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.
Spotweld Failure Prediction using Solid Element Assemblies
One current methodology for spotweld modelling utilizes a tied contact to connect the weld elements to the components. In order for this contact to be robust and acceptably mesh independent, multiple solid elements are needed to represent a single weld. Several studies were conducted which concluded that a cluster of eight hex elements provides significantly improved performance over a single beam or hex element. However, ease of use is critical to the application of these spotwelds since thousands of welds can be present in a single full vehicle model. Therefore, a single output is generated for each weld assembly rather than on an element basis. The time step for these hex clusters is controlled by the smallest edge length so using multiple elements does not result in a time step penalty since the thickness of the weld is usually the smallest edge length. This paper will present the development of the eight hex cluster weld, followed by the validation process of these cluster spotwelds. Failure parameters for the resultant-based Mat 100 Damage-Failure model were derived by simulating coupon tests of single welds in shear and tension failure. These failure parameters were then used in a component test model with dozens of welds, several of which failed under the applied load. Finally, these parameters were applied to a full vehicle model using automatic sorting of the welds by the pre-processing software. At both the component and full vehicle level, good agreement was found between simulation and test results. The additional mass scaling and run time penalties of the cluster spotwelds were not significant. Furthermore, the effort needed to apply automatic methods to organize the welds is small enough to be practical in the production CAE environment. 6th European LS-DYNA Users’ Conference 2.48 2.2.2
Stress Intensities Derived from Stresses Around a Spot Weld
The stress intensities (stress intensity factors and J-integral) around a spot weld between sheets of dissimilar materials and different thicknesses are generally expressed by the local stresses around the spot weld. The derivations are established on an interface crack model which represents the radial cross-sections of the spot weld with the local stresses varying from point to point along the periphery of the nugget. The J-integral is determined analytically by applying the equilibrium conditions and the elementary plate theory to the model. The stress intensity factors are then separated by following a load decomposition procedure and an analytic solution to interface cracks. The thermal stress intensities are also obtained for the spot weld. Application examples are given for a tensile-shear specimen.