A pragmatic approach for the fatigue life estimation of hybrid joints

Abstract

The need for more environmentally sustainable ways of transportation and for a reduction in emissions and fuel consumption make lightweight structures essential. Together with the use of lightweight materials and design optimisation, the use of hybrid joints represents one way to reduce the weight of components and it is becoming increasingly popular in the transportation industry. The name ‘hybrid joint’ refers to a connection where adhesive bonding is used in conjunction with traditional joining techniques, such as spot welds and rivets with the aim of combining and exploiting the advantages of the individual joining techniques. To optimise the design of hybrid joints and minimise 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 hybrid joints.

This work presents two practical methodologies for estimating the fatigue life of hybrid joints that can be easily adopted by companies in the transportation industry. The first methodology neglects the life given by the mechanical joints after failure of the adhesive (the joint is considered failed when the adhesive fails), while the second methodology considers the life of both the adhesive and the mechanical joints. In the first methodology, just one configuration would need to be analysed (‘hybrid’ joint or ‘purely bonded’ joint, if this simplification is considered reasonable). In the second methodology, instead, the analysis of two configurations would be required (the previous configuration followed by a configuration where only the mechanical fasteners are considered). The second methodology would produce more realistic fatigue life estimations compared to the first methodology, but it would be more onerous both in terms of modelling and computationally. For both methodologies, FE modelling guidelines to recover the required stresses are suggested. These guidelines require limited changes to the typical FE modelling strategies currently used, especially in the automotive industry. Furthermore, the proposed modelling guidelines provide FE models that are not computationally too onerous, reasonably mesh insensitive and that do not require congruent meshes. The relevant stresses recovered from the FE model are then used as an input into nCode DesignLife to estimate the fatigue life of the hybrid joints in the analysed structure. The fatigue life estimation is carried out using standard Stress-Life (SN) based nCode DesignLife analysis engines and bespoke SN curves obtained through testing of hybrid joint specimens, representative of the joints in the production parts.