S.R. Holdsworth

A Code of Practice for the determination of cyclic stress-strain data

Abstract There are no procedural standards for the determination of stress-strain properties where a reversal of stress is involved. The purpose of this Code of Practice is to detail the requirements for cyclic stress-strain (CSS) testing on uniaxial testpieces. CSS testing may entail the use of a single testpiece to produce data over several strain ranges. Alternatively, data from a number of constant strain range tests may be obtained, for example as the by-product of a series of low cycle fatigue (LCF) endurance tests. Procedures for LCF testing are covered by a number of existing Codes of Practice and Standards [1–6], and this document does not recommend any alteration to these. This Code of Practice has been prepared by the CSS Working Party of the ESIS TC11 High Temperature Mechanical Testing Committee. Historically, CSS results have been reported in terms of a relatively simple power law. However, engineers involved in design and assessment activities are now increasingly tending to use more advanced constitutive relations such as the Chaboche equations [7]. Hence, the model equations available to characterise CSS behaviour cover a range of complexities, with the approach selected being determined by the requirements of the end-user application. These will be influenced by such factors as the type and history of loading, the operating temperature and presence of thermal gradients, the variation of cyclic plastic strain within the component, and the need to determine absolute magnitudes or ranges of stress and strain. The laboratory test procedures defined in this Code of Practice are capable of generating the CSS data required for the full spectrum of model equations currently used in engineering assessment. In addition to recommending best laboratory practice, this document includes sections on engineering requirements, test data analysis (including the connection between alternative forms of model equation), and the exploitation of existing data. Advice is also given for those circumstances where testpiece material is limited, thus requiring quick methods of data acquisition using block loading techniques. In all cases, the use of cylindrical testpiece gauge lengths is recommended, and only isothermal testing at appropriate temperatures under strain-controlled conditions is covered.

Creep-fatigue properties of high temperature turbine steels

Abstract Cyclic/hold creep-fatigue properties for the new advanced 9–11%Cr steels are reviewed and shown to be significantly superior to those of 1%CrMoV turbine steels at 550°C. Moreover, cyclic/hold endurances for the creep resistant martensitic stainless steels at 600°C are at least as good as those for 1%CrMoV turbine steels at 550°C. An assessment of the creep-fatigue damage interaction characteristics of Grade 91 steel shows them to be no worse than those of 1%CrMoV turbine steels at their respective maximum application temperatures.

Stress regime-dependent creep constitutive model considerations in finite element continuum damage mechanics:

Structural analysis and the design of high-temperature components require the consideration of material inelastic deformation and damage accumulation characteristics for a wide range of stresses and temperatures. For many engineering alloys, the creep deformation/damage accumulation mechanism exhibited at high stresses are not the same as those at lower stresses. This article explains the importance of considering this stress regime dependency in the creep model formulations and introduces a new (primary–secondary–tertiary) creep model which considers a gradual change of creep deformation/damage accumulation mechanism with stress variation. Application of the new stress regime-dependent creep model formulation in finite element continuum damage mechanics to simulate creep deformation/damage accumulation in a series of creep crack incubation tests involving fracture mechanics compact tension specimens shows a good agreement with experimental observations which was not achievable by considering conventional single-regime creep model equations.

Code of practice for the measurement and analysis of high strain creep-fatigue short crack growth

Abstract There are no procedural standards for the determination of crack growth properties where a reversal of stress is involved, in particular at elevated temperatures. The purpose of this code of practice is to detail the requirements for fatigue and creep-fatigue short crack growth (CFSCG) testing, generally on uniaxial testpieces subject to high strain loading conditions. CFSCG testing may entail the use of a single testpiece to produce data over several strain ranges. Alternatively, data from a number of constant strain range tests may be obtained, as in conventional low cycle fatigue (LCF) endurance, but where a small starter (sharp) defect has been introduced. Procedures for long crack fatigue crack growth and LCF testing are covered by a number of existing codes of practice and standards, and this document does not recommend any alteration to these. CFSCG rates may be determined from testpieces with much larger notch acuities, e.g. uniaxial specimens of the Bridgman type, or reverse-bend specimens with a stress concentrating feature. Thus in addition to recommending best laboratory practice, this document includes sections on engineering requirements and test data analysis. Advice is also given for those circumstances where testpiece material is limited, thus requiring quick methods of data acquisition using block loading techniques. Only isothermal testing at appropriate temperatures under strain-controlled conditions is covered, although the techniques can be extended to non-isothermal (thermo-mechanical) fatigue testing conditions. An appendix covers in more detail: (i) the relation between the shape of short cracks (straight-fronted, thumbnail, etc.) and the specimen area thereby consumed by the fracture surface, and (ii) the relation between this fractional area change and the corresponding change in both the crack-monitoring potential drop signal and drop in peak tension load.

Developments in the assessment of creep strain and ductility data

Technical support to the European Creep Collaborative Committee (ECCC) is provided by WG1 and its subgroups. The main role of WG1 is to provide recommendations on procedures for data generation, collation and assessment to form the basis of common practices followed within ECCC [1]. However, the recommendations are published and are thereby available to other users. They are acknowledged to have influenced several important testing and assessment standards (e.g. [2–4]). Two WG1 sub-groups focus specifically on the provision of procedures relating to post service exposed materials (WG1.1) and creep crack initiation data (WG1.2). Following a short historical review of developments relating to the assessment of creep rupture data, the paper focuses on some present activities of WG1 (the membership of WG1 is given in Appendix C). During the ECCC ADVANCED-CREEP initiative (2001–2005), the main concerns of the group are the analysis of creep deformation and ductility properties for application to the assessment of components and multi-axial features.

The LICON methodology for predicting the long term service behaviour of new steels

Abstract The Brite-Euram funded LICON Project has developed a methodology for predicting the long-term creep-rupture behaviour of new generation steels, including their welded joints, in relatively short duration multi-axial specimen tests. The methodology relies on the acceleration of creep damage development under multi-axial loading conditions to enable extended extrapolation of rupture strength into the long time fracture regime. This approach provides similarity with the loading conditions experienced in real structures and enables a more accurate evaluation of the future in-service performance of welded components made of new generation steels for which no long-term service experience exists. This paper summarises the work performed and the results achieved in the project. The paper has been prepared on behalf of the LICON project consortium.

Creep constitutive model considerations for high-temperature finite element numerical simulations:

Finite element modelling is increasingly used as an integral part of creep analyses for the integrity assessment of high-temperature structures. An important consideration in such finite element simulations is the constitutive model used to represent the creep strain response of the component material as a function of temperature, stress and time. There are a variety of creep models which can be chosen by the analyst for implementation in finite element codes. In this study, five different creep models have been fitted to a set of experimental uniaxial creep curves for a 1%CrMoV turbine rotor steel at 550 °C. Subsequently, the derived constitutive equations have been implemented in finite element model representations of a series of fracture mechanics compact tension specimens manufactured from the same heat of the steel and loaded at the same temperature. The outcome clearly demonstrates the potential sensitivity of high-temperature numerical analyses of structures to the type of creep model adopted, and to the scope of the experimental data from which the model is derived. This is shown by comparing the load point displacement records from a number of compact tension specimen creep crack incubation tests with the results of finite element simulations employing the different creep deformation models. FE calculated steady-state creep stress/strain distributions ahead of the notches of compact tension specimens can also exhibit a strong sensitivity to the type of creep model adopted. Prior benchmarking the effectiveness of finite element simulation procedures for critical high-temperature components with the selected material creep model equation is therefore strongly recommended.

Exploring the applicability of the LICON methodology for a 1%CrMoV steel

Abstract The results of a series of creep crack incubation (CCI) tests have been used to examine the effectiveness of the LICON methodology for predicting long duration uniaxial rupture properties of a 1%CrMoV steel at 550°C. It has been demonstrated that good predictions can be made using this approach but that, for this to be possible, a knowledge of additional information is also required including uniaxial creep strain data, uniaxial and multi-axial rupture time properties and the results of finite element analysis. In order to apply the creep damage enhancement approach to the low alloy creep resistant steel, it has been necessary to establish the steady-state creep stress state in the compact tension testpiece geometry used in the investigation. The new evidence is described.

Temperature dependent representation for Chaboche kinematic hardening model

Abstract The assessment of high temperature components under cyclic deformation conditions increasingly relies on determinations of the stress–strain state at the critical location using non-linear finite element analysis. An important consideration in such finite element simulations is the used constitutive model. The Chaboche model has been widely accepted as an advanced model for such applications. This study evaluates the variation of Chaboche model parameters with temperature for low cycle fatigue conditions and introduces an approach to systematically calibrate the model for a range of temperatures, rather than for single temperatures. Furthermore, mathematical representations have been proposed to consider the effect of superimposed creep deformation on the Chaboche model parameters. Successful application of the proposed approach/formulation for representing the behaviour of a 10%Cr steel under low cycle fatigue and cyclic/hold deformation conditions for the temperature range of 20–625°C is presented.

Creep-Fatigue Failure Diagnosis

Failure diagnosis invariably involves consideration of both associated material condition and the results of a mechanical analysis of prior operating history. This Review focuses on these aspects with particular reference to creep-fatigue failure diagnosis. Creep-fatigue cracking can be due to a spectrum of loading conditions ranging from pure cyclic to mainly steady loading with infrequent off-load transients. These require a range of mechanical analysis approaches, a number of which are reviewed. The microstructural information revealing material condition can vary with alloy class. In practice, the detail of the consequent cracking mechanism(s) can be camouflaged by oxidation at high temperatures, although the presence of oxide on fracture surfaces can be used to date events leading to failure. Routine laboratory specimen post-test examination is strongly recommended to characterise the detail of deformation and damage accumulation under known and well-controlled loading conditions to improve the effectiveness and efficiency of failure diagnosis.