The assessment of high temperature components under cyclic deformation conditions increasingly relies on determinations of the stress-strain state at the critical location using nonlinear finite element analysis. An important consideration in such finite element simulations is the used constitutive model. The Chaboche model has been widely accepted as an advanced model for such applications. This study evaluates the variation of Chaboche model parameters with temperature for low cycle fatigue conditions and introduces an approach to systematically calibrate the model for a range of temperatures, rather than for single temperatures. Furthermore, mathematical representations have been proposed to consider the effect of superimposed creep deformation on the Chaboche model parameters. Successful application of the proposed approach/formulation for representing the behaviour of a 10%Cr steel under low cycle fatigue and cyclic/hold deformation conditions for the temperature range of 20-625uC is presented.

In high temperature engineering design, representations of the cyclic deformation behaviour of the material of the component are required as analysis data input to determine the stress-strain state at critical locations and ultimately component lifetime. While description of the cyclic stressstrain properties for a single heat of an alloy is relatively straightforward, even using complex constitutive model formulations, the same is not necessarily true for the derivation of mean or minimum alloy property representations. A novel method has recently been developed for this purpose. The concept is demonstrated for a high temperature steel for temperatures between 22uC and its maximum application temperature using the non-linear kinematic strain hardening model of Chaboche applied to the mid-life cyclic/hold data for a number of heats of the alloy.

A series of isothermal strain controlled creep-fatigue tests on fully instrumented cylindrical specimens with shallow chordal crack starters has been conducted for an advanced 9%Cr turbine rotor steel at 600 and 625uC. Cyclic/hold wave shapes involving a dwell period at peak strain in tension or compression were also performed with crack development being monitored by means of electrical potential drop instrumentation. It is found that temperature, total strain range and hold period are the most influential factors on short creep-fatigue crack propagation rates and specimen life. In order to establish a reliable relationship to represent subcritical crack development for high temperature component integrity assessment, the effectiveness of candidate correlating parameters such as cyclic strain range, cyclic J integral and strain energy density factor have been evaluated. Their application to circumstances involving short crack development due to fatigue, and interacting and non-interacting creep loading are evaluated with reference to the evidence determined from post-test metallurgical examination.

The paper reviews the methodology adopted in a Swiss Research Collaboration to devise a component-feature representative testpiece geometry and the thermo-mechanical fatigue (TMF) cycle parameters necessary to closely simulate the most arduous conceivable operating duties in steam turbines exposed to cyclic operation. Implementation of these service-like experimental conditions provides a practical indication of the effectiveness of deformation and crack initiation endurance predictions, in particular during fast turbine start-up conditions. Comprehensive post test inspection also provides evidence to demonstrate the physical realism of the laboratory simulations in terms of the creep-fatigue damage generated during the benchmark tests. Mechanical response results and physical damage observations are presented and their practical implications evaluated.

While the development of short cracks due to cyclic elastic loading has been relatively widely studied, in particular at room temperature, their consideration for cyclic inelastic loading at high temperatures and lower frequencies is not so common. Short creep-fatigue crack growth rates may be correlated in terms of cyclic strain range, cyclic J-integral or strain energy density factor, with appropriate allowance for associated creep damage accumulation. Candidate approaches are evaluated with reference to test results generated for an advanced 9 %Cr turbine rotor steel. This paper promotes the use of cyclic strain range and strain energy density factor relative to other candidate correlating parameters in relation to the results of a series of 30 min hold time creep-fatigue tests performed using fully instrumented uniaxial specimens with short crack starters. The focus of the testing campaign is an advanced 9 %Cr turbine rotor steel at temperatures of 600 and 625 C.