Non destructive inspection of aerospace composites by a fiber-coupled laser ultrasonics system

Abstract

Laser ultrasonics is a technique currently studied for nondestructive inspection of aerospace composite structures based on carbon fibers. It combines a pulsed laser impacting the surface generates an ultrasound inside the material, through the nondestructive thermoelastic effect. Second a detection interferometer probes the impacted point in order to measure the displacement of the surface resulting from the emitted ultrasound wave and the echo coming back from the different interfaces of the structure. Laser ultrasonics is of interest for inspecting complex shaped composites. We have studied the possibility of using frequency doubled YAG laser for the generation and which is fiber-coupled, together with a fibercoupled interferometric probe using a YAG laser in the NIR. Our final system is a lightweight probe attached to a robot arm and which is able to scan complex shapes. The performances of the system are compared for different wavelengths of generations. Also we have studied some experimental parameters of interest such as tolerance to angle and focus distance, and different geometries of generation beams. We show some examples of inspection of reference parts with known defects. In particular C-scans of curved composites structures are presented.

Laser Ultrasound Flexible System for Non-contact Inspection of Medium Size and Complex Shaped Composite Structures Made of Carbon Fiber Reinforced Polymer

“…Shallow defects (1 mm depth) are barely visible in both cases but at such depths, it is usually difficult to distinguish between both first pulse and first echo, which is a specificity of pulse-echo methods ("dead zone"). Other parameters of interest like the geometry of generation beam, and distance and angle with respect to the sample, have been discussed earlier in details in [15].…”