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Prediction of failure on high strength steel in seat mechanisms simulation

Tracks are the mechanisms which enable to translate the seat; they are key contributors in occupant safety as link between seat and car. With the current evolution of ecologic legislation, one of major automotive industry priorities is to decrease the product mass. To reach this objective, the use of high strength steels appears as a good solution with the drawback to be more brittle. In parallel, FEA models have to be more and more predictive in order to reduce the validation cost. In this context, rupture risk prediction appears as a strong need from design office and usual post-processing methods are not accurate enough to bring sufficient support to design teams. The solution chosen is a coupling between Ls-Dyna and the failure criteria crachFEM developed by MatFem Company. The evaluation of this risk is based on plastic strain evolution and stress state of the element. The methodology requires a specific characterization of the material to get information about the failure for different stress states. First application has been launched on ultimate strength subsystem on track. With dual- phase material, primary track failure mode is generally a profile rupture. First results highlighted correctly the area of rupture, but the ultimate strength was generally higher in FEA model than in the hard-test. This gap can be explained by the difference of scale between characterization of failure, which is a very local phenomenon, and the evolution of strain in simulation which is dependant of mesh size. Industrial crash model requirement (best compromise between accuracy and computation time: around 3mm mesh size) doesn’t permit to use mesh size needed for accurate rupture prediction. So “hybrid” modeling has been developed in order to have mesh size appropriate to MatFem analysis in useable computation time. With this approach the ruptures are well identified in term of areas, kinematics & ultimate strengths. Nowadays we are able, on this product, to predict with accuracy a risk of rupture on subsystem or on complete seat crash test.