Constitutive Behaviour and Crack Tip Opening Angle of Ultra High Strength Steel Sheets


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). The crack tip opening angle (CTOA), a fracture parameter for crack propagation resistance commonly used for Al sheet materials for aerospace applications, was measured for the two steels according to the recent ASTM E 2472-06 standard. CTOA was also measured for an advanced high-strength steel (AHSS), dual-phase DP780, for comparison. Critical CTOA decreased (i.e. propagation toughness decreased) with increasing volume fraction of martensite and yield strength, and decreasing elongation.
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