This paper presents an overview of the application of nonlinear ultrasonic (NLU)techniques to characterize materials; it has been demonstrated that NLU can providequantitative inputs to determine the material state and measure damage inengineering components. It has recently been shown that NLU can be used todevelop the framework for accurate life prediction of fatigue damaged components.These NLU measurements are done at the material level, before the formation ofmicro- and macro-cracks. The traditional NDE of damage of a material subject tofatigue starts from the time when a small crack initiates because there is nomeasurable macroscopic change in the material prior to the crack initiation. In mostmetallic materials, however, cracks in a measurable size appear quite late (after 80%)in the total life, while the material's integrity in terms of toughness and strengthgradually decreases due to the microplasticity (dislocations) and associated change inthe material' microstructure. Starting from mechanics fundamentals, we first developthe theoretical equations of wave motion in an elastic solid with quadraticnonlinearity. The next section considers measurement techniques for NLU, which isfollowed by examples of the assessment of fatigue damage in metals with NLU.
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