Adhesion and Fracture Behaviour of Curved FRP Composite Single-Lap Joints Using 1D and 2D Functionally Graded Adhesives: A 3D Finite Element Study

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Abstract
This paper presents Three-Dimensional (3-D) non-linear Finite Element (FE) analyses to assess the structural behaviour of curved laminated Fibre Reinforced Plastic (FRP) composite panel-based Single Lap Joints (SLJs) with Functionally Graded Adhesive (FGA) featuring pre-embedded adhesion failure. The principle of the fracture mechanics concept is used to investigate the behaviour of interfacial stresses (peel and shear stresses) along the adhesion failure fronts at critical locations. Corresponding three modes of Strain Energy Release Rates (SERRs) are considered as the damage growth parameters and are evaluated using Linear Elastic Fracture Mechanics (LEFM) based Virtual Crack Closure Technique (VCCT). Based on the stress analyses, it has been observed that the adhesion failure in the SLJ structure triggers at the interface between the adhesive and the lap adherend. Depending on the SERR magnitudes, it has been observed that interfacial damage propagates under mixed-mode conditions. As a result, the controlling parameter for damage propagation is total SERR (GT). Furthermore, instead of using a homogenous adhesive substance, FGA has been used to slow down the rate of damage progression. One of the most significant benefits of FGAs over homogeneous adhesives is that the material properties change continuously in the desired direction from point to point, although these materials are microscopically inhomogeneous. FGA bonded joints have a continuous change in the adhesive properties throughout the bonded region, allowing for the homogeneity of stress distribution and load transmission to increase the load-carrying capacity. Series of numerical simulations have been performed using different kinds of FGAs, like unidirectional (1D) and bidirectional (2D), with appropriate function profile in order to achieve significant minimization interfacial stresses and SERRs. Material gradation of adhesive indicates significant interfacial stresses and SERRs reduction at the incipient stage of failure, which necessitates the use of FGA for SLJ and prolongs the service life of the structure.

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