Fatigue crack monitoring in engine-grade titanium alloy by dynamic subtraction of surface acoustic wavemodulation

The initiation and evolution of fatigue cracks in forged titanium alloy samples are monitored ultrasonically during fatigue testing. An in-situ surface wave acoustic method is applied during fatigue with an overlaid small low-frequency periodic loading, resulting in a nonlinear modulation of reflected ultrasonic pulses. The acoustic wave time traces in the sample are collected for a range of applied fatigue and modulation load levels and for a range of spatial propagation positions within each fatigue cycle. These samples are characterised by strong microstructure-induced ultrasonic scattering. To improve the signal-to-noise ratio a post-processing subtraction technique is introduced with the aim of enhancing initiated crack detectability.