رفتار فرسودگی تکنیک های اتصالی مختلف
Abstract
1-Introduction and Motivation
2-Theoretical Background
3-Experimental Results
4-Numerical Analyses
5-Summary and Outlook
References
Abstract
The design of parts and structures in the vehicle development process is highly influenced by strength and stiffness requirements. Under cyclic loading a change in the local stiffness of joints can be observed which motivates analyses in order to tackle stiffness changes in structures. The fatigue and stiffness behaviour has been obtained for several joining techniques and material combinations: spot and seam welds, remote laser welds and high velocity bolting for steel and/or aluminium partner sheets. Specimens under shear and peel loading as well as hat profiles under torsion have been tested under constant amplitude loading. Quasi-static tests and micrographic evaluations have been conducted in order to determine material parameters and the setup of FE models. Online stiffness measurements during fatigue life have been performed. In this paper an approach to describe degradation in specimens representing a joint detail is presented. The stiffness degradation curves for the various joining techniques are compared and a numerical description of the stiffness degradation behaviour for further simulations is derived. The concept of simulating stiffness changes in conventional joining techniques is introduced. First numerical analyses of stiffness changes in spot welded specimens are presented. Several sensitivity analyses, e.g. with regard to R-ratio or loading amplitude, are performed in order to check the numerical stability of the implemented method.
Introduction and Motivation
Strength and stiffness requirements highly influence the dimensions and therefore also the weight of components and structures. In order to meet increasing customer demands and to reduce the overall vehicle emissions, light weight construction is an essential aspect in the vehicle development process. To achieve a lightweight construction a diversity of materials and joining techniques is applied. To illustrate the usage of connections in the automotive industry, Figure 1 shows the spot and seam welds in the current BMW 7 series. More than 5000 spot welds are used in the vehicle chassis. Especially joints are known to change their stiffness during fatigue life which can have an impact on the local stress state. As a result of local stiffness changes, fatigue life may be affected positively or negatively. An elongation of fatigue life can occur if, due to changes in the local stiffness, stresses redistribute in areas which have observed lower stresses in the previous fatigue life. Under different conditions a reduction of fatigue life may also be a consequence of degradation. The stiffness behaviour often depends on loading direction and load level, material combination or the used joining technique, respectively. In the first part of the paper a general comparison of the fatigue and stiffness behaviour of various joining techniques, material combinations and loading conditions is performed.