On the crack front curvature in bonded joints

Standard tests of adhesively bonded specimens are likely to produce heterogeneous stress distribution along the crack front and its vicinity. High separation rate mode I dominated fracture test is performed. Observation of post mortem fractured surfaces with an optical microscope reveals characteristic features of mixed mode I/III fracture near the sides of the specimen but not in the middle. At first, finite elements calculations are presented to highlight that the adhesive layer is not loaded in pure mode I. We focus on the effect of mode III contribution near the side of the test pieces. To accommodate the anticlastic curvature of the adherends and the stress heterogeneity in the bondline, the crack front is curved during propagation. The anticlastic curvature of the adherend is evidenced with interferometric profilometry. These experimental observations are compared with finite element calculations in case of straight and curved crack front. Mode mixity along the crack front, local strain energy density and global energy release rate are evaluated. Difference between local and global approaches are discussed. It is found that the local strain energy density criterion suffices to allow for predicting the crack front shape.