Nonlinear Acoustic Ndt: Approaches, Methods, and Applications

Conventional acoustic instruments widely used in industry and technology for material characterisation and quality assessment make use of so-called linear acoustic response of materials that generally results in the amplitude and phase variations of the input signal. The nonlinear approach to acoustic NDT is concerned with non-linear material response, which is inherently related to the frequency changes of the input signal. Recent nonlinear NDT-methods are based on anomalously high nonlinearity of microand macro-scale defects and appeal directly to the vulnerable (faulty) areas within material or a product. On this background, various NDT methods have been proposed which use the nonlinear effects as a signature of damage. The paper reviews the nonlinear mechanisms and experimental results obtained with the methodologies of nonlinear laser vibrometry and nonlinear air-coupled emission. Numerous case studies are given to demonstrate their applicability for nonlinear NDT and defect-selective imaging in various materials. Particular examples include quality assessment and NDT of materials and components applicable in automotive, aviation, and construction industries.