Strain Rate and Stress-State Dependence of Gray Cast Iron

An investigation of the mechanical strain rate, inelastic behavior, and microstructural evolution under deformation for an as-cast pearlitic gray cast iron (GCI) is presented. A complex network of graphite, pearlite, steadite, and particle inclusions was stereologically quantified using standard techniques to identify the potential constituents that define the structure-property relationships, with the primary focus being strain rate sensitivity (SRS) of the stress-strain behavior. Volume fractions for pearlite, graphite, steadite, and particles were determined as 74%, 16%, 9%, and 1%, respectively. Secondary dendrite arm spacing (SDAS) was quantified as 22.50 mu m +/- 6.07 mu m. Graphite flake lengths and widths were averaged as 199 mu m +/- 175 lm and 4.9 mu m +/- 62.3 mu m, respectively. Particle inclusions comprised of manganese and sulfur with an average size of 13.5 mu m +/- 9.9 mu m. The experimental data showed that as the strain rate increased from 10(-3) to 10(3) s(-1), the averaged strength increased 15-20%. As the stress state changed from torsion to tension to compression at a strain of 0.003 mm/mm, the stress asymmetry increased similar to 470% and similar to 670% for strain rates of 10(3) and 10(3) s(-1), respectively. As the strain increased, the stress asymmetry differences increased further. Coalescence of cracks emanating from the graphite flake tips exacerbated the stress asymmetry differences. An internal state variable (ISV) plasticity-damage model that separately accounts for damage nucleation, growth, and coalescence was calibrated and used to give insight into the damage and work hardening relationship.