Initiation of fatigue crack growth in austenitic stainless steel detected by ultrasound: Role of in-plane orientation

We discuss herein the initiation of crack growth in austenitic stainless steel in its early stage of fatigue life, which we study via strain range controlled low-cycle fatigue experiments. The manner in which the intensity of ultrasound back reflected from grain boundaries in fatigued austenitic stainless steel depends on the in-plane orientation of the incident ultrasound wave was investigated experimentally for four different directions of the incident ultrasound wave. With the material under fatigue loading, we measured the intensity of ultrasound waves back reflected from grain boundaries within the material surface. The intensity of back-reflected ultrasound decreases as the number of cyclic loads increases in going from the early stage of fatigue to just before the onset of crack growth. This is because of the formation of persistent slip bands and the average dislocation density increases within the specific crystal grain where the crack initiates. The attenuation of back-reflected ultrasound is due to vibrations of dislocations, which we consider in the analysis. During the fatigue process, the intensity of the back-reflected ultrasound varies as a function of in-plane orientation of the incident ultrasound wave. By using multiple in-plane incident-wave orientations, we increase the probability to detect a decrease in back-reflected ultrasound that occurs during fatiguing. We tentatively attribute such decreases in back-reflected intensity to differences in crystal-grain orientation and in the shape and size of grain boundaries.