Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage, part I: Nonlinear wave modulation spectroscopy (NWMS)

The level of nonlinearity in the elastic response of materials containing s tructural damage is far greater than in materials with no structural damage . This is the basis for nonlinear wave diagnostics of damage, methods which are remarkably sensitive to the detection and progression of damage in mat erials. Nonlinear wave modulation spectroscopy (NWMS) is one exemplary meth od in this class of dynamic nondestructive evaluation techniques. The metho d focuses on the application of harmonics and sum and difference frequency to discern damage in materials. It consists of exciting a sample with conti nuous waves of two separate frequencies simultaneously, and inspecting the harmonics of the two waves, and their sum and difference frequencies (sideb ands). Undamaged materials are essentially linear in their response to the two waves, while the same material, when damaged, becomes highly nonlinear, manifested by harmonics and sideband generation. We illustrate the method by experiments on uncracked and cracked Plexiglas and sandstone samples, an d by applying it to intact and damaged engine components.