ISBN 9783843933179

978-3-8439-3317-9, Reihe Ingenieurwissenschaften

Sinu Joseph
Improvements in Finite Element Spot-weld Modelling and Fatigue Life Estimation

149 Seiten, Dissertation Technische Universität Dresden (2016), Softcover, A5

Zusammenfassung / Abstract

Resistance spot-welding is commonly used to join sheet metal parts in the automotive industry. Durability of the vehicle body is largely determined by the fatigue lifes of the spot-welds. Finite element (FE) based methods are used to assess structural performance early on in the vehicle development process. In the FE model of a vehicle body, the spot-welds are represented by simplified structures. Currently, the area contact model 2 (ACM2) is used which consists of a solid element placed between the shell element layers, with interpolation elements joining shell and solid nodes to constrain relative displacement. The present method of using the interpolation elements makes the ACM2 stiffness very mesh-sensitive.

In order to minimise the mesh-sensitivity of ACM2, an alternate method of using interpolation elements is developed. It ensures that a uniform area of shell mesh around the spot-weld is constrained. Further, the constraint level at each shell node is determined by a suitable function. It is observed that the stiffness of the modified spot-weld model remains uniform for different mesh sizes. The stiffnesses of these simplified spot-weld models are nevertheless not comparable to that of a detailed model of the spot-weld, particularly for bending and torsion.

A method to independently adapt these stiffnesses by using transversely isotropic material for the solid element is proposed. The material constants are obtained based on deviation of stiffness from a detailed model as well as relative compliance of the spot-weld within a double cup specimen. Application in a component-like specimen gives weld loads that are comparable to that of the detailed model.

The stress parameter for fatigue life prediction is currently estimated based on an analytical plate model of the spot-weld. The outer diameter of the circular plate has a fixed ratio to the spot-weld diameter which overestimates the calculated stress for coach-peel loads. By comparing the stresses from detailed shell element models to analytically obtained stresses, a relation between flange widths and outer diameter was derived. Scale factors to modify analytical stresses from tensile-shear loads were also obtained which improved accuracy of the estimated stress. The modifications reduce the scatter of S-N curves in most specimens that were investigated.