Adhesive thickness influence on the shear fracture toughness measurements of adhesive joints

To increase the confidence in the design of bonded structures, it is important to accurately predict their mechanical strength. In this context, Fracture and Damage Mechanics approaches have significant advantages over Continuum Mechanics. The Fracture Mechanics approach is often applied by an energetic analysis, where the main parameter to predict the evolution of damage and failure is the critical strain energy release rate (GC). This parameter can also be divided into the tensile (GIC) and shear (GIIC) components. Cohesive Zone Models (CZM) are a powerful strength prediction technique within the scope of Fracture Mechanics and take advantage of cohesive laws to induce crack growth in the numerical models. This work aims to estimate GIIC of adhesive joints of composite structures by the End-Notched Flexure (ENF) test, considering different values of adhesive thickness (tA). In this context, different data reduction methods to estimate GIIC are compared. On the other hand, the shear cohesive laws of the adhesive layer are obtained by the Finite Element Method (FEM) and CZM using an inverse technique, enabling to understand the influence of tA on the joints' shear behaviour. The numerical models accurately captured the observed experimental behaviour of the adhesive. A clear dependency was found regarding both GIIC and the shear cohesive strength (ts0), depending on tA.