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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.

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 cyclic plasticity of a typical exhaust manifold material has been successfully characterized using a unified constitutive model based on the models developed by Sehitoglu and his co-workers [1-2]. Both monotonic tensile and cyclic strain controlled fatigue tests were performed at temperatures ranging from ambient to 800 °C to evaluate the material constants in the model. In this model, the isotropic hardening is ignored due to its minor effect upon the selected cast iron, which simplified the data processing. The model predictions are satisfactory for a wide temperature range (23-800 °C), and the strain rates covered (0.5-50 %/min).

A study has been conducted to quantify the typical internal surface roughness of a cast iron exhaust manifold. In addition, the range of surface roughness values that can be obtained with various manufacturing methods was measured. Initial investigations were conducted to measure the effect of a range of surface roughness values on the performance of the engine system, specifically torque and the thermal losses through the exhaust manifold walls. Several manifold geometries were used to represent a variety of actual manifold applications, including designs that were subjected to tight packaging constraints. Physical tests were used to show that large variations in surface roughness resulted in modest changes in manifold component pressure losses. A simulation tool was used to predict that modest improvements in manifold pressure losses have little impact on engine output.