Synthetic Aperture Imaging of Contact Acoustic Nonlinearity

Abstract

Contact acoustic nonlinearity (CAN) effect based on the generation of higher order harmonic components in the ultrasonic wave reflected at closed interfaces is effective to detect closed cracks which are not detectable by conventional linear ultrasonic techniques. However, the field application of CAN effect has been limited since it is difficult to interpret the signals received from closed interfaces. For this reason, CAN imaging techniques should be required to enhance the industrial applicability. As an ultrasonic imaging methods, the phased array ultrasonic testing (PAUT) has been widely employed in various fields such as medical and nondestructive testing (NDT). However, PAUT has disadvantages to apply for imging of CAN effect. First, each small transducer of PAUT has too low acoustic power to activate the CAN effect. Moreover, a high speed switch to apply a high power electric signal to each transducer has not been commercialized. To overcome these limitations, this study proposes a synthetic aperture imaging (SAI) of CAN effect. The SAI technique is a post processing method which can supply the high power electric signal by a comparatively lower speed switch such as relay. In order to verify the effectiveness of the proposed technique, the SAI technique was compared with the imaging based on PAUT in the experiment, in which solid-solid contact interfaces were tested with respect to increasing contact pressure. The experimental results proved that the SAI technique could visualize CAN effect at the closed interfaces which can not be inspected by using conventional PAUT. The proposed technique will be able to improve the possibility for the detection of closed interfaces and consequently be applied to industrial fields in order to evaluate the closed interfaces accurately.