A novel approach for damage assessment in adhesively bonded composite joints using backface strain technique

In this study, the backface strain (BFS) is measured by both digital image correlation (DIC) and fiber optic sensors (FOS) to detect the crack initiation and propagation in adhesively bonded composite single-lap joints (SLJ). BFS measures the resultant strain deriving from the positive strain, due to tensile load, and negative strain related to the bending moment. A point, called zero-strain point (ZSP), can be detected on the substrate surface of SLJ due to the concurrent effect of these positive and negative strains. The experimental activity shows that the value of the ZSP changes when the crack starts to propagate. Thus, this point can be used to monitor the service conditions of adhesive joints. The effect of joint dimensions on the position of the ZSP is investigated when the joint is subjected to quasi-static loading. In addition, the applicability of the method is investigated under a cyclic loading condition. The work shows that the ZSP can be used as an index to monitor joint healthiness. Furthermore, FOSs can be used for an in-situ monitoring of the joint.