Laser Doppler imaging of delamination in a composite T-joint with remotely located ultrasonic actuators

This paper presents an experimental study on the interaction of ultrasonic guided waves with delamination in the web–flange interface of a co-cured, co-bonded composite T-joint. A complex interaction of waves is induced, first, by one, and, later, by two piezoceramic wafer(s) bonded on the inner surface of the flange. The flange surface velocity distributions are reconstructed from the scanning laser Doppler velocimeter (LDV) measurements to study the effect of a delamination overlapping the web–flange interface. The surface-bonded piezoelectric actuator(s) placed remotely on the flange produce primarily anti-symmetric wave incidence to the delamination. A two-stage wave mode conversion process, one due to the web and joint filler, and the other due to the delamination underneath the flange is observed. First, we address the problem of identification of delamination by ultrasonic contrast imaging of the web–flange interface using a single actuator. This study is extended further to enhance the ultrasonic contrast imaging at the web–flange interface using a standing wave. A quantitative methodology based on mode conversion strength is developed to monitor different structural features like free-edge, web–flange interface and delamination. The results show the complex nature of the conversion of flexural waves to in-plane waves through the deltoid and along the web–flange interface.