Imaging defects in laminate composite plates using focused shear waves generated by air-coupled transducer

This research develops a nondestructive evaluation (NDE) technique to obtain images of two types of defects, delamination and debonding, in a thick laminate composite plate as a laboratory material system for wind turbine blades. The technique uses focused shear waves which are generated and detected by a set of noncontact, air-coupled ultrasonic transducers. The noncontact nature of the air-coupled method facilitates rapid scanning/inspection of large composite structures. Ray-tracing simulations are performed which visualize the focusing behavior of the focused shear waves in the specimen, from which the experimental parameters are determined. The proposed focused shear wave technique is compared with a Lamb-wave based technique, showing that the present technique is more appropriate for direct imaging of these defects. The obtained images of delamination and debonding defects agree well with the estimated defect locations and dimensions. This demonstrates the feasibility of further developing the present technique for specific field applications in wind turbine blades. (C) 2016 Elsevier Ltd. All rights reserved.