Fatigue Evaluation of Flow Drill Screw Joints Using the Equilibrium-Based Structural Stress Method 2020-01-0189
According to the increasing demands for light-weight design in the automotive industry, the use of thinner and lighter materials such as aluminum alloys for automotive parts has led to significant weight reduction. The joining of these materials has required development of new technologies in joining/fastening rather than welding. Flow drill screwing is one of the latest technologies created to fasten sheet metal panels.
This paper discusses results of an evaluation of fatigue characteristics of flow drill screw (FDS) joints based on experimental data and observations from the literature. It was observed that the important fatigue-related geometric parameters of FDS joints were the gap between sheets and the extruded (or bulged) zone during screwing. Major failure modes were observed such as sheet failures where cracks grow from the inner surface of the sheet and around the extruded zone.
In this paper, the fatigue evaluation procedure for FDS joints using the mesh-insensitive equilibrium-based structural stress (ESS) method was used. The ESS-based simplified modeling procedure was used for the fatigue-evaluation procedure for FDS joints. The effectiveness and applicability of the ESS-based simplified procedures for modeling and simulating resistance spot welds and self-piercing rivet (SPR) joints have been proven by previous investigations. After accounting for geometric and fatigue characteristics of FDS joints, an ESS-based simplified procedure and a master S-N curve approach-based procedure were established. It was observed that the fatigue data in the interim design S-N curve became consolidated with a small scatter band regardless of sheet thickness and sheet preparation (with and without a clearance hole).
Additionally, the design S-N curve for FDS joints is a little bit lower than that for SPR joints from the interim design S-N curve comparison. From the residual stress observation for FDS joints and SPR joints, it can be inferred that the compressive residual stress distribution around the SPR helps to improve fatigue life compared with FDS joints.