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Session 2

Failure modeling of a self piercing riveted joint using LS-DYNA
Besides the basic product requirements, the aspect of energy efficiency is in the center of automobile engineering. A mixture of different light weight materials like aluminium and higher strength steels, called multi-material mix, is used increasingly to fulfill these requirements and reduce the weight of the vehicles. Hence the challenges for the joining technique are increasing. Mechanical joining techniques like self piercing riveting have great potential to fulfill this challenge. In particular the joints are the highest loaded parts during crash loading and overloading situations and have to be modeled in crash simulations. Joints are modeled with simplified elements in crash simulations due to efficiency. The simplified models should be able to reproduce the deformation and failure behavior as well as the energy absorption of the joints with less computational cost but with adequate accuracy. In this paper the modeling possibilities in LS-Dyna are investigated for a self piercing riveted joint of aluminium sheets. Beams, eight-noded hexahedrons, hexahedron clusters and constrained elements have been used for a simplified modeling of the riveted connection. The material models MAT_SPOTWELD, MAT_SPOTWELD_DAIMLER, MAT_ARUP_ADHESIVE, MAT_COHESIVE_ MIXED_MODE_ELASTOPLASTIC_RATE and the constrained models CONSTRAINED_SPR2 and _SPR3 have been tested with the simplified rivet model. The failure models are based on forces and moments, on normal, shear and bending stresses, on stresses and fracture energies and on forces and displacements for the constrained SPR models. The model parameters were determined by simulation of specimen tests under tension, lap-shear, peel and combined loading and by fitting the measured force vs. displacement curves. The different numerical results are compared concerning the measured load bearing capacities and energy absorption. The comparison showed that the hexahedron element with MAT_COHESIVE_MIXED_MODE_ELASTOPLASTIC is the most promising model for self piercing riveted joints in aluminium sheets because of the good description of the measured force vs. displacement curves and energy absorption under tension and lap-shear loading. The weakness of this model is the insufficient modeling of the peel loading and the lack of a possibility to control mixed mode loading. The paper gives a recommendation for further developments of modeling self piercing riveted joints.
Development of an improved screw model at faurecia
Some years ago Faurecia used a very simple screw model. In this model the screw holes are filled by a rigid body and the screw shaft is modeld by a simple spring beam with an unrealistic stiffness. But this model doesn’t represent the behavior of screws in real test. With this simple model it was not possible to get the peeling effect of the holes. And the deformation of the part and the screw was unrealistic. Also the forces inside the screw were too high. Due to the goal to reduce the time to market and the number of prototypes to develop a new product it was necessary to develop a better screw model. At the end of 2008 a new keyword was available in LS-Dyna with which it was easy to implement a defined preforce on a beam element. This was the point to start with an improved screw model.
Phenomenological driven Modeling of Joints
In the construction of automobiles different technologies are used to join sheets. Today the most common method for connections is resistant spot welding. There are thousands of spot welds in body-in-whites, which are determining the behavior of the structure under crash conditions. New research is striving after replacing these thermal joints by adhesives or rivets for optimization of the production process. It is essential to ensure a cost saving and time optimized car development to reduce the required number of experiments by using precise simulations. For the quality of such simulation models accurate reproduction of the mechanical joint behavior is necessary. Because of the fact that the element size is bound by the time step in explicit finite elements schemes, a detailed model for the joints is not applicable in typical body-in-white simulations under crash conditions. For this reason simplified models using beam or solid elements have to be used to represent the connection. In the majority of load cases current modeling strategies do not show the required accuracy needed for design decisions. Therefore new ways of spot weld modeling and approaches for rivets modeling should be investigated. For spot welds several strategies of modeling exist. They all are based on the effort to reproduce the spot weld behavior by using specially adopted constitutive or structural models. For these models parameters have to be determined by comparing the maximum load under tensile, shear and peel conditions with corresponding numerical investigations. In the present paper it will be shown that a model with a relative small number of elements driven by a rigorous phenomenological approach can achieve better results. The quality of the proposed model will be evaluated by comparison of tests with KS2-specimen. In the field of self-piercing rivets an established modeling technique doesn’t exist. In this paper firstly capabilities of several modeling techniques will be investigated and secondly they will be compared with a model based on the aforementioned approach.
Process development for multi-disciplinary spot weld optimization with CAX-LOCO, LS-OPT and ANSA
The number of connection entities in modern car constructions is growing continuously. From that point of view, the identification of the most suitable structural behaviour of various car body configurations with respect to the number and the arrangement of connections becomes a challenge in automotive development. A standard simulation and optimization process was developed and established in a common project with the Audi AG and DYNAmore GmbH. The simulation model assembly process consists of a car body without any connection entities, a structured data format that describe the connections in detail and an automated process that realizes the connections using ANSA. All of these components are administrated and provided through the AUDI specific simulation data management tool CAx Load Case Composer (LoCo). This software is developed by DYNAmore and provides, among other innovative features, the possibility to parameterize components of the simulation model. With that ability at hand, it becomes possible to introduce parameters for the number of spot welds on a specified line. With the automated assembly process, the simulation engineer becomes able to investigate a number of spot weld configurations with a minimal amount of time and specific process knowledge. Connecting this parameterized assembly process with a structural optimization software like LS-OPT, provides the possibility to set up a systematic investigation of spot weld configurations with respect to any simulation response representing structural performance. The reduction of the total amount of connections under consideration of constraints can be one goal of such an investigation. Also the adjustment of a desired structural stiffness or the control of the deformation behaviour by the connection setup might be possible objectives in that context.