A dedicated ultrasound transmission method is used for in-situ detection and monitoring of fatigue cracks. For this purpose, piezoelectric transducers are embedded at the ends of hourglass-shaped specimens of HSLA steel. One transducer emits constant-amplitude time-limited pulses which travel as longitudinal and surface waves. The waves are received by the other transducer and their times-of-flight and amplitudes are readily monitored by a digitizing oscilloscope. During constant amplitude fatigue cycling at constant R, a decrease in surface wave amplitude indicates crack initiation while crack propagation is sensed by the change in the amplitude of both waves. During initial fatigue cycles, the transmitted wave amplitudes may decrease due to cyclic plastic strain and the accumulation of dislocations. In subsequent cycles, the lattice defects reach a saturation level giving rise to a steady state level of the transmitted wave amplitudes. Wave amplitudes monitored during saturation stage serve as a reference for subsequent measurements of amplitude decrease that indicates the growth of a fatigue crack. Changing the applied load in the early stage of fatigue cycling has a little effect on the saturation level. Such an effect diminishes with fatigue cycling and does not interfere with the ability of detecting crack initiation. The method is capable of monitoring the entire history of fatigue damage evolution from initial strain hardening, through strain saturation, crack nucleation, crack growth to failure.
