Failure mechanisms and damage model of ductile cast iron under low-cycle fatigue conditions

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\ub0C. 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\ub0C, albeit the deformation is mainly plasticity, its LCF is limited by intergranular embrittlement. When the temperature is increased above 600\ub0C, rate-dependent stress-strain behaviour manifests due to creep, and the synergetic interaction of creep with oxidation dominates the LCF process. As a result of such interaction, a crossover-behaviour between room temperature and high-temperature (>600\ub0C) 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.Peer reviewed: YesNRC publication: Ye

Failure mechanisms and damage model of ductile cast iron under low-cycle fatigue conditions

Strain-controlled low-cycle fatigue (LCF) tests were conducted on ductile cast iron (DCI) at strain rates of 0.02, 0.002, and 0.0002/s in the temperature range from room temperature to 1073 K (800 \ub0C). A constitutive-damage model was developed within the integrated creep-fatigue theory (ICFT) framework on the premise of strain decomposition into rate-independent plasticity and time-dependent creep. Four major damage mechanisms: (i) plasticity-induced fatigue, (ii) intergranular embrittlement (IE), (iii) creep, and (iv) oxidation were considered in a nonlinear creep-fatigue interaction model which represents the overall damage accumulation process consisting of oxidation-assisted fatigue crack nucleation and propagation in coalescence with internally distributed damage (e.g., IE and creep), leading to final fracture. The model was found to agree with the experimental observations of the complex DCI-LCF phenomena, for which the linear damage summation rule would fail.Peer reviewed: YesNRC publication: Ye

Simulation of thermomechanical fatigue of ductile cast iron and lifetime calculation

In this paper, both standard and constrained thermomechanical fatigue (TMF) tests were conducted on a high silicon ductile cast iron (DCI). The standard TMF tests were conducted with independent control of mechanical strain, out-of-phase (OP) and in-phase (IP) strain, and temperature in the range from 300 to 800\ub0C. The constrained TMF tests were conducted with various constraint ratios of 100%, 70%, 60% and 50% at the temperature ranges of 160 to 600\ub0C and 160 to 700\ub0C. Based on a material model as calibrated with low-cycle fatigue (LCF) data of DCI, finite element analyses (FEA) of the above TMF tests were carried out with Abaqus. A damage mechanism-based lifetime model was integrated into a C++ API code to post-process the Abaqus output results. Simulation predictions show good agreement with experiments for stress-strain responses and lifetime under different TMF conditions.Peer reviewed: YesNRC publication: Ye

Simulation of thermomechanical fatigue of ductile cast iron and lifetime calculation

In this paper, both standard and constrained thermomechanical fatigue (TMF) tests were conducted on a high silicon ductile cast iron (DCI). The standard TMF tests were conducted with independent control of mechanical strain, out-of-phase (OP) and in-phase (IP) strain, and temperature in the range from 300 to 800\ub0C. The constrained TMF tests were conducted with various constraint ratios of 100%, 70%, 60% and 50% at the temperature ranges of 160 to 600\ub0C and 160 to 700\ub0C. Based on a material model as calibrated with low-cycle fatigue (LCF) data of DCI, finite element analyses (FEA) of the above TMF tests were carried out with Abaqus. A damage mechanism-based lifetime model was integrated into a C++ API code to post-process the Abaqus output results. Simulation predictions show good agreement with experiments for stress-strain responses and lifetime under different TMF conditions.Peer reviewed: YesNRC publication: Ye

Thermomechanical fatigue of ductile cast iron and its life prediction

Thermomechanical fatigue (TMF) behaviors of ductile cast iron (DCI) were investigated under out-of-phase (OP), in-phase (IP), and constrained strain-control conditions with temperature hold in various temperature ranges: 573 K to 1073 K, 723 K to 1073 K, and 433 K to 873 K (300 \ub0C to 800 \ub0C, 450 \ub0C to 800 \ub0C, and 160 \ub0C to 600 \ub0C). The integrated creep-fatigue theory (ICFT) model was incorporated into the finite element method to simulate the hysteresis behavior and predict the TMF life of DCI under those test conditions. With the consideration of four deformation/damage mechanisms: (i) plasticity-induced fatigue, (ii) intergranular embrittlement, (iii) creep, and (iv) oxidation, as revealed from the previous study on low cycle fatigue of the material, the model delineates the contributions of these physical mechanisms in the asymmetrical hysteresis behavior and the damage accumulation process leading to final TMF failure. This study shows that the ICFT model can simulate the stress\u2013strain response and life of DCI under complex TMF loading profiles (OP and IP, and constrained with temperature hold).Peer reviewed: YesNRC publication: Ye

Thermomechanical fatigue of ductile cast iron and its life prediction

Thermomechanical fatigue (TMF) behaviors of ductile cast iron (DCI) were investigated under out-of-phase (OP), in-phase (IP), and constrained strain-control conditions with temperature hold in various temperature ranges: 573 K to 1073 K, 723 K to 1073 K, and 433 K to 873 K (300 \ub0C to 800 \ub0C, 450 \ub0C to 800 \ub0C, and 160 \ub0C to 600 \ub0C). The integrated creep-fatigue theory (ICFT) model was incorporated into the finite element method to simulate the hysteresis behavior and predict the TMF life of DCI under those test conditions. With the consideration of four deformation/damage mechanisms: (i) plasticity-induced fatigue, (ii) intergranular embrittlement, (iii) creep, and (iv) oxidation, as revealed from the previous study on low cycle fatigue of the material, the model delineates the contributions of these physical mechanisms in the asymmetrical hysteresis behavior and the damage accumulation process leading to final TMF failure. This study shows that the ICFT model can simulate the stress\u2013strain response and life of DCI under complex TMF loading profiles (OP and IP, and constrained with temperature hold).Peer reviewed: YesNRC publication: Ye