Using digital image correlation to automate the measurement of crack length and fracture energy in the mode I testing of structural adhesive joints

•A DIC method is proposed to measure the mode I crack length for adhesive joints.•Effective crack lengths including microscopic damage and root rotation effects were measured.•Crack length extended to the compression zone beyond the FPZ leads to the determination of accurate Gc values.•A method to correct the Gc measurement using crack length dependent LEFM methods is proposed. In this study, the crack lengths in adhesively bonded double cantilever beam (DCB) test specimens have been determined using the digital image correlation technique in combination with an elastic foundation model. This method facilitates the continuous measurement of the crack length and offers significant advantages over conventional visual observation including improved accuracy and the potential for automation. This method has been applied to three joints bonded with different structural adhesives, and fracture energy Gc values calculated with the effective crack lengths determined by this method have been shown to be accurate by comparison to Jc values. Finally, a new method is proposed to correct the crack lengths that are visually measured and the Gc values determined using the standard analysis. This scheme is shown to improve the accuracy of the Gc values appreciably.