Study And Application Of Influences Of Spot Weld Layout On Structural Crashworthiness

Spot welding is still a commonly used joining method in vehicle structures for its advantages in stiffness, durability and cost. There are usually thousnands of welds on a signle vehicle body. These welds form discrete joining points and produce stress concentration areas on the vehicle structure. The welds to some extent may be considered as imperfections of the vehicle body. Therefore, spot weld layout is an influential factor to the crashworthiness, durability and NVH performance.Energy absorbing structures in vehicle body are designed to absorb as much kinetic energy as possible in a limited deformation displacement for protecting the occupants. Most typical energy absorbing structures on vehicle body are close-hat thin-wall sections. The vehicle crash performance is mainly dependent on the deformation mode and energy absorbing efficiency of these structures.In this thesis, the relationship between spot weld layout and deformation mode of typical energy absorbing structures, such as single-hat, double-hat and curved tubes, is studied. Key parameters of spot weld layout have been determined. Using finite element analysis, the deformation mode and energy absorbing characteristics under different spot weld layout design parameters have been studied. With the simulation results and the plastic hinge theory, the influence of spot weld layout on the deformation stability and energy absorbing capability of the structures has been discussed. It is found that, as the extent of the imperfections increases in the structures, the spot weld layout design would exert less influence on the structural performance. Meanwhile, the influence of spot weld layout is also closely related to the loading types and the boundary constraint conditions on the structure.In the automotive industry, with the current technologies, it is still not possible to consider all the vehicle design aspects, such as styling, performance, manufacturability, etc., in a single design optimization process. Iterations between design for performance and design for manufacturing are typical. After the performance design, the spot weld layout in vehicle body often needs to be adjusted due to certain kind of geometrical constraints in welding process, and thus the structural performances would have to be reassessed accordingly. Such design iterations are a barrier to increasing the vehicle development efficiency and shortening the development cycle. In this thesis, a new approach has been proposed for the concept of Design for Manufacturing. The manufacturing constraints of welding are included into the structural performance design stage. With such a concept, the structural design would achieve performance in a manufacturable way and aforementioned issue could be resolved. In the meantime, the crash performance of energy absorbing structures under different spot weld layouts can be predicted through the response surfaces generated in the study. And the sensitivity of spot weld layout under different boundary conditions has also been presented.