Search

The relevance of incorporating "threshold stress" in models for the dependence of rupture time on applied stress is discussed in the context of data for 2.25Cr1Mo and 1CrMoV steels, with particular reference to a recent publication on this subject (Evans, 2023) [1]. • Highlights of "Concerning the use of 'threshold stress' in the modelling of creep rupture data". • Reviews recently published rupture models including threshold stress. • Examines validity of threshold stress in those models of rupture data. • Presents and compares alternative models excluding threshold stress. [ABSTRACT FROM AUTHOR]

This addendum to Reliable analysis and extrapolation of creep rupture data, Int J Press Vessels Pip 2017 , covers matters that were insufficiently addressed in the original article. Uncertainties that obscure evaluation of the P-NID procedure by analysis of multi-batch data are addressed by blind testing the procedure for individual batches of an 18%Cr steel, Type 316 SS. Uncertainties that arise in the extrapolation of near-linear rupture curves are addressed by the inclusion of temperature dependence in parametric stress equations. The use of Tensile Strength data as equivalent short-term rupture data is demonstrated and discussed. • Discusses testing the reliability of an extrapolation procedure by modelling data for individual batches of material. • Shows that P-NID models of short-term data for individual batches of 18%Cr steel predict long-term data accurately. • Discusses difficulties in extrapolating data that show a near-linear relationship between log stress and log rupture time. • Illustrates the advantages of a model that includes a temperature-dependent parametric stress. • Discusses and demonstrates the use of UTS data as equivalent short-term rupture data. [ABSTRACT FROM AUTHOR]

The P-NID (Parametric, Numerical Isothermal Datum) method of extrapolating creep rupture data has been applied to the four large datasets recently analysed by the European Creep Collaborative Committee in order to re-evaluate its own recommended procedure. It is demonstrated from an analysis of these and other datasets that the P-NID method provides a very reliable basis for extrapolation. [ABSTRACT FROM AUTHOR]

In this study, the datum temperature method (DTM) is applied to the Wilshire-Cano-Stewart (WCS) model to predict rupture time, minimum-creep-strain-rate, and creep deformation. The datum temperature method (DTM) is an alternative calibration approach that can be applied to existing models to improve accuracy and extrapolation reliability. The availability of creep data is limited due to the time and costs associated with testing. Often data is not available at desired operating conditions or creep life. Although accelerated testing methods have been developed, there is still a need to extrapolate creep data to conditions of interest. Conventional time-temperature-parametric (TTP) models have been used but the need for a decision which TTP model to use for specific materials, the point of convergence, and the inflection points limit their applicability. As an alternative, the DTM is proposed, where all data is transferred to a datum temperature for ease of calibration. In this study, the DTM is combined with the novel continuum damage mechanics based WCS model. To accomplish this, a single heat of alloy P91 from the National Institute of Material Science (NIMS) is considered and the WCS model is calibrated by DTM, and post-audit validated using the remaining heats of materials and an additional creep deformation dataset from Kimura. This calibration approach allows the model to predict rupture, minimum-creep-strain-rate, and creep deformation from a single datum temperature and single heat. It is found that DTM can significantly reduce the time for materials qualification and effort needed for model calibration. • The datum temperature method (DTM) has two additional equations to predict minimum-creep-strain-rate and creep deformation. • The DTM accurately calibrates the WCS model for the prediction of alloy P91 stress-rupture, MCSR, and creep deformation. • The datum temperature method is proven to have similar accuracy as conventional calibration methods with a simpler approach. [ABSTRACT FROM AUTHOR]

The efficient and accurate prediction of creep rupture limit poses a huge challenge for high-temperature engineering such as aerospace, nuclear and chemical industries. It is important to investigate the applicability of mainstream assessment approaches and related creep rupture failure mechanisms through theoretical and numerical views. In this study, major creep rupture assessment techniques are comparatively investigated for the first time, including the isochronous stress-strain (ISS) curve-based creep rupture limit analysis, the Omega creep damage model-based creep analysis and the direct method-based creep rupture assessment by an extended Linear Matching Method (LMM). New virtual creep test curves are generated from the Omega creep model and chosen as the unified creep source data to derive the key material parameters used for different methods. For proposing a reasonable strategy for evaluating high-temperature structures in terms of creep rupture, the balance between computational efficiency and accuracy is comprehensively analyzed. Through a practical engineering application of a high-temperature pressure vessel component, a profound insight into the techniques of creep rupture evaluation is delivered from different views. Moreover, several assessment curves are built based on a new understanding of creep rupture failure mechanism, with an effective numerical plan to validate the creep rupture boundary illustrated. It is demonstrated that the LMM direct creep rupture analysis is more suitable for calculating the structural creep rupture limit, with both monotonic and cyclic load conditions considered. • Comparative investigation of creep rupture evaluations is discussed. • Unified material data is introduced for different analyses. • Deep understanding of creep rupture failure mechanisms for high-temperature structures is provided via user subroutine. • Constructed monotonic and cyclic creep boundaries of structure for design purposes. • An effective verification for creep rupture boundary is illustrated. [ABSTRACT FROM AUTHOR]

Abstract: Statistical analysis is an efficient method for the optimisation of any candidate mathematical model of creep rupture data, and for the comparative ranking of competing models. However, when a series of candidate models has been examined and the best of the series has been identified, there is no statistical criterion to determine whether a yet more accurate model might be devised. Hence there remains some uncertainty that the best of any series examined is sufficiently accurate to be considered reliable as a basis for extrapolation. This paper proposes that models should be validated primarily by parametric graphical comparison to rupture data and rupture gradient data. It proposes that no mathematical model should be considered reliable for extrapolation unless the visible divergence between model and data is so small as to leave no apparent scope for further reduction. This study is based on the data for a 12% Cr alloy steel used in BS PD6605:1998 to exemplify its recommended statistical analysis procedure. The models considered in this paper include a) a relatively simple model, b) the PD6605 recommended model and c) a more accurate model of somewhat greater complexity. [Copyright &y& Elsevier]

Abstract: The worked example presented in BSI document PD6605-1:1998, to illustrate the selection, validation and extrapolation of a creep rupture model using statistical analysis, was independently examined. Alternative rupture models were formulated and analysed by the same statistical methods, and were shown to represent the test data more accurately than the original model. Median rupture lives extrapolated from the original and alternative models were found to diverge widely under some conditions of practical interest. The tests prescribed in PD6605 and employed to validate the original model were applied to the better of the alternative models. But the tests were unable to discriminate between the two, demonstrating that these tests fail to ensure reliability in extrapolation. The difficulties of determining when a model is sufficiently reliable for use in extrapolation are discussed and some proposals are made. [Copyright &y& Elsevier]

Abstract: The finite element-based Continuum Damage Mechanics (CDM) software DAMAGE XXX has been developed: to model high-temperature creep damage initiation, evolution and crack growth in 3-D engineering components; and, to run on parallel computer architectures. The driver has been to achieve computational speed through computer parallelism. The development and verification of the software have been carried out using uni-axial crosswelded testpieces in which the plane of symmetry of the V-weld preparation is orthogonal to the tensile loading axis. The welds were manufactured using 0.5Cr–0.5Mo–0.25V ferritic parent steel, and a matching 2.25Cr–1Mo ferritic steel weld filler metal. The Heat Affected Zones (HAZ) of welds were assumed to be divided into three sub-regions: Coarse grained-HAZ (CG-HAZ); Refined grained-HAZ (R-HAZ); and, the inter-critical HAZ regions (Type IV-HAZ). Constitutive equations and associated parameters are summarised for weld, CG-HAZ, R-HAZ, Type IV-HAZ, and parent materials, at 575, 590, and 600°C. These are used to make finite element-based predictions of crossweld testpiece lifetimes and failure modes using the newly developed 3-D parallel computer software, and independent 2-D serial software, at an average minimum cross-section stress of 69.5MPa. Crossweld testpiece analyses, done using the newly developed 3-D parallel software, have been verified using independent results of 2-D serial software; and, of laboratory experiments. [Copyright &y& Elsevier]

Application of the P-NID procedure for modelling and extrapolation is demonstrated for a sparse set of rupture test results for Inconel 740H. Models based on data restricted to 8000 h are extrapolated and compared to longer-term data at up to 70,000 h. Further extrapolations are presented for lifetimes in the range 100,000 h to 200,000 h, and the reliability of these extrapolations is discussed. Comparisons are made to published extrapolations based on conventional models. Highlights of draft paper Rupture modelling and extrapolation for a sparse dataset for Inconel 740H. • Demonstrates extension of the P-NID procedure for rupture modelling to a sparse dataset. • Presents alternative models for Inconel 740H and metrics of their accuracy. • Identifies most reliable model for extrapolation to over 100,000 h. • Demonstrates somewhat better long-term properties for Inconel 740H than published previously. [ABSTRACT FROM AUTHOR]

Abstract: Creep-strength enhanced ferritic steels such as Gr.91, Gr.92 and Gr.122 have been introduced for power plant applications recently, and some have experienced creep failure in boiler tubes and thick wall components after several years of operation. In order to use these steels safely in power plants, establishment of creep life prediction and design factors for base metal and weldment is essential. In this paper, creep rupture strengths and lives obtained by means of uniaxial creep testing and internal pressure component testing for the above-mentioned high-strength ferritic steels are presented comparatively. Design life and weld reduction factors are discussed based on the data and on ASME criteria for establishing allowable stresses in the time-dependent temperature region. [Copyright &y& Elsevier]

Abstract: Creep life assessment technology has not yet been developed for Mod.9Cr-1Mo steel welded joints due to the unique degradation and failure mechanisms in the heat affected zone. Nevertheless, there is strong demand from power plant operators for the development of nondestructive damage detection and life assessment technology for weldments of this steel. In this study creep rupture testing using a large-size welded joint was conducted, and creep and damage detection tests were carried out to elucidate the degradation mechanism. Also the microstructure and hardness changes in the heat affected zones of the welds were investigated to clarify the degradation mechanisms of such welds in comparison with base metal during creep. In general, the changes in hardness distribution along the welds were difficult to correlate with the life consumption, although degradation in welds due to creep was successfully detected. Accordingly, a new approach to degradation evaluation and creep life assessment by hardness measurement method for Mod.9Cr-1Mo steel welds is proposed. [Copyright &y& Elsevier]

Abstract: This paper is concerned with reliability analysis and residual life prediction of HK40 austenitic steel based on their creep rupture data by introducing a Z-parameter method. A normal distribution is supported for the value of the Z-parameter and the distribution curves of tubes after service exposure have been shown to deviate to the right with an increase in service time. According to creep rupture data both from different segments of a long-term service tube and from the same segment locations after different service time, it is proven that the Z-parameter reflects the deterioration in creep rupture properties. Residual life prediction and damage evaluation by Z-parameter method based on confidence level are also presented. [Copyright &y& Elsevier]

Abstract: Design and life assessment procedures for high temperatures are based on ‘expert knowledge’ in structural mechanics and materials science, combined with simplified methods of structural analysis. Of these, R5 is one of the most widely used life assessment methods internationally with procedures based on reference stress techniques and shakedown calculations using linear elastic solutions. These have been augmented by full finite element analysis and, recently, the development of a new programming method, the linear matching method (LMM), that allows a range of direct solutions that include shakedown methods and simplified analysis in excess of shakedown. In this paper, LMM procedures are compared with calculations typical of those employed in R5 for cyclic loading problems when the assumption of a constant residual stress field is appropriate including shakedown and limit analyses, creep rupture analysis and the evaluation of accumulated creep deformation. A typical example of a 3D holed plate subjected to a cyclic thermal load and a constant mechanical load is assessed in detail. These comparisons demonstrate the significant advantages of linear matching methods for a typical case. For a range of cyclic problems when the residual stress field varies during the cycle, which include the evaluation of plastic strain amplitude, ratchet limit and accumulated creep strains during a high temperature dwell periods, the corresponding LMM and R5 procedures are discussed in an accompanying paper. [Copyright &y& Elsevier]

A Z-parameter method based on the Larson–Miller relationship is proposed to assess the remaining life of a CrMoV steel. It is found that the non-linear master curve of stress σ vs Larson–Miller parameter P of the steel can be expressed as: P=26.68-2.49 log σ-0.01σ. A family of curves parallel to the master curve can be expressed as: P=Z-2.49 log σ-0.01σ, where the value of Z represents the deviation from the master curve which corresponds with the deterioration in rupture properties. The deterioration in rupture properties is connected with the degradation of microstructure, namely the spheroidization of carbides in the pearlitic steel. A relationship between the value of the Z-parameter and the level of spheroidization E can be expressed as: Z=27.32-0.54E. A normal distribution is supported for the value of the Z-parameter by both normal probability plotting paper and Shapiro–Wilk normality tests. [Copyright &y& Elsevier]

Creep rupture tests on uniaxial and double notched bar specimens have been performed at 550 °C to understand the multiaxial stress rupture behaviour in two different materials Durehete 1055 and 2.25Cr1Mo steel. The criterion for governing multiaxial rupture stress for the materials was established and compared from the relationship between the representative stress and the skeletal point stresses. Different multiaxial stress rupture criterion were produced.Semi-quantitative metallurgical evaluation of creep damage on the representative tested specimens has been carried out to investigate the stress-state sensitivity of creep damage distribution in two different materials. The steady state stress distribution from finite element solutions was correlated with the semi-quantitative damage map of the test specimens. The different governing multiaxial stress criterion between 2.25Cr1Mo steel and Durehete 1055 steel resulted in the significant difference in creep damage distribution across the notch throat of the materials. [Copyright &y& Elsevier]

The recently developed linear matching method (LMM), which is easily implemented within commercial FE codes, has been successfully used to evaluate elastic and plastic shakedown loads. In this paper, the method is extended to the prediction of the creep rupture life of a structure, based upon a bounding method currently used in the life assessment method R5. The method corresponds to the requirement that, for the operating load history, the structure should shakedown where the yield stress is given by the lesser of the plastic yield stress and a high temperature rupture stress corresponding to a rupture time. A holed plate subjected to cyclic thermal load and constant mechanical load is assessed in detail as a typical example to confirm the applicability of the above procedures. The examples show that the method remains numerically stable, even when the method is inverted. [Copyright &y& Elsevier]