Service improvement of high-strength formed sheet steel structures 'Bodylife'

High strength sheet steel provides great potentials for modern light weight automotive body structures. Due to their high strength under static, dynamic as well as under fatigue loading thinner sheets can be used resulting in lower weight of the structure in comparison to traditional mild steel. By cold forming the sheets are deformed to the complex structures within the new design concepts by innovative forming technologies. Such forming technologies increase the capabilities of high strength steels by exploiting the strengthening behavior of the materials due to forming. However, the forming itself changes the material strength and the thickness of the sheets locally. To ascertain a realistic evaluation of the forming on the strength, monotonic and fatigue tests were performed on specimens with different deformation grades, both uniaxially and biaxially. For all materials a significant increase of the yield strength due to prestraining can be noticed. Under cyclic loading with plastification the prestrained materials softens significantly nearly to the level of the as received materials. The degree of the softening depends on the prestrain state and the level of the plastification of the material under cyclic loading. In general for the materials tested under this prestrain state the bi-axial strain state can preserve more of its increased yield strength due to prestraining than the other prestrain states. Elasto-plastic Finite Element simulation of the material specimens was performed to understand the softening effect under cyclic loading and to evaluate the effects due to tensile and compression loading. The significant strengthening of the high strength sheet steels due to forming will be preserved in the structure as long as there are no strain amplitudes above the elastic limits of the as received material. This strengthening phenomena are reflected in higher fatigue strength of roughly 10% in the high life cycle range over 10 6 cycles. The tests were evaluated as the respective Manson Coffin and Ramberg Osgood coefficients for probabilities of survival of Ps = 50% and are presented for the calculation engineer in a data bank also including the scatter. Variable amplitude tests were performed on the material specimens as well. Applying linear damage accumulation hypothesis according to Palmgreen Miner leads to an allowable damage sum of D=0,5, which should be used by the designer of automotive body structures during fatigue calculation under service loads. An idealized structure was developed to systematically investigate the reliability of using the fatigue data from specimen testing for the durability evaluation of automotive sheet steel structures. To optimise the geometries of the model part with a defined forming state, FE simulations were made and test specimens manufactured. This permitted to evaluate the major and minor strains which vary in dependence of the material, the sheet thickness, the punch radius and the indentation, as compared with the forming limit curve, as well as the thinnings. On the basis of these results, a model part and the pertaining tool were designed. The thus developed model part consists of two top-hat section cups made of edged blanks.