Prediction of stress relaxation from creep data in terms of average creep rate
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Abstract:
A new prediction model of stress relaxation from creep data in terms of average creep rate was established. An incremental calculation procedure has been further established to obtain stress relaxation data from creep data. To validate the effectiveness of the proposed conversion models, the predicted results were compared with those obtained by the previous continuum damage mechanics model as well as the stress relaxation experimental data using 1Cr10NiMoW2VNbN steel. Results showed that the creep–stress relaxation conversion model based on the average creep rate is better than that based on the continuum damage mechanics model because the former avoids the dispersion effect of fracture strain data, although the predicted results from the two models are both well in agreement with the experiment data.Keywords:
Stress relaxation
Experimental data
An estimation method to predict creep performances of high temperature structural materials has been proposed. A Stress relaxation equation is obtained by fitting stress relaxation testing curves and modifying Tanaka-Ohba reloading stress relaxation constitutive equation. Based on the relationship between stress relaxation and creep, a unified prediction equation of creep is deduced. The method is to use the unified equation to derive creep strain rates or creep strain vs. time curves from stress relaxation measurements through some specified time increments. In order to validate the approach, the predicted results are compared to the experimental results of uni-axial isothermal creep tests conducted on 1Cr10NiMoW2VNbN steel. Good agreement between results of creep tests and the predicted results indicates that the developed method can be recommended in the creep behavior evaluation of high temperature materials.
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Isothermal process
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An estimation method to predict creep performances of high temperature structural materials has been proposed. The method is to use a simplified and normalized model of stress relaxation to derive creep strain rates and creep strain vs. time curves from stress relaxation measurements through an integrated analytical procedure according to the relationship between stress relaxation and creep. In order to validate the approach, the predicted results are compared to the experimental results of uni-axial isothermal creep tests conducted on 1Cr10NiMoW2VNbN steel with the same temperature of stress relaxation tests. Good agreement between results of relaxation tests and the predicted results indicates that the developed method can be recommended in the creep behavior evaluation of high temperature materials.
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Stress relaxation
Diffusion creep
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Many materials exhibit a stress range dependent creep behavior. The power-law creep observed for a certain stress range changes to the viscous type creep if the stress value decreases. Recently published experimental data for advanced heat resistant steels indicates that the high creep exponent (in the range 5-12 for the power-law behavior) may decrease to the low value of approximately 1 within the stress range relevant for engineering structures. The aim of this paper is to confirm the stress range dependence of creep behavior based on the experimental data of stress relaxation. An extended constitutive model for the minimum creep rate is introduced to consider both the linear and the power law creep ranges. To take into account the primary creep behavior a strain hardening function is introduced. The material constants are identified for published experimental data of creep and relaxation tests for a 12% Cr steel bolting material at 500°C. The data for the minimum creep rate are well-defined only for moderate and high stress levels. To reconstruct creep rates for the low stress range the data of the stress relaxation test are applied. The results show a gradual decrease of the creep exponent with the decreasing stress level. Furthermore, they illustrate that the proposed constitutive model well describes the creep rates for a wide stress range.
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Diffusion creep
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Short time creep and creep recovery were investigated by means of the endochronic theory of viscoplasticity. It is shown that the constitutive equations for constant-strain-rate stress-strain behavior, creep, creep recovery, and stress relaxation can all ber derived from the general constitutive equation by imposing appropriate constraints. In this unified approach, the effect of strain-hardening is naturally accounted for when describing creep and creep recovery. The theory predicts with reasonable accuracy the creep and creep recovery behaviors for Aluminum 1100-0 at 150 C. It was found that the strain-rate history at prestraining stage affects the subsequent creep. A critical stress was also established for creep recovery. The theory predicts a forward creep for creep recovery stress greater than the critical stress. For creep recovery stress less than the critical stress, the theory then predicts a normal strain recovery.
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Diffusion creep
Viscoplasticity
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With the development of ultrasupercritical power generation technology, creep strength of high-temperature materials should be considered for safety evaluation and engineering design. However, long-time creep testing should be conducted by traditional creep assessment methods. This paper established a high-efficient prediction method for steady creep strain rate and creep strength based on short-term relaxation tests. Equivalent stress relaxation time and equivalent stress relaxation rate were defined according to stress relaxation characteristics and the Maxwell equation. An accelerated creep prediction approach from short-term stress relaxation tests was proposed by defining the equivalent relaxation rate as the creep rate during the steady stage. Stress relaxation and creep tests using high-temperature material 1Cr10NiMoW2VNbN steel were performed to validate the proposed model. Results showed that the experimental data are in good agreement with those predicted solutions. This indicates that short-term stress relaxation tests can be used to predict long-term creep behavior conveniently and reliably, and the proposed method is suitable for creep strength design and creep life prediction of 9–12%Cr steel used in ultrasupercritical unit at 600 °C.
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Stress relaxation
Diffusion creep
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Abstract Many materials exhibit a stress range dependent creep behavior. The power–law creep observed for a certain stress range changes to the viscous type creep if the stress value decreases. Recently published experimental data for advanced heat resistant steels indicate that the high creep exponent (in the range 5–12 for power–law behaviour) may decrease to the low value of approximately 1 within the stress range relevant for engineering structures. The aim of this paper is to confirm the necessity of the assumption of the stress range dependent power–law–viscous creep transition for the solution of stress relaxation problems affected by creep behavior at elevated temperatures. A constitutive model for the minimum creep rate is introduced to describe both the linear and the power law creep depending upon the stress level. The proposed constitutive model includes a strain hardening function to describe the primary creep stage. To demonstrate the existence of the linear creep behaviour in the low stress range of application area and the influence of the primary creep behaviour on relaxation, several solutions of a uniaxial stress relaxation problem are presented for the loading values relevant to engineering applications. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Diffusion creep
Hardening (computing)
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A previously proposed theory is specialized for the uniaxial state of stress and its prediction for creep and relaxation is analyzed in detail. Constant true stress and constant load creep tests are simulated in the presence and absence of thermal aging together with the constant strain relaxation test. The signs of the creep rate and its time derivative as well as the relaxation rate and its time derivative are introduced as criteria. The constant load creep test can reproduce the normal creep curves and nonclassical creep curves (ε˙ > 0; ε¨ > 0 for all ε and for σ0 > 0). The capabilities of the constant true stress test are limited to primary creep if the work-hardening slope is positive. When aging is introduced almost any creep curve can be reproduced in both tests. The importance of initial strain is discussed and demonstrated by room temperature creep tests on Type 304 Stainless Steel. It is suggested that the initial strains together with the creep curves be reported in the future. Poisson’s ratio in creep needs to be measured and tests are proposed which will enable a quantitative assessment of aging.
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Viscoplasticity
Constant (computer programming)
Work hardening
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It was analyzed that the intrinsic limitation of present accelerated creep test methods,especially creep design method through stress relaxation tests with an exceeding pre-strain to elastic-strain.A set of short term stress relaxation tests of 1Cr10NiMoW2VNbN steel were carried out.Based on the relationship between stress relaxation and creep,the creep strain rate corresponding to the steady state creep stage were achieved through the stress relaxation test by using numerical analysis and a cross-plotting approach.And then,the second stage creep curves were constructed for high temperature strength design.An integrated accelerated creep test methodology through stress relaxation tests was therefore established.Results of creep tests indicate that the developed method can lead to a good agreement with the experimental data and thus is recommended in the creep performance prediction and strength design of high temperature components.
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