FAILURE MODELING OF A SELF PIERCING RIVETED JOINT USING LS-DYNA

摘要

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.