Accumulation of creep damage in X10NiCrAlTi 32 20 (alloy 800 H) in test with stress or temperature changes

The applicability of damage accumulation rules and of a constitutive equation derived by Cords to the description of the creep behaviour of X10NiCrAlTi 32 20 (Alloy 800 H) in creep tests with stress or temperature changes has been investigated. The tests were carried out at 800deg C and 900deg C and in order to ensure that the creep damage in the tests was the same as that obtained during actual service, creep tests for durations of up to 10 000 h were carried out. Metallographic examination of tested specimens showed that typical creep damage was occurring under the selected test conditions. It was shown that thermally aged specimens exhibited with increasing ageing time a decrease in time to rupture. Therefore the use of the linear damage accumulation rules for predicting rupture time is only possible if large margins of safety are introduced or if the structural changes due to thermal ageing effects during the creep test are included. Investigations into the applicability of the non-linear damage accumulation model developed by Kachanov were carried out, using different methods for the determination of the necessary materials parameters. By the introduction of a logarithmic time scale for the fitting of all the measured strain/time data at a given temperature, the best prediction of rupture life was obtained. Evaluation of the test results using the Cords set of equations showed that the equations needed to be extended to include an additional damage term at the low stresses of the experiments. (orig.)