Scattering-based Defect Characterisation using Laser Ultrasound Arrays

Assessment of defect size, shape and orientation is critical in the field of Non-Destructive Evaluation. This assessment can be performed with high accuracy by measuring the ultrasonic scattered wave field generated by a defect as a function of incidence and reflected angles, and producing the associated scattering matrix. In this work the scattering matrix is measured for defect characterization using non-contact, laser ultrasound. A synthetic laser induced phased array (LIPA) is used to record the scattered wave field at varying angles, on an aluminum sample for four crack-like defects, and the scattering matrix is extracted. The directivity of laser ultrasound is accounted for by a new concept proposed, called the sensitivity matrix. The experimental scattering matrices are compared to numerically modelled ones to characterize the orientation of the defects. Good agreement is observed between the true and measured defect orientations especially when compared to an imaging-based approach, where one of the defects could not be imaged accurately.