In situ ultrasonic monitoring of surface fatigue crack initiation and growth from surface cavity

Abstract A surface acoustic wave method for in situ monitoring of fatigue crack initiation and evolution from a pit-type surface flaw is described. The method is demonstrated for fatigue tests on Al 2024-T3 and Inconel 718 samples with different surface pit sizes. The surface acoustic wave signature is acquired continuously during the fatigue cycle without stopping the fatigue test. Crack initiation and propagation are identified clearly from the ultrasonic surface wave reflection signals. Crack initiation in the Inconel 718 sample is observed to occur at a much later stage of fatigue life than in the Al-2024-T3 sample. The ultrasonic results are supported by fractographs of fracture surfaces. Small crack sizing is performed from the ultrasonic signatures using the low frequency scattering model.