An experimental study for identifying crack initiation forces in single-lap bonded joints

•This study focuses on determining the crack-initiation force of adhesives.•Failure progression of adhesive is analyzed by integrating multiple methods.•The extended finite element is combined with the virtual crack closure technique.•Digital image correlation is incorporated in the relative displacement method (RD).•The RD is experimentally validated to accurately detect crack-initiation force. This paper highlights a method for determining crack initiation force in adhesives used for single-lap joints (SLJs). Crack onset behavior was investigated by combining multiple techniques of failure progressive analysis. The proposed relative displacement method (RD) incorporated digital image correlation (DIC), focusing on bending strain behavior responses to the RD of substrates. Deformation acceleration calculated from the strain distribution drastically increased with increasing crack initiation force and lied within 3% accuracy range, as compared with the behavior of bending compliance measured by a strain gage. The RD was confirmed practical for detecting crack initiation force in SLJs.