Nonlinear ultrasonic reciprocal method for damage localisation

Nonlinear ultrasonic techniques are based on the measurement of nonlinear elastic effects in the material, such as the magnitude of second harmonic response, in order to reveal the presence of surface and sub-surface flaws. These methods have shown to be more sensitive to damage-related structural alterations than any other ultrasonic technique, which typically relies on the investigation of linear material parameters such as wave speed, damping and wave scattering. This paper presents a novel nonlinear ultrasonic technique, here named as nonlinear ultrasonic multi-path reciprocal method, for the localization of material damage based on the measurement of nonlinear elastic effects using a spatially distributed array of surface mounted ultrasonic transducers. In the proposed methodology, the reciprocal nonlinear material response is measured between each pair of sensors acting both as transmitter and receiver. The resulting damage location is mapped to tangent circumferences centred at the transmitter location for each pair, and all contributions (multi-path) from all pairs are summed to achieve the final defect location. Both the second order nonlinear parameter and the bispectrum coefficient are used as a nonlinear ultrasonic signature due to signal noise effects hiding the nonlinear harmonic response. The proposed methodology is validated on a damage composite plate. Unlike traditional ultrasonic methods, this technique does not require any knowledge of the mechanical properties and layup, nor any baseline with the undamaged component.