Nonlinear ultrasound: A novel approach to flaw detection and imaging

Abstract

Conventional ultrasonic NDT equipment is normally a mono-frequency instrument which makes use of the amplitude and phase variations of the input signal due to its scattering by defects. The nonlinear approach to ultrasonic NDT (NNDT) is concerned with the nonlinear response of defects, which is related to extreme frequency changes of the original input signal. These spectral changes are caused by a high nonlinearity of micro- and macro-scale defects. In the paper, basic mechanisms responsible for frequency conversion by nonlinear defects are discussed and major features of the nonlinear spectra derived. Experimental methodologies of nonlinear scanning laser vibrometry (NSLV) and nonlinear air-coupled emission (NACE) are used to study nonlinear elastic wave-defect interactions. Applications for defect-selective imaging and NNDT are demonstrated for a series of hi-tech materials and industrial components. © RILEM 2013.