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In this talk, I will first introduce recent models that relate the measured nonlinear ultrasonic signals to pertinent microstructural parameters that characterize the state of damage in structural materials. This will be followed by discussions of several ultrasonic measurement techniques including the generation of second harmonic and nonlinear wave mixing. The talk will conclude with several case studies involving fatigue damage in high-strength superalloys, radiation damage in stainless steels, plastic deformation in aluminum alloys, ASR-induced damage in concrete, etc. These studies clearly demonstrated that nonlinear ultrasonic measurements provide quantitative inputs to determine the material state and measure damage in structural materials.
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