From thin to extra-thick adhesive layer thicknesses: Fracture of bonded joints under mode I loading conditions

The fracture behaviour of joints bonded with a structural epoxy adhesive and bond line thicknesses of 0.1-4.5 mm has been studied. However, limited research is found on similar joints with thicker bond lines, which are relevant for maritime applications. Therefore, the effect of the adhesive bond line thickness, varying from 0.4 to 10.1 mm, on the mode I fracture behaviour of steel to steel joints bonded with a structural epoxy adhesive was investigated in this study. An experimental test campaign of double-cantilever beam (DCB) specimens was carried out in laboratory conditions. Five bond line thicknesses were studied: 0.4, 1.1, 2.6, 4.1 and 10.1 mm. Analytical predictions of the experimental load-displacement curves were performed based on the Simple Beam Theory (SBT), the Compliance Calibration Method (CCM) and the Penado-Kanninen (P-K) model. The P-K model was used to determine the mode I strain energy release rate (SERR). The average mode I SERR, G(I av.), presented similar values for the specimens with adhesive bond line thicknesses of 0.4, 1.1 and 2.6 mm (G(I av.) = 0.71, 0.61, 0.63 N/mm, respectively). However, it increased by approximately 63% for 4.1 mm (G(I av.) = 1.16 N/mm) and decreased by about 10% (in comparison with 4.1 mm) for the 10.1 mm (G(I av.) = 1.04 N/mm). The trend of the G(I av.) in relation to the bond line thickness is explained by the combination of three factors: the crack path location, the failure surfaces features and the stress field ahead of the crack tip.