Nonlinear ultrasonic techniques for material characterization

Abstract

This chapter considers both the theoretical aspects of the nonlinear ultrasonic phenomena in elastic solids and their applications to materials characterization; it has been demonstrated that nonlinear ultrasound (NLU) measurements can provide quantitative inputs to determine the material state and measure damage in engineering components. It has recently been shown that NLU can be used to develop the framework for accurate life prediction of components under mechanical and thermo-mechanical loading. These NLU measurements are done at the material level, before the formation of macroscopic damage. The traditional NDE of damage of a material subject to, for example, fatigue starts from the time when a small crack initiates because there is no measurable macroscopic change in the material prior to the crack initiation. In most metallic materials, however, cracks of a measurable size appear late in the fatigue life (typically after 80%), while the material toughness and strength decreases gradually due to the microplasticity (dislocations) and associated change in the material’s microstructure. Starting from mechanics fundamentals, we first develop the theoretical equations of wave motion in an elastic solid with quadratic nonlinearity, covering bulk, surface, and guided waves. Various nonlinear acoustic phenomena occurring in the infinite and bounded elastic solids are described in a consistent mathematical framework. The next section considers measurement techniques for NLU, including examples of the assessment of fatigue and thermal damage in metals with NLU.