Abstract

Cyclic loading of mechanical components promotes the formation of dislocation substructures in metals as precursors to crack nucleation leading to a final failure of the metallic components. It is well known within the ultrasonic community that the acoustic nonlinearity parameter is a meaningful indicator of microstructural damage accumulation. However, current nonlinear ultrasonic techniques suffer from response saturation and limited resolution after 50% fatigue life of the metallic medium. The present study investigates the feasibility of incorporating collinear wave mixing interactions into second harmonic assessments to improve the sensitivity of the nonlinear parameter to microstructural accumulation of damage precursors (DP). To this end, a decomposition technique was explored to obtain higher harmonics from short time-domain pulses propagating through thin metallic components such as jet engine turbine blades. The results demonstrate the effectiveness of the decomposition technique to measure the acoustic nonlinearity parameter as an early and continuous indicator of fatigue damage precursors throughout the service life of critical aircraft components. A micrographic study showed a strong correlation between the nonlinearity parameter and the increase in damage precursors throughout the life of the specimens.

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