Detecting contact acoustic nonlinearity in TOFD measurements via quasistatic loading

Abstract

Abstract. In this paper, we propose a novel method to detect and quantify contact acoustic nonlinearity using conventional time of flight diffraction (TOFD) non-destructive testing (NDT) equipment, with the aim to improve the sensitivity and robustness of TOFD measurements. This new method involves applying an external cyclic quasi-static load while simultaneously taking TOFD measurements. The applied load causes modulation of contact surfaces within damaged areas of the material, which can be observed as changes in the time-domain TOFD response. Additional processing extracts any load-dependent features from the signals, allowing the identification and quantification of damage and defects that exhibit contact acoustic nonlinearity. Importantly, this new quasistatic contact acoustic nonlinearity (QS-CAN) technique maintains time-resolution and localisation capability of conventional TOFD. It is shown that the technique can differentiate between different types of damage such as fatigue cracks or voids within samples. The new QS-CAN nonlinear ultrasonic methodology is a fundamental extension to all existing nonlinear measurement techniques. It allows for the first time to use time signals captured from conventional NDT equipment to extract nonlinear material characteristics.