Adhesive bond testing by laser shock waves and laser interferometry

Abstract

Adhesive bonding of structural components presents many practical advantages when compared to other joining methods, but its application for critical components is limited by the absence of reliable nondestructive methods that can assure the bond strength of the joint. In this paper, a method based on shock waves produced by a pulsed laser is applied to the evaluation of bond strength of two plates joined with an adhesive. Different adhesives were tested. A shock wave, produced by an energetic short laser pulse can cause a delamination at the adhesive/plate interface when it propagates through them. A good bond is unaffected by a certain level of shock wave stress whereas a weaker or kissing bond is damaged. The method is made quantitative and in-situ by optically measuring the sample back surface velocity with a Doppler or velocity interferometer. The interferometer signals allow distinguishing interfaces that pass the test from the ones that fail. The measured back surface velocity is related to the internal stress by a simple equation. Experimental results show that the proposed test is able to differentiate bond quality and give a value of the bond strength. Laser-ultrasonic inspection made on laser shock tested samples confirms that weak bonds are revealed by the method. The proposed testing approach may help a broad adoption of adhesive bonding throughout the aerospace industries and its use for joining primary aircraft structures.Peer reviewed: NoNRC publication: Ye