Materials at High Temperature Vol 19, Issue 4, 2002

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Simple modelling of the constitutive behaviour of a 1%CrMoV rotor steel in service-like thermo-mechanical fatigue tests

F. Colombo1, B. Masserey1, E. Mazza1 and S.R. Holdsworth2

1Institute of Mechanical Systems, Centre of Mechanics, CLA J25, ETH Zurich, 8092 Zurich, Switzerland

2Alstom Power, Steam Turbine R & D, Rugby CV21 2NH, UK

Thermo-mechanical fatigue tests have been performed to determine the cyclic constitutive behaviour of a 1%CrMoV rotor steel. The mechanical strain and temperature control profiles adopted in these tests were selected to represent loading conditions which could occur at critical rotor locations in a high temperature turbine rotor in service. The cyclic evolution of the elastic-plastic and creep stress/strain states is analysed. Cyclic deformation influences creep behaviour as well as the yield characteristics of this steel, the resistance to time independent and time dependent plasticity being reduced by approximately 20% at midlife. A constitutive model based on monotonic data (with time dependent and time independent plasticity components) is developed to determine midlife cycle behaviour and predictions are compared with measured TMF stress-strain curves. Simple rules are proposed which are suitable for application in turbine component lifetime calculations.

Keywords: thermo-mechanical fatigue test, 1%CrMoV rotor steel

Thermo-mechanical fatigue: the route to standardization

Peter Hähner* and Johan Bressers

European Commission, Joint Research Centre, Institute for Energy, Mechanical Performance Characterization, NL-1755 ZG Petten, The Netherlands

Thermo-mechanical fatigue (TMF) testing plays an increasingly important role in the design of various safety critical components which are exposed to thermal and mechanical loads. At present, however, a standard for TMF testing does not exist, causing a lack of comparability of TMF data. In view of this situation, the European Commission is funding a project, entitled Thermo-Mechanical Fatigue: The Route to Standardization (TMF-Standard). The objectives of the TMF-Standard project are to provide, at the European level, the technical basis that will enable to define in a comprehensive way all testrelated aspects of a standard for strain-controlled TMF, to draft the findings in the form of a Code-of- Practice for TMF testing, to co-ordinate the European input into the ISO Working Group on TMF, and to disseminate the Code-of-Practice among all interested parties with a view to promoting the implementation of a European TMF testing platform consisting of a chain of laboratories measuring TMF properties according to sound metrology principles, thus improving the industrial competitiveness and promoting free trade. In the present paper, the context, scope and content of the TMF-Standard project are described in some detail.

Keywords: thermo-mechanical fatigue

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

R. Hales1, S. R. Holdsworth2, M. P. O’Donnell3, I. J. Perrin2, 4 and R. P. Skelton5*

1Consultant, Gloucester, UK

2Alstom Power, Steam Turbine R & D, Rugby CV21 2NH, UK

3British Energy Generation Ltd., Barnett Way, Barnwood, Gloucester GL4, 3RS, UK

4Now at: Alstom Power, Power Plant Laboratories, 2000 Day Hill Road, Windsor Connecticut 06095, USA

5Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

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.

Keywords: cyclic stress-strain data, single step, multiple step, incremental step tests, elastic modulus, cyclic yield stress, hardening, softening, process equations.

Use of conventional stress-strain data to develop parameters for an advanced constitutive model

M. P. O’Donnell1, S. K. Bate2, I. Bretherton2, D. N. Gladwin1 and J. P. Hayes2

1British Energy Generation Ltd., Barnett Way, Barnwood, Gloucester GL4 3RS, UK

2Serco Assurance, Walton House, 404 The Quadrant, Birchwood, Warrington, Cheshire WA3 6AT, UK

For structural integrity assessment of power generating plant components it is becoming increasingly important to take advantage of inelastic constitutive models. This is particularly important for cases such as, severe loading conditions, postulated fault conditions, complex loading histories, complex geometries, and/or to reduce the conservatisms inherent in simplified assessment or design approaches. The inelastic model outlined in this paper has been developed within this framework and with a view to providing a robust representation of material behaviour with a practical approach to the number of material constants required. The model has the capability to capture the evolving nature of the materials being considered over a range of loading conditions and temperatures. Additionally, its incremental form is applicable for use under complex loading histories where simple process equations such as the power law are not capable of representing the path dependence of the load history, which results from the dissipation of energy during the plastic deformation process. This paper focuses on plasticity and provides details of a combined isotropic non-linear kinematic model, the fast reactor state variable (FRSV) model. The material parameters required to represent the evolutionary plasticity behaviour in Type 316 stainless steel from 400 to 650°C and total strain ranges from 0.4 to 2% have been determined. Details of the material testing requirements and the methodology used to derive the plasticity constants are outlined. Validation of the model against the experimental data is provided via finite element calculations.

Keywords: constitutive modelling, cyclic plasticity

Cyclic stress-strain properties of serviceexposed ferritic steels for use in thermal fatigue assessments

R. P. Skelton

Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

This paper is concerned with the laboratory isothermal (low cycle) fatigue response of sixteen ferritic steel samples taken from power station components which have seen between 70,000 h and 170,000 h service exposure to high temperatures. After suitable machining into specimens the cyclic stress-strain properties between 300 and 600°C at strain rates between 6 _ 10–4/s and 5 _ 10–6/s were established using the multiple step technique (which is discussed in some detail). Ferritic steels generally cyclically soften, and by evolutionary tests undertaken at 550°C it is demonstrated that start/up and shut/down procedures in service have accelerated this process in material subjected to temperature transients. The steady-state cyclic stress-strain response has shown a marked loss in strength compared with data on a selection of unexposed steels e.g. material trepanned from turbine casings at the manufacturing stage or from specially cast test blocks. The data have been fitted to the Ramberg–Osgood law and are presented in Tables for direct application to remanant life assessments requiring stress analysis. The effects of artificial ageing are compared with the response of service-exposed material. Full chemical analyses and abbreviated service histories of the materials are presented together with graphical illustrations of trends in the data extracted from the Tables.

Keywords: cold/hot starts, cyclic softening, multiple step test, Ramberg-Osgood law, service ageing, strain rate effect

Elevated temperature cyclic stress-strain behaviour in nickel based superalloys

I.M. Wilcock, D.G. Cole, J.W. Brooks and M.B. Henderson1

QinetiQ, Cody Technology Park, Farnborough, Hampshire GU14 0LX, UK

1ALSTOM Power Technology Centre, Whetstone, Leicester LE8 6LH, UK

A number of nickel superalloys which respond differently to cyclic stress-strain have been investigated by means of strain controlled low cycle fatigue testing. Fifteen tests have been conducted on cylindrical specimens of the powder metallurgy nickel disc alloy Udimet 720Li (U720Li) at temperatures of 650°C and 700°C. Comparisons are made to the behaviour seen in LCF tests on the commonly used gas turbine combustor material Nimonic alloy C263 which is seen to cyclically harden at 300°C and soften at 800°C. The test procedure and subsequent data analyses have been detailed, referring constantly to that recommended by the Code of Practice for the Determination and Interpretation of Cyclic Stress-Strain Data. A selection of hysteresis loop data is presented and details are provided for the method used to calculate elastic modulus. For U720Li, the evolution of maximum stress and total stress range throughout the tests is investigated and it is seen in general that neither hardening nor softening occurs during these tests, and it is concluded that this is due to the relatively small plastic strains applied. The C263 has been tested under conditions that show a change from cyclic plastic hardening to time-dependent softening behaviour, and this can be clearly seen in the cyclic stress-strain data.

Keywords: nickel-based superalloy, cyclic stress-strain, fatigue