Lamb Wave Based Monitoring of Delamination Growth in Mode I and Mode II Fracture Tests

A new method to monitor the delamination onset and growth in Mode I and Mode II interlaminar fracture tests using Lamb waves is proposed. Double cantilever beam (DCB) and end notch flexure (ENF) specimens made of 8-layer glass-fiber/epoxy (GF/EP) reinforced composite laminates were used as standard samples in the experimental studies. Fundamental A0 mode was actuated in all samples with a pair of collocated piezoelectric transducers and sensing was performed in a pitch-catch configuration using a single surface bonded piezo-element on the opposite side with delamination. It was determined that the amplitude and phase of the received A0 mode wave packets had high sensitivity to both Mode I and Mode II delamination onsets. Besides, the crack length was continuously monitored using the mode conversion effect and relative group velocity change of the A0 mode. The transition points, extracted from the ultrasonic waveforms, correlated well with load–displacement curves used for determining interlaminar fracture toughness. Similar approach can be easily extended to study/monitor adhesively bonded joints.