DETECTION AND MEASUREMENT OF FATIGUE-CRACK IN HSLA STEEL WITH A DEDICATED ULTRASONIC PULSE TRANSMISSION METHOD

A dedicated 'ultrasound transmission' method was used for detection of fatigue cracks. The measurements were done in-situ on hourglass-shape d specimens of HSLA steel that had piezoelectric transducers embedded in each end. Time-limited pulses were emitted from one transducer and received by the other. Longitudinal and surface waves resulted from ea ch pulse and were monitored with a storage oscilloscope. During fatigu e cycling between zero and a constant tensile load, the changes in the rime-of-flight and in the transmitted amplitudes of the ultrasonic wa ves were used to monitor the elastic and plastic strains in order to d etect fatigue damage and crack initiation. During the initial fatigue cycles, a decrease in the transmitted wave amplitude occurred due to p lastic deformation. In subsequent cycles, during which the stress-stra in hysteresis loops indicated essentially zero plastic strain, the tra nsmitted wave amplitudes remained constant. Eventually, a fatigue crac k nucleated and grew. This was detected by a decrease of the transmitt ed wave amplitudes. From here on, further decrease of the transmitted amplitudes measured the crack growth. Crack growth was also measured f rom marked crack fronts in the final fracture surface. The entire hist ory of fatigue damage evolution from initial strain hardening, through strain saturation, crack nucleation and growth could be monitored wit h the present technique.