A Unified Elastic-Viscoplastic Constitutive Model of Grey Iron

The strength requirement of grey cast iron for diesel engine applications continues to increase in response to the demand for increasing power and fuel economy while simultaneously decreasing green house gas emission. Production of high strength grey iron usually involves heavy alloy addition and increased cooling rates to achieve finer microstructure and higher strength. The increased cooling rates often result in higher residual stresses. Excessive thermal stress may lead to cracked castings or compromised fatigue strength if the residual stresses are tensile. Application of a thermal stress model could help engineers to properly design the process to avoid excessive residual stresses and estimate service life more accurately. In order to accurately predicted the thermal stress evolution during the entire manufacturing process, a unified elaustic-viscoplastic constitutive model is developed. This model features strain-rate dependency to model the creep and plasticity in a unified manner. It. also features an asymmetric flow surface to address the strength difference between tension and compression of grey cast iron. The parameters of the model are identified by a series of uniaxial tension and compression tests front room temperature to 1100 degrees C. The model is implemented using the ABAQUS UMAT user subroutine and used to predict the force and displacement evolutions of a stress bar casting with in-situ force and displacement measurements. Comparison of the predictions to the measured data shows a substantial improvement in accuracy of the new model over a standard elastic-plastic model.