*Time-dependent creep induced failure is a major concern for structural components operating at elevated temperatures. The likelihood of a failure is aggravated by randomness in several sources (operating conditions variability, priori and pre-load damage, heat-to-heat variation, geometric parameters) of uncertainty. Creep rupture data shows expanding scatter bands for long-duration creep tests where uncertainty can span multiple logarithmic decades of life. This experimental uncertainty is exacerbated by the uncertainties that exist during service. Traditional continuum damage mechanics (CDM) based creep-damage model is calibrated by a single best-fit line through the experimental result; consideration of the presence of substantial scatter bands are often disregarded. It is ideal to convert deterministic models to probabilistic ones where reliability is a tunable parameter in design.*

*The objective of this paper is to develop a probabilistic model for Minimum-creep-strain-rate (MCSR) and stress-rupture (SR) that is capable of predicting experimental uncertainty and extrapolating the expanded scatter bands observed in long-duration creep data. The Sine-hyperbolic (Sinh) CDM model is selected. Multi-isotherms of MCSR and SR data for 304 (18Cr-8Ni) and 316 (18Cr-12Ni-Mo) stainless steel are gathered from the NIMS material database. A deterministic calibration is performed where the optimal material constants are obtained with no initial damage and perfect loading conditions. Probabilistic calibration begins with adding ASTM-specified stress and temperature tolerances (±X% MPa, ±Y°C) to capture a portion of the experimental uncertainty. The initial damage tolerances are then calibrated to capture the remaining uncertainty in the data. Probability distribution functions (pdfs) are assigned to each uncertain parameter. Monte Carlo simulations are performed over a range of stress and temperature. The probabilistic Sinh model is shown to predict the expanding scatter band observed in long-term MCSR and SR data. Parametric simulations are performed where service condition uncertainty is added to the probabilistic model. It is determined that service condition uncertainties further degrade the creep resistance of a material.*


Probabilistic Minimum-Creep-Strain-Rate and Stress-Rupture Prediction for the Long-Term Assessment of IGT Components

Division / Sponsor:
International Gas Turbine Institute (IGTI)

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

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