Backface strain as an index to detect damage initiation in composite single-lap bonded joints: Effects of adhesive type and joint dimensions

In this study, the backface strain (BFS) method applied by digital image correlation (DIC) is used to detect crack initiation and propagation in adhesively bonded single-lap joints (SLJ). By comparing the positive strain, due to the 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 the SLJ. Using the Bigwood and Crocombe analytical model, the presence of the ZSP on the backface is explained and the experimental results are used to detect it. The monitoring of the ZSP reveals useful information about the health condition of the joint. The main aim of this research is to investigate how the ZSP position varies by changing adhesive type (epoxy and polyurethane) and bonding area dimensions both in elastic conditions and damage progression. The results illustrate that the position of the ZSP in polyurethane SLJs is closer to the middle of the joint compared to epoxy SLJs. Additionally, the ZSP is more easily recognizable in epoxy adhesive SLJs when substrates are thicker. Finally, the ZSP showed negligible sensitivity to joint width for both types of adhesive joints regardless of the adhesive type. In conclusion, it is shown that the ZSP can be used as a monitoring index to detect damage initiation and propagation in SLJ specimens.