Strain rate-temperature dependency of impact tensile properties and ductile fracture behavior in ductile cast iron

Impact tensile tests of as-cast and ferritic annealed ductile cast irons ar e carried out at strain rates up to 10(3)/s using a servo-hydraulic high sp eed testing machine. The fractography of the tensile specimens is carefully carried out with a scanning electron microscope. The experimental results demonstrate that the 0.2% proof stress and ultimate tensile strength of the materials used increase with increasing strain rate and lowering temperatu re. For the as-cast material with higher pearlite content in the matrix, th e strain rate sensitivities of the strengths at room temperature are found to be higher than those of ferritic material. According to the relationship s between the tensile properties and strain rate-temperature parameter, R-v alue, it is considered that for the deformation mechanisms of ductile cast irons at strain rates up to 10(3)/s, the thermally activated process is pre dominantly. Also, the constitutive equations of the impact tensile strength s for these irons are made based on the strain rate-temperature parameter R . It is believed that the proposed equations can be appropriate for enginee ring applications. While both the work-hardening exponent and the tensile d uctility increase by decreasing the R-value. However, the tensile ductility decreases by decreasing the R-value less than 4500K, because of the over-s trengthening by deformation constraining occurs: It is supported by observa tions of fracture surfaces in the tensile specimens that micro-dimple fract ure surface area induced from debondings at a large number of small inclusi ons near the eutectic cell boundaries increases with decreasing R-value.