Interface crack model using finite fracture mechanics applied to the double pull-push shear test

An analytical procedure predicting a debond (interface crack) onset and growth in an adhesive joint between two beams or plates is developed and applied to a specific configuration often used in reinforcement tests in civil engineering. The procedure is based on Timoshenko beam theory and Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) combined with the Coupled Criterion of the Finite Fracture Mechanics (CC-FFM) for mixed-mode fracture. First, a sixth order differential equation in the shear stresses along the adhesive layer is deduced and solved, leading to closed form expressions for both shear and normal stresses in the adhesive. Then, the critical value of the applied load necessary to produce debonding is predicted by coupling a stress and an energy condition based on: (i) the stress distribution produced in the interface before the debond onset and (ii) the energy released during the debonding process along the interface. Although the developed procedure can be applied to several types of joints with different geometries, materials and loads (e.g., double lap joint tests including adherents made of steel or composites), herein it is applied to the double pull-push shear test where the debond onset and growth between a Carbon Fibre Reinforced Polymer (CFRP) laminate and a concrete block occurs. For such a case, the debond is produced under predominant fracture mode II; nevertheless, it is shown that relevant normal (peeling) stresses associated to mode I may appear as well. A comparison of the present solution with a previous one by the shear-lag model is provided as well.