An Investigation on Spot Weld Modelling for Crash Simulation with LS-DYNA
Because of weight reduction necessities, high-strength steels are more and more widely used in bodyin-white (BiW) structures in recent years. Concerning the behaviour of such structures during a car crash the joints between high-strength materials are seen as critical points. Therefore the properties of welded joints, especially the failure behaviour at high velocities have to be taken into consideration during the development phase. In this paper, we present an overview on the current activities at the DaimlerChrysler AG regarding investigations on spot weld failure behaviour. A suitable model that is able to represent the failure behaviour of spot welds in BiW structures and that is independent of mesh sensitivity has been developed. An elasto-plastic material model based on von Mises plasticity (MAT_100) has been further enhanced with a new failure criterion. The model has been implemented into LS-DYNA. The material as well as the failure behaviour is verified and calibrated through precision experiments conducted on specimen level and later validated on component level.
Investigation of Spot Weld Behavior Using Detailed Modeling Technique
Summary: The development of suitable and robust simplified spot weld m odels for full vehicle crash simulation meets one of the major demands of the development process regarding efficient and appropriate full car crash models. Normally a high number of experiments are required to derive and validate accordant material and failure parameters for each material and gage combinations. Concerning systematical investigations, the effort for manufacturing and testing c an increase immoderately. In this paper, we describe a method to reduce the effort of characterizing the joint failure by using a highly detailed simulation model. Different loading conditions and geometrical aspects can be analyzed by using this model in combination with the Gurson’s materia l model as an appropriate visco-plastic material model which includes already a damage formulation. Especially the experimental investigation of strain rate effects at higher velocities of joints is quit e sophisticated due to dynamic effects of the test arrangement ‐ the magnitudes are often overlaid by oscillations. Using the simulation to evaluate the behavior of a spot weld allows to study and predict the strain rate effect for higher velocities. As a second point, we use the detailed model for a systematic analysis to determine the effect of the sheet thickness with respect to the failure parameters. The influe nce of imperfections which occurs due to the production process can be eliminated, so that a self-contai ned approach can be obtained.
Shape and surface texture of molten droplets deposited on cold surfaces
When a molten droplet impacts on a cold surface, the interaction between solidification and fluid dynamics can lead to different shapes and surface textures of deposited droplets. This article presents an experimental study on deposition of small droplets (39 μm diameter) of molten wax ink on solid surfaces. Our results show that both the final shape and the surface texture of deposited droplets are extremely sensitive to deposition conditions, which include initial temperature of droplets, type of substrate surface and distance between substrate and printhead. Different surface textures are observed and shown to depend on the thermal and flow histories of ink droplets during the process of impact. A test using crystalline wax reveals crystallization as one of the major factors in determining the final texture.