Damage detection by using features of nonlinear ultrasonic modulation in vibrating structures

Abstract: Due to the high need for structural health monitoring in aerospace applications, numerous, quite mature linear ultrasonic NDT techniques are available for the detection of defects. Linear ultrasonic NDT e.g. by using guided waves is based on mode conversion and reflection of probe waves by a delamination or 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 in differentiation between sources of nonlinearity makes defects undistinguishable from e.g. nonlinearity induced by mechanical contacts. Here, we aim to detect damage, addressing the practical difficulties of monitoring vibrating structures. Defects were created and detected in aluminum 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.