Effects of Mechanical Load History on Lamb Wave Interactions with Fatigue Cracks in Aluminum Plates
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The average age of the USAF fleet is now 27 years and climbing. Currently, the USAF assesses structural health of aircraft primarily using schedule-based inspections. Automated structural health monitoring systems could enable a transition to condition-based maintenance, reducing costs and increasing aircraft availability. A study on helicopter structures employing a commercial-off-the-shelf SHM system using elastic ultrasonic waves found that such systems may demonstrate reduced sensitivity to high-cycle versus low-cycle fatigue cracks. This dissertation reports on experiments to study the interaction of ultrasonic Lamb waves with both high- and low-cycle fatigue cracks in aluminum plates. Three similar cracks (one high-, two low-cycle) were interrogated with broadband Lamb wave excitations and measured using a laser Doppler vibrometer. The data was post-processed using chirp and frequency-wavenumber filtering to determine the structural response for symmetric and asymmetric Lamb wave modes at frequencies from 50 kHz to 500 kHz. Transmission and reflection coefficients were calculated representing the amount of wave energy that passed through or was reflected from each crack. The results demonstrated quantifiable differences in the amount of Lamb wave energy transmitted through and reflected from high- versus low-cycle fatigue cracks.