A nonlinear ultrasonic SHM method for impact damage localisation in composite panels using a sparse array of piezoelectric PZT transducers

Structural health monitoring techniques (SHM) for material damage identification have demonstrated higher sensitivity and accuracy when relying on the assessment of nonlinear features exhibited in the material response under ultrasonic wave propagation. In this paper, a novel nonlinear ultrasonic SHM method is introduced for localisation of impact damage in composite laminates using an array of surface-bonded sensors. Unlike existing algorithms, this method enables quick selection of a suitable signal transmission frequency based on the combined sensor-material response, it does not rely on baseline data or complex measurements of signal arrival time, and it allows identification of malfunctioning sensors to minimise damage localisation errors. The proposed technique is based on the transmission and reception of ultrasonic waves through the inspected panel. Initially, the functionality of the transducers is inspected by comparing the signal amplitude in both directions of sensor to-sensor paths. Then a planar map of material nonlinearity parameter beta is created, and the damage position is defined as the point of highest beta amplitude. Experimental tests on three CFRP panels confirmed successful positioning of barely visible impact damage (BVID) within a range of 4-22 mm. Sensor functionality check was demonstrated on one of the composite laminates, and a malfunctioning transducer was detected. The results suggested that the presented method could be considered an improved alternative to existing SHM techniques for localisation of BVID in composite panels.