Applications of non-linear acoustics for quality control and material characterization

Abstract

For several decades, nonlinear acoustic methods have been used for material characterization, quality control, and biomedical diagnostics. This approach is based on a second or higher-order phenomenon. Most nondestructive evaluation tasks employ conventional first-order ultrasonic techniques. Utilizing a nonlinear regime may bring new essential information and improve the characterization of materials with defects or flaws that are challenging to detect using traditional acoustical methods. Such defects inexhaustibly include thin cracks and dislocations through which sound passes without reflection; filled cracks or glue layers with acoustical contact between surfaces, voids, and agglomerations thereof with a dimension less than the wavelength; inclusions with a subtle acoustical difference from surrounding media; and multilayer structures with various boundary conditions between layers. For such cases, defects can be detected, visualized, and evaluated using a nonlinear reflection effect. This effect accompanies a typical sound wave reflection at interfaces between media, producing reflected and refracted waves. In the nonlinear regime, these waves have components with double frequency. The nonlinear properties of both media determine the wave amplitude. The nature of the evaluated medium determines the type and number of parameters that describe the nonlinear properties. These parameters' magnitude and spatial distribution provide valuable information about the material properties and object structure. Inspection instruments that utilize the effect of nonlinear reflection can be effective tools for quality control.