Elastic waves provide a number of methods to detect damage or material degradation. Ultrasonic guided waves are elastic waves that propagate in bounded geometries. The complex constructive and destructive interference patterns enable the waveguide cross-section to be fully energized and the waves to propagate long distances. Linear analysis of guided waves permits detection of changes in linear elastic constants and acoustic impedance changes that cause reflections and scattering. Nonlinear analysis of guided waves enables detection of small changes in the microstructure of the material that do not affect the linear elastic constants or result in detectable scattering. It is the distortion of the guided wave resulting from the microstructure changes that causes the generation of higher harmonics, which are then representative of the early stages of degradation. The ability of nonlinear ultrasonic guided waves to detect early degradation, sometimes referred to as damage precursors, is extremely attractive for structural health monitoring enabled condition based maintenance. The basis and methodology for utilizing guided waves for early damage detection is discussed. Then as an example, the ability of the fundamental shear horizontal mode to characterize fatigue damage prior to the initiation of a macroscale crack is demonstrated on a set of 2024-T3 aluminum plates.
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