Nondestructive detection of fatigue cracks in PM 304 stainless steel by internal friction and elasticity

A resonant frequency mechanical vibration method was used to nondestructively detect fatigue crack initiation in notched 304 stainless steel samples prepared by powder metallurgy. This method allowed the determination of an effective elastic modulus and the direct measurement of internal friction. Changes in the modulus and internal friction were found to correlate well with the presence of 50 μm long fatigue cracks. The length of the through cracks initiated at the notch was measured using surface replicas, which were examined in a scanning electron microscope. Small crack growth rate data were also obtained and plotted versus the stress intensity factor. The crack growth rates were compared with long crack data performed on compact tension samples of this material. The short cracks grew at intensity factors below the long crack threshold stress intensity factor.