Exploring the Feasibility of High Repetition Laser Ultrasound Non-Destructive Examination of Carbon Fibre Reinforced Polymer Composites

This thesis reports a reliable approach for Non-Destructive Examination (NDE) of the composite structure to inspect delamination with lasers, inspired by the conventional ultrasonic testing technique. This novel technique consists of a setup wherein a target is subjected to a calibrated mechanical stress, generating compressional and shear waves that propagate within the laminate by a laser source. These waves reflect off the back surface or if they encounter a discontinuity. Such a mechanism can be obtained by shock/stress waves induced by a laser that generates a high strain rate. The laminates are subjected to a wide range of duration and amplitudes of impact using lasers, according to the properties, particularly geometrical and mechanical. Coupled with the measurement of velocity/displacement of the free surface and the surface of impact, the study provides the understanding of the real-time diagnosis of a possible delamination consequent to the mechanical effects of the impact. The simulation validates the characteristics of the loading and material, in correlation with the experimental results, and postulates the complete cognisance of interpretation of results. The numerical study reveals constraints and propagation of stress waves within the target, and allows for quantification of the existence of a delamination and its depth. The relation between the displacement/velocity results and the material properties, to characterize the depth of delamination is established. The experiments are conducted on carbon/epoxy composite laminates with and without delamination at different depths. The delamination in the laminates is introduced using an aluminium foil. The experiments are conducted in the testing arrangements similar to conventional technique(s). An optical phase modulation system is used to capture the ultrasonic signals arriving as shock/stress waves. It is also reported how the numerical model can be used as a predictive tool for the rapid estimation of the shock/stress propagation, to carry out a feasibility of NDE by varying the laser parameters. One such variation is achieved for a laser that has high repetition rate and low energy. All the investigations reveal certain possibilities and some associated challenges, which are explained by the theoretical and numerical means. These observations are useful for a user to come to a resolution on adapting this technology