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-shaped 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 each pulse and were monitored with a storage oscilloscope. During fatigue cycling between zero and a constant tensile load, the changes in the time-of-flight and in the transmitted amplitudes of the ultrasonic waves were used to monitor the elastic and plastic strains in order to detect fatigue damage and crack initiation. During the initial fatigue cycles, a decrease in the transmitted wave amplitude occurred due to plastic deformation. In subsequent cycles, during which the stress-strain hysteresis loops indicated essentially zero plastic strain, the transmitted wave amplitudes remained constant. Eventually, a fatigue crack nucleated and grew. This was detected by a decrease of the transmitted wave amplitudes. From here on, further decrease of the transmitted amplitudes measured the crack growth. Crack growth was also measured from marked crack fronts in the final fracture surface. The entire history of fatigue damage evolution from initial strain hardening, through strain saturation, crack nucleation and growth could be monitored with the present technique. This revised version was published online in August 2006 with corrections to the Cover Date.