Finite element modelling and updating of structure of sheet metal with bolted and welded joints

A large and complex structure such as a car body-in-white typically consists of several major components that are produced from thin metal sheets. The components are joined together by different types of mechanical joints such as rivets, spot welds and bolted joints. These mechanical joints are highly influenced the overall dynamic behaviour of structures. The finite element method has been widely used for predicting the dynamic behaviour of structures. However, to model the local effects (such as slip, loosing, or clearance effect) arising from the joints is cumbersome and time consuming. Moreover, the predicted model is often found to be inconsistent with the measured data. The discrepancies are believed to be arisen from the invalid assumptions about the model and properties data of the initial finite element model. This thesis puts forward the idea of using a simple and practical bolted joint modelling considering local effects of the area of the bolted joints of thin metal sheet structures. CFAST element and initial stress ratio are used in modelling the bolted joints and the local effects of the mating area between bolt, washer and surface of the structure. The properties of the parameters of the CFAST element and the initial stress ratio are used in the model updating procedure. The advantage of this technique allows the local effects of the bolted joints to be modelled in a simple way and it proved to be successful in modelling bolted joints. The influence of the stiffness of suspension springs which is used in simulating free-free boundary conditions in the experimental work especially for the structure that is made from thin metal sheet is investigated as well in this thesis. In the investigation, CBUSH element is used to model the suspension springs and the stiffness of the spring is taken into account as the updating parameter for the finite element model of the full welded structure with free-free boundary conditions. In this thesis, the use of the simple and practical modelling is adopted in the development of the finite element model of the full welded structure that consists of ten components made from thin metal sheets joined by spot welds and bolted joints. The model is updated using the results obtained from the experiments via the application of two model updating methods. They are iterative method and response surface method (RSM). In the iterative method, the NASTRAN SOL200 is used to improve the finite element models of the components and of the welded structures. The work in the iterative method is divided into two parts. The first one, the finite element models of components are updated in order to reduce the discrepancies of the natural frequencies before they are assembled together. Meanwhile, the second part is the updating process of the finite element models of the welded structures by concentrating on joint modelling and updating. Finally, the response surface method (RSM) is used in updating the model of the full welded structure with fixed boundary conditions due to bolted joints. The Latin hypercube sampling is used to generate numerical samples. The accuracy and efficiency of both methods (iterative method and RSM) are presented and discussed.