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Effects of strain rate on mechanical behavior and fractographic features are elucidated in Usibor® 1500 hot-stamped at extreme conditions. Uniaxial tensile tests have been performed at strain rates ranging from 10−4 to 103 per second. It is observed that the ductility and ultimate tensile strength are higher at the highest strain rate as compared to the lowest strain rate studied. Unbiased quantitative fractography has been applied to characterize the fracture surfaces after tensile tests. Numerous quantitative fractographic attributes including area fractions of the fractures surfaces generated by different fracture micromechanisms and number density of dimples in the dimpled fracture regions have been estimated from the quantitative fractographic data. These data reveal that ductile dimpled fracture is the dominant fracture micromechanism at all strain rates. Nonetheless, the extent of the fracture surface generated by ductile dimpled fracture increases with increase in the strain rate. Therefore, increase in the strain rate facilitates dimpled ductile fracture or increases resistance to brittle fracture (or both).

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Dual phase (DP) steels are being increasingly used as structural components in the automotive industry owing to their combination of high strength and moderate ductility. Formability and crash resistance are important mechanical properties that expose the steel to high strain rates. Fracture characteristics of a DP steel at strain rates spanning seven orders of magnitude are investigated using quantitative fractography. The DP steel investigated in this study is comprised of a continuous matrix of martensite with interspersed ferrite islands. The data reveal that the flow stress, uniform strain, and ductility are substantially higher at the higher strain rates. Quantitative fractography, employed to understand the fracture characteristics, shows an increase in the extent of ductile fracture at high strain rates. In this presentation, the results demonstrating high strain rates facilitating dimpled ductile fracture and/or increased resistance to faceted fracture are discussed.