Theoretical study based on 2D assumptions of the influence of small pores on crack initiation in adhesively bonded joints

The intrinsic performance of epoxy adhesives seems to designate them as the ideal solution for assembling certain materials. Pore can have an influence on the load at which the first crack is initiated and on the final strength. In the context of damage tolerance, this work aims to explore the effect of small voids that cannot be detected by conventional non-destructive testing (NDT). To attain this goal, a method based on a criterion coupling strength and toughness is used. 2D finite-element calculations are performed on simple configurations to understand the steps leading to failure when pores are present. When the defects are far from the edges, the loss in load at the initiation of the first crack is a function of the minimum distance between two pores. When the location of the defect interacts with the stress concentration due to a free edge, this loss is a function of the minimum spacing between pores and the free edge. The knock-down factor on the load at failure for a bonded joint with epoxy adhesives remains small (close to the defect area divided by the total area of the bondline). The presence of pores may change the fracture surface (from an adhesive one without pore to a cohesive one with pores).