefore,the free surface velocity profile of target or the stress profile of the interface between target and low impedance buffer depends strongly on the damage evolution on the spall plane.It is revealed that the traditional determination of spall strength without taking into account the stress relaxation is invalid measure of the spall strength.