From matrix-graphite interface mechanics to the multiaxial yield surface of ductile cast iron: a micro-to-macro numerical modeling approach

Ductile cast iron (DCI) exhibits tension–compression asymmetry and pressure-sensitive yielding that conventional J2-plasticity cannot reproduce. Micromechanical models resolving graphite and interface effects have generally been assessed under uniaxial loading, whereas multiaxial studies have commonly idealized graphite nodules as voids. This work develops an experimentally validated micro-to-macro framework for a ferritic–pearlitic EN-GJS-600-3. A three-dimensional periodic Representative Volume Element (RVE) with statistically distributed graphite nodules combines radially oriented transversely isotropic graphite elasticity, unilateral matrix–graphite contact, and cooling-induced residual stresses. The matrix response is independently assessed through Vickers microindentation, while the homogenized RVE is validated against an experimental axial–torsional yield locus. The model incorporating graphite transverse isotropy and residual stresses reproduces this locus with an RMSE of 5.54 % and a maximum error of − 9.27 %. The simulations show that interface opening under tension makes graphite nodules behave approximately as voids, whereas interface closure under compression transfers load to the nodules and increases the macroscopic yield strength. Cooling-induced compressive residual stresses further delay interface opening. Using an energy-based definition of yielding, general triaxial stress states are explored, and the yield surface is mapped in stress-invariant space. The predicted surface is closed, strongly pressure-sensitive and weakly dependent on the Lode angle; neither the Von Mises nor the conventional Drucker–Prager criterion reproduces this behavior. The micromechanical predictions are therefore condensed into a three-invariant Drucker–Prager Cap criterion suitable for component-scale Finite Element analyses. The proposed framework establishes a direct connection between graphite-scale mechanisms and a practical macroscopic constitutive description of DCI yielding.