[en] Friction welds are formed by the mechanisms of diffusion as well as mechanical inter-locking. The severe plastic flow at the interface by the forge action of the process brings the subsurface so close together that detection of any unbonded area becomes very difficult. No reliable method is available so far to determine the weld quality nondestructively. This paper presents an attempt to determine weld strength quantitatively using the ultrasonic pulse-echo method. The new approach calculates the coefficient of reflection based on measured amplitudes of the echoes. This coefficient provides a single quantitative measurement which involves both acoustic energy reflected at the welded interface as well as transmitted across the interface. As a result, it was known that the quantitative relationship between the coefficient and the weld strength(torsional strength) could be drawn. (Author)