Applying features of nonlinear ultrasonic modulation for defect detection in vibrating structures

Abstract. Aircraft structures are subjected to various external factors that influence their lifetime. Due to the high need for structural health monitoring in aerospace applications, numerous, quite mature linear ultrasonic NDT techniques have been developed for the detection of defects. Typically guided waves are used, which are based on mode conversion and reflection of probe waves by a defect, provided the defect is open, resulting in an acoustic impedance mismatch. However, in practical applications defects are often ‘closed’ when not under substantial stress. Moreover, in most nonlinear ultrasonic NDT techniques the lack of differentiation between sources of nonlinearity makes defects indistinguishable from e.g. nonlinearity induced by mechanical contacts. Here, we aim to detect damage, addressing the practical difficulties of monitoring vibrating structures. Typical defects were created by fatigue facilities and detected in aluminium plate-like samples using PZT transducers to generate and detect probe waves. The presented diagnostic algorithms compare features of the nonlinear relation between the amplitude of the transmission probe wave and the load on the sample with a threshold value, in order to assess the state of the sample. The applications are robust to environmental changes, are based on durable components, while being sensitive to vibrating defects