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A mechanism-based approach—the integrated creep-fatigue theory (ICFT)—is used to model low cycle fatigue behavior of 1.4848 cast austenitic steel over the temperature range from room temperature (RT) to 1173 K (900 °C) and the strain rate range from of 2 × 10−4 to 2 × 10−2 s−1. The ICFT formulates the material’s constitutive equation based on the physical strain decomposition into mechanism strains, and the associated damage accumulation consisting of crack nucleation and propagation in coalescence with internally distributed damage. At room temperature, the material behavior is controlled by plasticity, resulting in a rate-independent and cyclically stable behavior. The material exhibits significant cyclic hardening at intermediate temperatures, 673 K to 873 K (400 °C to 600 °C), with negative strain rate sensitivity, due to dynamic strain aging. At high temperatures >1073 K (800 °C), time-dependent deformation is manifested with positive rate sensitivity as commonly seen in metallic materials at high temperature. The ICFT quantitatively delineates the contribution of each mechanism in damage accumulation, and predicts the fatigue life as a result of synergistic interaction of the above identified mechanisms. The model descriptions agree well with the experimental and fractographic observations.

This letter attempts to comment on the article by Chen et al. [Mater. Sci. Eng. A 688 (2017) 416] in the aspects of microstructure and tensile properties of ductile iron with niobium additions.

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 °C to 800 °C, 450 °C to 800 °C, and 160 °C to 600 °C). 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–strain response and life of DCI under complex TMF loading profiles (OP and IP, and constrained with temperature hold).

Heat-resistant ferrous cast alloys are generally divided into four groups: ferritic cast iron, austenitic ductile iron, ferritic stainless steel, and austenitic stainless steel. There is an active debate about whether or not heat treatment should be imposed on these ferrous castings prior to placing them in service. In this investigation, a series of experiments were conducted to evaluate the effect of heat treatment on the microstructure, tensile properties, hardness, linear growth, and thermal fatigue life for alloys of each of the four groups. In general, heat treatment appeared to cause similar microstructural changes in these alloys such as the formation of precipitated particles adjacent to the cell boundary regions, but it also affected each material’s properties in a different fashion. In spite of a slight increase in tensile elongation at room temperature, annealing had no impact on the ductility minima at medium temperatures for ferritic cast iron. Heat treatment did not appreciably alter the tensile properties of austenitic ductile iron. The Brinell hardness of ferritic stainless steel samples was reduced, but the hardness was increased for austenitic stainless steel samples after heat treatment.

Cast austenitic stainless steel 1.4848 is used to manufacture automotive exhaust system components. Low cycle fatigue (LCF) of 1.4848 austenitic steel was investigated through strain-controlled fatigue testing at strain rates of 0.02/s, 0.002/s, and 0.0002/s in the temperature range from room temperature (RT) to 900°C. Its cyclic behavior was characterized in relation to deformation mechanisms. At RT, the material behavior was rate-independent and cyclically stable, which occurred by plasticity. The material exhibited significant cyclic hardening at intermediate temperatures, 400°C to 600°C, with negative strain-rate sensitivity. In this temperature range, dynamic strain aging (DSA) presumably occurred due to slip dragging solute atoms. At high temperatures, 800°C and 900°C, the material exhibited positive rate-dependence in the hysteresis behavior, and the cyclic stress response tended to stabilize with increasing cycles. The high-temperature behavior was presumably controlled by a combination of plasticity and dislocation-glide creep. The integrated creep-fatigue theory (ICFT) was used to describe the deformation and life behaviors based on the identified mechanisms, which were corroborated by fractographic observations. Copyright © 2017 by ASTM International., Symposium on Fatigue and Fracture Test Planning, Test Data Acquisitions and Analysis 2016, 4-5 May 2015, San Antonio, United States

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°C. The constrained TMF tests were conducted with various constraint ratios of 100%, 70%, 60% and 50% at the temperature ranges of 160 to 600°C and 160 to 700°C. 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., SAE 2015 World Congress and Exhibition, September 22-24, 2015, Seattle, Washington, USA

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 °C). 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.

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 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°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., SAE 2013 World Congress and Exhibition, 16 April 2013 through 18 April 2013, Detroit, MI