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.
FE-MODELING OF SPOTWELDS AND ADHESIVE JOINING FOR CRASHWORTHINESS ANALYSIS
The increasing demands with regard to the predictive capabilities and the exactness of crash simulations require more and more investigations into numerical models in order to capture the physical behavior reliably. Steps towards this goal are the usage of finer meshes which allow for a better geometrical representation and more sophisticated material models which allow better prediction of failure scenarios. Another important playground towards improved crash models is the area of connection modeling. Validation in this area is usually closely related to very detailed models which cannot be easily translated into a crash environment due to time step restrictions. Therefore, representative substitute models have to be developed and foremost validated. The aspect that failure of the connections has to be considered as well adds another dimension to the complexity of the task. The present paper highlights the conflict between predictive capability, capture of physical reality and manageable numerical handling. Another aspect of the paper is the attempt to raise the awareness of the topics verification and validation of numerical models in general. This concept is illustrated using latest developments for modeling of spotwelds and adhesive bonding in LSDYNA.
Stress Intensities at Spot Welds
The stress intensities (notch stress, stress intensity factors and J-integral) at spot welds under typical loads of tensile-shear, cross-tension and coach-peel are derived as a number of simple formulas on the basis of an analytic solution where the stress intensities at spot welds are generally determined by the stresses around the spot welds and of some analytic solutions to circular rigid inclusions in plates with the inclusions simulating the weld nuggets. The derived formulas show consistently the trends in the stress intensities with the design parameters for spot welds such as nugget diameter and sheet thickness and additionally with spacing of force for cross-tension spot welds and load eccentricity for coach-peel spot welds. The stress intensities at spot welds under general loading conditions are estimated in terms of the forces and moments transferred by the spot welds based on the derivations. The theoretical predictions from the formulas are compared favorably with the finite element results. As an application example, some fatigue test data for spot welds in the form of load range versus life to failure are transferred into the form of stress intensities range versus life to failure with the scatterband of the fatigue test data being substantially reduced.