Applications of Laser-Ultrasonics and Laser-Tapping to Aerospace Composite Structures

Laser-ultrasonics is a well known and mature nondestructive technique for inspecting polymer matrix composites used in aerospace. This technique uses a pulse echo interrogation mode, in which ultrasound is first generated by a pulsed laser and then detected by a second laser coupled to an optical interferometer. While very successful to find delaminations in laminates, difficulties are found for reliably detecting disbonds in honeycomb and foam core structures, particularly when the detachment occurs at the back skin. For this purpose, a novel technique called LaserTapping is proposed. Laser-Tapping is based on the thermoelastic excitation by a pulsed laser of the top skin which bulges and is driven into vibration if it is detached from the material underneath. Laser-Tapping uses essentially the same hardware as laser-ultrasonics but probes in a lower ultrasonic frequency range. Laser-ultrasonics and Laser Tapping can then be advantageously used concurrently. This paper presents examples of applications of a combined Laser-Ultrasonic Laser-Tapping system to honeycombs structures with defects, including delaminations in the skin and skin detachments at the front side and the back side.