The effects of microstructure, strain rates and geometry on dynamic impact response of a carbon–manganese steel

The effects of strain rates, microstructure and geometry on the occurrence of adiabatic shear bands in AISI 1340 steel during impact loading were investigated. Cylindrical and cuboidal specimens of the steel were heat treated to have microstructure consisting of tempered martensite, ferrite + martensite (dual-phase) or ferrite + pearlite. Depending on the applied impact momentum, both deformed and transformed shear bands were observed in the specimens with martensitic and dual-phase structures. Deformed bands first formed and then metamorphosed into transformed bands as the intensity of localized shear strain increased. The tendency for formation of a transformed band is higher in the martensitic steel specimens. Within the range of the applied impact momentum of between 30 and 42 kg m/s, which generated strain-rates of between 630 and 3400/s, only deformed bands were observed in the normalized steel specimens containing ferrite and pearlite. The width of these deformed bands increased as the applied impact momentum was raised. Whereas circular shear bands were observed on the transverse section of the impacted cylindrical specimens, the shape of shear bands in the same section of the cuboidal specimens was rectangular. The hardness of the shear bands, which was measured to be higher than that of the bulk material, increased as the impact momentum was raised. Fractographic study of fractured specimens showed ductile shear failure and knobby fracture modes along the transformed bands.