Interfacial damage in composite strengthened beams - Derivation of a non-linear model and extraction of material parameters

The bonding of composite plates to the tensile side of concrete beams or slabs has recently been accepted as an effective retrofitting method. A concrete member retrofitted in this manner often fails through the delamination of the composite. Most existing models for the analysis of delamination assume linear elastic material behaviour until ultimate failure of the component. This is contradictory to experimental observations that clearly indicate the occurrence of non-linear damage processes at load levels far below the ultimate load. In this paper, a novel nonlinear fracture mechanics approach for the analysis of delamination failure is introduced. The delamination is modelled as an interfacial crack, with progressive interfacial damage represented by material softening at the crack wake. The focus of the paper is on the determination of the interfacial shear stress vs slip (tau(s) - s) relation. A new testing method to study progressive delamination is first described. A finite element model for the specimen is then set up, with softening elements at the plate/beam interface. By comparing experimental measurements to finite element results using different softening behaviour at the interface, the constitutive relation of the interface elements, in terms of shear stress vs sliding displacement, can be obtained. From our results, the softening behaviour exhibits a sharp drop followed by more progressive weakening, indicating sudden loss of cohesion/bond at a critical stress level, and a more gradual reduction in friction with increased sliding and damage.