High Temperature Flow Response Modeling of Ultra-Fine Grained Titanium
This work presents the mechanical behavior modeling of commercial purity titanium subjected to severe plastic deformation (SPD) during post-SPD compression, at temperatures of 600-900 °C and at strain rates of 0.001-0.1 s−1. The flow response of the ultra-fine grained microstructure is modeled using the modified Johnson-Cook model as a predictive tool, aiding high temperature forming applications. It was seen that the model was satisfactory at all deformation conditions except for the deformation temperature of 600 °C. In order to improve the predictive capability, the model was extended with a corrective term for predictions at temperatures below 700 °C. The accuracy of the model was displayed with reasonable agreement, resulting in error levels of less than 5% at all deformation temperatures.
A Modified Johnson-Cook Model for Ferritic-Pearlitic Steel in Dynamic Strain Aging Regime
In this study, the flow stress behavior of ferritic-pearlitic steel (C45E steel) is investigated through isothermal compression testing at different strain rates (1 s−1, 5 s−1, and 60 s−1) and temperatures ranging from 200 to 700 °C. The stress-strain curves obtained from experimental testing were post-processed to obtain true stress-true plastic strain curves. To fit the experimental data to well-known material models, Johnson-Cook (J-C) model was investigated and found to have a poor fit. Analysis of the flow stress as a function of temperature and strain rate showed that among other deformation mechanisms dynamic strain aging mechanism was active between the temperature range 200 and 400 °C for varying strain rates and J-C model is unable to capture this phenomenon. This lead to the need to modify the J-C model for the material under investigation. Therefore, the original J-C model parameters A, B and n are modified using the polynomial equation to capture its dependence on temperature and strain rate. The results show the ability of the modified J-C model to describe the flow behavior satisfactorily while dynamic strain aging was operative.
Effect of Low Temperature on Fatigue Life and Cyclic Stress-Strain Response of Ultrafine-Grained Copper
Fatigue life curves and cyclic stress-strain curves of ultrafine-grained (UFG) copper of purity 99.9 pct produced by equal-channel angular pressing (ECAP) were determined under stress control at room temperature (RT) and at a temperature of 173 K. The obtained curves were compared to the corresponding curves obtained on conventional-grain (CG) copper. At both temperatures, the lifetime of UFG copper is longer than that of CG copper. The S-N curve of UFG copper is temperature dependent, while its cyclic stress-strain curve is temperature insensitive. To explain this temperature effect, two mechanisms of cyclic plastic deformation were proposed: the temperature-independent bulk dislocation mechanism taking place in the entire loaded volume and the temperature-dependent localized mechanism consisting of cooperative grain boundary (GB) sliding along the shear plane of the last ECAP pass taking place in the surface layer and leading to formation of surface fatigue markings.