Spot-Weld Fatigue Optimization

Abstract: The purpose of this thesis project is to develop a methodology that can be used to minimize the number of spot-welds in a mechanical structure, this is done in a reliable manner via optimization methods. The optimization considers fatigue life in spot-welds and also stiffness and eigenfrequency values. The first chapter of this thesis presents a spot-weld fatigue model proposed by Rupp (1995), common FEmodels of spot-welds and also important aspects about structural optimization in general. The second chapter further describes how topology optimization and size (parameter) optimization are applied on a simple multi-weld model with respect to the aforementioned structural constraints. The topology optimization is later used on a full-size car model, while the size optimization is used to optimize the multiweld model by adding an non-linear structural constraint - a crash indentation constraint. The spot-weld fatigue model proposed by Rupp (1995), is also verified by comparing FE results using different FE-models of spot-welds compared to fatigue data by Long and Khanna (2007). Both optimization methods successfully minimize the total amount of spot-welds on the multi-weld model. The topology optimization,accompanied with thegradient based MFD algorithm,minimizes th etotal spot-welds with around 15% and 3% on the multi-weld model and car body respectively. The size optimization, using design of experiments and response surfaces, manage storeduce the number of welds in the multi-weldmodel by 25%. However, with the size optimization the computational time is several orders of magnitude longer-even without the formulation of the crash constraint. The fatiguespot-weld model fares reasonably well compared to the experimental fatigue data, regardless of the FE model of the spot-weld. It is concluded that the ACM model would be recommended based on its compatibility with fatigue and optimization methods, mesh-independence and also other studies have shown its ability to represent stiffness and eigenfrequency correctly.