Automated procedures for the determination of high temperature viscoplastic damage constitutive equations

Methods have been developed for the determination of the material parameters in the Chaboche viscoplasticity model, and in the evolution equation for cyclic plasticity damage. The matrix of cyclic plasticity tests required for parameter determination consists of nine tests to be carried out for three strain ranges, and for each strain range, three strain rates. A programme of uni-axial cyclic plasticity tests has been carried out on cast copper (nominal composition: 99.99% Cu, 0.005% O$_{2}$, B.S. 1035-1037) at strain ranges $\pm $0.3%, $\pm $0.6%, and $\pm $1.0%, each at strain rates 0.6% s$^{-1}$, 0.06% s$^{-1}$, and 0.006% s$^{-1}$. The matrix of tests has been carried out at each of the following temperatures: 20, 50, 150, 250 and 500 degrees C. At elevated temperature, strain rate has been found to have a significant effect on specimen lifetime. Low strain rates lead to increased creep damage evolution at high temperature, and hence lead to reduced cycles to failure. Cast copper has been found to be a rate sensitive material at temperatures above 150 degrees C. No strain rate effect was observed at temperatures below 150 degrees C. Ratchetting tests have been carried out at 20, 150, 250 and 500 degrees C. The effect of mean stress and stress rate on ratchet rates and lifetimes has been examined. Mean stresses of the order of 1% of the applied stress range have been found to lead to significant ratchet rates for this material, resulting in failure by plastic collapse at elevated temperature. The methods presented for the determination of the material parameters and the results of the cyclic plasticity testing programme have enabled the viscoplastic damage model to be developed.