An interface damage model accounting for the confinement effect

The strain concentration in narrow zones can be satisfactorily simulated adopting interface models. The interface reproduces the mechanical response of a thin layer of material where high strain concentration occurs. Interface models are successfully adopted to reproduce several damaging phenomena at different scales, from the micromechanical scale to the geological and tectonic scale. An actual and interesting field of application of interface models is the stress analysis of bond response of external strengthening made of Fiber Reinforced Polymer (FRP) laminates applied on quasi-brittle, i.e. cohesive, substrates (masonry or concrete), with the simulation and prediction of the detachment phenomenona. The decohesion processes initiate and propagate in a narrow region (the adhesive and a thin layer of substrate cover) separating two well defined domains. In fact, experimental evidences demonstrate that as the cohesive substrate is weaker than the glue and the reinforcement,the debonding usually occurs with the removal of a thin layer of the support material, which remains glued on the reinforcement. This thin layer of cohesive material is subjected not only to the classical traction vector but also to in-plane stress components; it is expected that, in presence of tensile or compressive in-plane stresses in the layer of cohesive material, the failure of the interface occurs for lower or higher values of the traction, respectively. In fact, the complexity of the state of stress in the cohesive layer, due to the presence of in-plane effect, can change the failure mechanism and the maximum debonding force. As consequence, the in-plane deformation of this thin layer of cohesive material can play an important (if not fundamental) role in the failure mechanism [1]. An enhanced formulation for cohesive interface that keep into account the effect of in-plane deformations of the gluing surface is presented [2]. Starting from this kinematic assumption an enriched formulation of interface model is proposed. In fact, the kinematics is defined not only by the relative displacement occurring between the two surfaces of the interface, but the strain arising in the plane of the interface is also accounted for. Simple numerical simulations are presented in order to illustrate the capabilities of the model. Later on, some numerical applications are carried out in order to assess the performances of the proposed model in reproducing the mechanical behavior of concrete elements strengthened with external FRP reinforcements. In particular, comparisons between numerical predictions and experimental results are shown.