A practical methodology for the fatigue life estimation of adhesive joints

Abstract

The use of adhesives in structural joints as an alternative to traditional joining techniques, such as welding and rivets, is becoming increasingly popular in the transportation industry, including (but not limited to) automotive and aerospace. This trend is driven especially by the increasing need and focus on greener ways of transportation and the consequent light weighting requirements. Furthermore, adhesives have the significant advantage, over traditional joining techniques, of allowing joining of dissimilar materials, including composites. Therefore, to optimise the design of adhesively bonded structures and reduce the risk of in-service fatigue failures, the transportation industry needs efficient, robust, and easy-to-use approaches for the modelling and fatigue life estimation of adhesively bonded joints.This work presents a practical methodology for estimating the fatigue life of adhesively bonded joints that can be easily adopted by companies in the transportation industry. The proposed methodology includes Finite Element (FE) modelling guidelines to recover the required stresses along the bond lines, with limited changes to the typical FE modelling strategies currently used, especially in the automotive industry. It is proposed to model the adherends using shell elements and the adhesive layer with solid elements. The exposed edges of the adhesive are then wrapped with membrane shell elements used to extract the peel stresses. The solid elements and the shell elements are connected using ‘bonded contacts’ (or similar tools, depending on the FE software used). The proposed modelling guidelines provide FE models that are not computationally too onerous, with a good level of mesh insensitivity and that do not require congruent meshes. The peel stresses recovered from the FE model along the bond lines are then used as an input into nCode DesignLife to estimate the fatigue life of the bonded joints in the analysed structure. The fatigue life estimation is carried out using the standard nCode DesignLife SN analysis engine and bespoke SN curves obtained through testing of adhesively bonded specimens, representative of the joints in the production parts.