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Use of ultra high strength steel (UHSS) sheet in automotive components has potential to simultaneously reduce weight and increase crashworthiness. For crashworthiness design and simulation, constitutive equations are required; however, these are scarce for UHSS. Also, UHSS sheets may suffer unexpected fracture such as shear fracture, and toughness data for UHSS sheets is very limited. In this work, effects of strain rate and temperature on flow stress of two UHSS sheet steels (a dual-phase ferritic/martensitic DP980 and a martensitic boron (B) steel) are experimentally investigated and compared to a simple constitutive equation for structural steels based on thermal-activation theory of dislocation motion. The flow stress of the two UHSS steels obeys a constitutive equation similar to that of structural steels of other microstructures (ferrite, ferrite/pearlite, pearlite, ferrite/bainite, and bainite).

Determination of forming limit of tubes can be difficult when it involves many process related variables and the use of costly hydroforming equipment. Therefore, the industry currently relies on sheet or tube longitudinal properties to estimate tube hydroformability. This paper introduces a simple method for experimentally assessing formability of tubes as material intrinsic characteristics for tube hydroforming, and presents results on the correlation between mechanical properties and failure location as well as forming limit at a small negative minor strain state. One commercial hydroforming tube, and one aluminum trial tube, both made by a continuous tube mill and seam welded using high-frequency (HF) induction welding, were evaluated for their forming characteristics.

This study determines the correlation between microstructure, fracture mechanism and formability of Al tailor-welded blanks (TWBs). It is found that porosity in the welds is detrimental to the formability of non-vacuum electron beam (NVEB) welded AA5754-O temper Al TWBs. It is also found that the degree of influence of the weld characteristics on formability can be minimal in one test condition, but critical in another, depending on the loading condition, strain state (related to blank geometry) and relative position of the weld seam to the major strain during forming. Metallography, microhardness measurement, electron probe chemical analysis and fractography for 1-2 mm NVEB AA5754-O temper of Al TWBs were used to analyze precipitation, porosity, grain structure, microhardness profile and microchemistry profile across the weld of Al TWBs and their fracture mechanisms during forming tests. Formability of the NVEB welded AA5754 Al TWBs has been previously evaluated.