Analysis of the substrate nature on the strength of a single-lap joint

Nowadays, the adhesive bonding process takes an important place in several industrial fields, especially in aeronautics. Given its advantages over other conventional mechanical processes, this process is being extended to be applied in composite materials and, in recent years, more particularly in the bonding of functionally graded materials. In bonded assemblies, the adhesive is the weak link due to its mechanical properties which are very weak compared to those of the two substrates. To this end, current research aims to reduce the concentration of stresses in the adhesive joint by choosing substrates whose mechanical properties are adequate with those of the adhesive in order to minimize load transfer and ensure a robust assembly. The present work analyzes, by the finite element method, the mechanical behavior of a single-lap joint with varying nature of the substrates. The analysis takes into account the variation of stresses in the adhesive of a bonded joint of different types (metal/metal, composite/composite, FGM/metal, FGM/FGM). On the other hand, an attempt has been made to introduce an adhesive with graded mechanical properties made up of two types to ensure efficient joining and reduce stress concentrations at the bond edges. The introduction of the FGM substrate mechanical properties is done using a USDFLD (User subroutine to redefine field variables at a material point) subroutine implemented in the ABAQUS computer code. Different damage approaches were used for the adhesive, namely the virtual crack closure technique (VCCT) and cohesive zone models (CZM) techniques. The effect of the mechanical properties of the substrates and adhesive were considered. The results show clearly that the value of the different stresses can be reduced if the mechanical properties of the substrates are optimized. On the other hand, the different techniques used to model the bonded joint converge towards the same results, emphasizing the agreement of the load-displacement curves with the experimental test, and the variation of the stresses according to the lap length with analysis by analytical models.