Failure Mechanisms and Damage Model of Ductile Cast Iron under Low-Cycle Fatigue Conditions 2013-01-0391

Strain-controlled low-cycle fatigue (LCF) experiments were conducted on ductile cast iron at total strain rates of 1.2/min, 0.12/min and 0.012/min in a temperature range of RT ~ 800°C. An integrated creep-fatigue (ICF) life prediction framework is proposed, which embodies a deformation mechanism based constitutive model and a thermomechanical damage model. The constitutive model is based on the decomposition of inelastic deformation into plasticity and creep mechanisms, which can describe both rate-independent and rate-dependent cyclic responses under wide strain rate and temperature conditions. The damage model takes into consideration of i) plasticity-induced fatigue, ii) intergranular embrittlement, iii) creep and iv) oxidation. Each damage form is formulated based on the respective physical mechanism/strain. The overall damage accumulation follows a nonlinear interaction mechanism that represents the nucleation and propagation of a surface crack in coalescence with internally distributed damages (cracks/voids). For ductile cast iron (DCI), the model predicates that the room temperature deformation and LCF life are primarily driven by cyclic plasticity; but at 400°C, albeit the deformation is mainly plasticity, its LCF is limited by intergranular embrittlement. When the temperature is increased above 600°C, rate-dependent stress-strain behavior manifests due to creep, and the synergetic interaction of creep with oxidation dominates the LCF process. As a result of such interaction, a crossover-behavior between room temperature and high-temperature (≻600°C) strain-life relationships may occur, as observed in the experiments. The model prediction corroborates with the LCF test results and fractographic observations on the test coupons, which further substantiates the validity of the model.