Materials and Fabrication

Primary Creep Modeling Based on the Dependence of the Activation Energy on the Internal Stress

[+] Author and Article Information
Luca Esposito

e-mail: l.esposito@unicas.it

Nicola Bonora

Department of Civil and Mechanical Engineering,
University of Cassino,
Via G. Di Biasio 43, 03043 Cassino, Italy

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNALOF PRESSURE VESSEL TECHNOLOGY. Manuscript received February 24, 2011; final manuscript received April 24, 2012; published online October 17, 2012. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 134(6), 061401 (Oct 17, 2012) (6 pages) doi:10.1115/1.4006856 History: Received February 24, 2011; Revised April 24, 2012

In high temperature design, the accumulation of creep strain during the primary stage has to be considered since most of the allowable design strain occurs in this stage. In this work, assuming that the creep rate in the transient regime can be given as a fraction of the steady state creep rate and function of the internal stress, a mechanism based model for primary creep has been derived. Taking into account that the apparent activation energy varies with the internal stress, which evolves with creep strain, an exponential form of the creep rate versus creep strain has been obtained. The proposed model for primary creep requires the identification of two material parameters only which are shown to be function of the applied stress and independent of temperature. The proposed model has been validated for high chromium steel P91.

Copyright © 2012 by ASME
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Fig. 1

Comparison of the predicted creep rate as a function of the creep strain with experimental data for P91 steel at 130 MPa and T = 600 °C.

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Fig. 2

Comparison of the predicted creep curves with experimental data at T = 550 °C

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Fig. 3

Comparison of the present model prediction with creep strain rate data at T = 550 °C

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Fig. 4

Evolution of the decay constant k as a function of the applied stress for both T = 550 °C and T = 600 °C

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Fig. 5

Evolution of the scaled activation volume as a function of the applied stress T = 550 °C

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Fig. 6

Sensitivity indices SΩ¯σ and Skσ as a function of creep strain

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Fig. 7

Scatter plots of the creep rate versus Ω¯ and k at ɛ = 0.002 and ɛ = 0.02. Ellipses delimit the 90% confidence interval.

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Fig. 8

Effect of the input parameters variation on the model response. P90 and P10 indicate 90% upper bound and 10% lower bound prediction, respectively.

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Fig. 9

Effect of the input parameters variation on the model response: calculated creep curves versus experimental data



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