Principles and applications of nonlinear acoustic nondestructive testing

Abstract

Nonlinear acoustic techniques have recently been introduced as a new tool for nondestructive inspection and evaluation of fatigued, defective, and fractured materials. Various defects such as cracks, debonding, fatigue, etc., lead to anomalously high levels of nonlinearity as compared with flawless structures. One of the acoustic manifestations of such nonlinearity is the modulation of ultrasound by vibration. This effect is explained by using a model of the defect as contact between two rough surfaces. The vibration varies the contact area modulating the ultrasonic probing wave passing through the defect. A theoretical model considering interaction of different modes of vibrations and probing waves is discussed. Two methods employing the nonlinear interaction of ultrasound and vibration were developed, namely vibro-modulation (VM) and impact-modulation (IM) methods. The VM method employs forced harmonic vibration of the tested structure, while the IM method uses impact excitation of the structures natural modes of vibration. The crack detection tests were carried out for different objects made out of steel, plastic, glass, concrete, and carbon. These nonlinear techniques demonstrated certain advantages as compared with the conventional linear acoustic technique, specifically high sensitivity and applicability to highly inhomogeneous structures. [First author was supported in part by RFBR, Grant No. 96-05-64459.]