A finite-difference model with mixed interface laws for shear tests of FRP plates bonded to concrete

In the last years the strengthening of existing structures by means of externally glued fiber-reinforced polymer has become increasingly popular. This stimulated the study of new numerical and analytical models to understand the ultimate behavior of this reinforcement technique. Simultaneously, many experimental tests permitted to identify the main parameters involved in the failure process, which often occurs by debonding (i.e. with a crack propagating into the support underneath the reinforcement). Recent studies clarified the role of the coupling between normal and tangential stresses, pointing out that tensile stresses orthogonal to the bonding plane (peeling stresses) could be detrimental to the bond behavior. The effects of the peeling stresses are substantially related to a local reduction of the shear strength along the bonded length and to a variation of the failure mechanism. To investigate these issues, here a numerical model for debonding of FRP reinforcements is proposed by adding the effects of normal stresses to the classic pure shear cohesive zone model. The interaction between the reinforcement and the support is accounted for using an interface cohesive law, coupling tangential and normal behaviors. Finally, comparisons with experimental evidences and already validated models are presented to assess the capabilities of the proposed approach.