Constitutive model for high temperature deformation of SiMo cast iron: Application to thermo-mechanical fatigue of exhaust manifolds

Abstract

A dislocation density-based constitutive modeling framework is proposed for modeling the elevated temperature deformation of ductile SiMo cast iron, which is used for automotive exhaust manifolds. The constitutive model, implemented in a finite deformation framework, has considerations for inelastic deformation mediated by dislocation plasticity and dislocation creep. While a Kocks type thermally activated flow rule is used to model dislocation plasticity, a power law is used to model dislocation creep. Kinematic hardening due to backstress is modeled using a Chaboche type model. Further, the model also has considerations for thermal strains. The model parameters are calibrated using available experimental data from tensile, cyclic, stress hold and strain hold tests. The calibrated model is then validated by comparing predictions with the experimental data from cyclic strain hold and out of phase thermo-mechanical fatigue tests. Further, the constitutive model is used for component level finite element simulation of an exhaust manifold subjected to thermal cycling. Model predictions of the accumulated inelastic strain are correlated with the experimentally observed crack locations and a qualitative concurrence is observed. This serves as qualitative validation of the constitutive model and also establishes its credibility for modeling thermo-mechanical deformation at the component level.