Material degradation often starts with changes in the microstructure that do not affect the bulk linear elastic material behavior. Degradation may continue to form macroscale damage and eventually cause failure. Few nondestructive methods are available to assess microstructural features. Such methods would enable condition-based life prediction at a much earlier point in the service life of the structure and change the way life cycles are managed. The potential of mixing primary guided wave modes to generate combinational higher harmonics that are sensitive to the early stages of damage development is demonstrated. The method is based on the interaction of two ultrasonic guided waves at a point. The interaction of these waves generates higher harmonics at combinational frequencies due to the lattice anharmonicity. Combinational frequencies are much preferred to integer multiples of the excitation frequency from a measurement system perspective. The spectroscopy method is applicable for plates and shells (including pipes) and will be demonstrated on an aluminum plate using magnetostrictive transducers and the fundamental shear horizontal mode. The results indicate that localized microstructural damage can be detected without the use of a historical baseline.

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