Search Results

Utilizing the Finite Element Method (FEM), two evolving CAE methods have recently reached a high level of efficiency and accuracy to optimize the properties of components in respect of stiffness, stress level or fatigue resistance. The first type of CAE-methods is a family of optimization methods known as (parameter-free) shape and topology optimization. Very remarkable and useful results can be generated with these methods to reach considerable stiffness increases or stress and weight reductions. The second CAE-method is fatigue life prediction which gives reasonable accurate outputs for component life if input data like stress history and material properties are well known. State of the art algorithms and software can handle a combination of complex load histories, detailed material description and large FEM-data to give reliable results in short time.

Self-piercing rivets are a young technology of spot joining which becomes more and more important in car body engineering because of some advantages compared to other spot joining technologies as e.g. spot welding. For Finite Element analysis a local model for self-piercing rivets has been developed. The requirements of this model are (1) to represent the correct local stiffness behavior of self-piercing riveted joints and (2) to be suitable for fatigue analysis. This model was developed analogously to a local model which is already in use for the fatigue assessment of spot welds since several years. The local stiffness behavior of the model has been adapted to test results of different self-piercing riveted specimens. The local mesh refinement is performed automatically by the SPOT preprocessor module of the fatigue software FEMFAT. Both node dependent and node independent remeshing are supported.

For lightweight automotive structures, the stiffness and the fatigue behaviour is greatly influenced by the properties of its joints. The used joining technology, the number and locations of the spot joints are of high importance for both engineers and cost accountants. An overview of common computational procedures including European and national standards is given for the assessment of the stiffness and fatigue behaviour of thin sheet structures with spot joints and arc welds. The influence of the quality and the size of finite shell elements on the fatigue result is investigated and it is shown, how this influence can be minimized.

Fatigue life prediction has reached a high level in respect to practical handling and accuracy in the last decades. As a result of insecure or lacking input data unacceptable deviations between numerical results and test results in terms of cycles till crack initiation are possible. On the one hand, the accuracy of Finite Element results gets better and better because of greatly increasing computer power and mesh density. Whereas on the other hand, the situation is much more critical regarding load data and especially regarding local material properties of the components (compared to specimen data). But in the last few years also the possibilities of process simulation have improved in such, that at least a few local material properties or quality indicators can be predicted with sufficient reliability.

Structural optimization and fatigue simulation became standard applications in the various phases of design development processes within the last years. Introducing fatigue simulation into the optimization procedure enables to use all the available methods from the field of fatigue also for structural optimization. This means, beside correct consideration and combination of static and dynamic load portions, also the option for assessing different joining technologies like spot welds. Beside gaining better performance of spot welded structures, economic aspects drive the development of optimization in the field of spot welds. Therefore different methods like sensitivity based approaches or genetic algorithms have been used for some time. The application of standard software for topology optimization was investigated for the purpose of automated finding optimal spot weld distributions.

In today’s automotive industry, mechanical engineers are encouraged to develop lightweight vehicles to reduce the consumption of energy. At the same time, the service life and safety standards, which become m ore and more rigorous, must be fulfilled. Numerical analysis of the component’s lifetime in an early stage of the development process can increase the reliability of automotive structures, and lead to shorter development periods and cost reductions due to a decrease in testing expenditures. Most cracks in fatigue testing originate in notches, welds or spot-weld joints. The dimension of the notches, the design and the position of the weld seams, as well as the number and the location of the spot weld joints have a significant technical and economical impact. In order to achieve an optimum use of the material, an optimization of these critical areas has to be performed.