*In conventional creep testing (CCT) a specimen is subject to constant load and temperature for a long-duration until creep rupture occurs. Conventional testing can be costly when considering the number of experiments needed to characterize the creep response of a material over a range of stress and temperature. To predict long-term creep-rupture properties, the time-temperature stress superposition principle (TTSSP) approach has been employed where stress and/or temperature is applied at an elevated level; the result of which are extrapolated down to low stress and/or temperature conditions. These methods have been successful in predicting minimum-creep-strain-rate (MCSR) and stress-rupture (SR) but suffer from an inability to predict the creep deformation curve or account for changes in deformation mechanisms or aging that occurs at long-duration. An accelerated technique termed the stepped isostress method (SSM) allows the accelerated testing of materials to determine their creep deformation response. Unlike TTSSP tests, the SSM test employs a single specimen where the stress is periodically step increased until rupture. The SSM creep deformation curve is processed (time and strain shifted) to produce an accelerated creep deformation curve that represents the creep deformation curve at the initial stress level in SSM. The processing procedure has yet to be proven.*

*The pre and post-processed SSM creep deformation curves can be exploited to enable the calibration and validation of a creep constitutive law across multiple stress levels. Hence the research objective of this study is the application of pre-processed SSM test data to calibrate a creep-damage constitutive model and the validation of the model and processing procedure using the post-processed SSM data. Three SSM tests were conducted on Ni-based superalloy Inconel 718 at 650°C with stress being periodically increased until rupture. The Sine-hyperbolic (Sinh) creep-damage model is employed in this study. The Sinh creep-damage constitutive model is based on coupled creep strain rate and damage evolution equations; where both rates are dependent on the current state of damage. Calibration is two-step: quasi-analytical and numerical optimization. Each stepped creep deformation curve is tackled quasi-analytically to determine fixed MCSR and SR related material constants and accumulated damage. The damage accumulated at the end of each step is then passed onto subsequent steps to calibrate the ‘Normalized’ MCSR, rupture prediction, and damage evolution. Numerical optimization is applied to optimize model constants involved in the creep strain constitutive equations in order to generate best-fitted Sinh creep deformation curves. The pre and post-processed creep deformation curves are compared to Sinh model predictions. The Sinh model satisfactorily predicts the pre- and post-processed creep deformation curves. The goodness of fit accredits the feasibility of the application of the Sinh model across multiple stress levels overcoming the empiricism of the processing steps. Calibration using SSM data reduces the number of tests needed to calibrate a model; significantly reducing costs. A single SSM test replaces numerous creep tests at different stresses. In future work, the stepped isothermal method (SIM) will be employed for calibration across temperature.*


Calibration of CDM-Based Creep Constitutive Model Using Accelerated Creep Test (ACT) Data

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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