Creep Dwell Research Articles

Recent developments in the R5 procedures for assessing the high temperature response of structures

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and for assessing components containing defects (Volume 4/5).The current R5 Volume 2/3 procedure for assessing creep-fatigue initiation in weldments involves analysis assuming homogeneous parent material properties with a single Fatigue Strength Reduction Factor (FSRF) used to account for both the reduction in fatigue endurance and enhancements in strain due to material mismatch and local geometry effects. A new development of this approach involves splitting the FSRF into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhancement Factor (WSEF), which accounts for material mismatch and local geometry effects. The new approach results in less conservative predictions of creep damage than are obtained using the existing approach as it is only the WSEF that affects the stress at the start of the creep dwell rather than the larger FSRF in the existing approach. However, the new approach has little effect on the calculated fatigue damage as the combined effects of the WER and the WSEF are essentially equivalent to the previous FSRF.In addition to methods for predicting creep and creep-fatigue crack growth for load-controlled situations, R5 Volume 4/5 gives advice on the prediction of crack growth for situations involving combined primary and secondary loading. This advice has recently been extended to cover cases involving significant amounts of crack growth, C(t) estimation methods for elastic-plastic-creep behaviour and explicit advice on the treatment of combined loading for situations involving significant welding residual stresses. This extended advice can be used to predict the growth of defects in non-stress relieved austenitic weldments operating at elevated temperatures.This paper briefly outlines the current R5 Volume 2/3 and Volume 4/5 procedures and then focuses on these two significant recent developments in R5, both of which are highly relevant to real instances of creep and creep-fatigue cracking that have been observed in high temperature austenitic plant components.

Effects of dwell location on the creep-fatigue endurance of cast Type 304L

Thermo-mechanical cycling leads to creep-fatigue cycles in which the dwells can be in a variety of positions. In-phase cycling gives dwells at the peak. Out of phase cycling (by 180°) gives compressive dwells and 90° phase cycling gives a cycle in which the dwell is at an intermediate position within the cycle. Furthermore, components often experience cycles with a variety of different strain amplitudes and the creep dwells do not experience pure strain control but may contain elastic follow-up. To investigate the effects of these different factors on creep-fatigue endurance a series of isothermal tests have been conducted on Cast 304L steel at 650°C. These tests were then analysed using the British Energy R5 procedure, which was found to be overly conservative for tests with compressive dwells and for tests with dwells at an intermediate position within the cycle. This paper describes alternative approaches, for these types of cycles that give improved predictions of failure.

Effects of dwell location on the creep-fatigue endurance of cast Type 304L

Thermo-mechanical cycling leads to creep-fatigue cycles in which the dwells can be in a variety of positions. In-phase cycling gives dwells at the peak. Out of phase cycling (by 180°) gives compressive dwells and 90° phase cycling gives a cycle in which the dwell is at an intermediate position within the cycle. Furthermore, components often experience cycles with a variety of different strain amplitudes and the creep dwells do not experience pure strain control but may contain elastic follow-up. To investigate the effects of these different factors on creep-fatigue endurance a series of isothermal tests have been conducted on Cast 304L steel at 650°C. These tests were then analysed using the British Energy R5 procedure, which was found to be overly conservative for tests with compressive dwells and for tests with dwells at an intermediate position within the cycle. This paper describes alternative approaches, for these types of cycles that give improved predictions of failure.

An improved method for calculation of creep damage during creep–fatigue cycling

Thermal mechanical cycling can result in creep–fatigue cycles in which the creep dwell has a wide variety of positions within the cycle. The effects of creep dwell position have been investigated using tests on three austenitic stainless steels: Type 316H, cast Type 304L and Type 347 weld metal. The analysis of these tests has resulted in the proposal of a new method to calculate creep damage. This method has been shown to give better predictions for the creep damage at failure than both the standard ductility exhaustion approach and the time fraction approach. In particular, the new method gives improved predictions of creep damage at failure for cycles with intermediate dwells and for cycles with low strain ranges. The present paper summarises the validation of this new method, the effects of multiaxial states of stress and the effects of compressive dwells.

Modeling of the Behavior of a Welded Joint Subjected to Reverse Bending Moment at High Temperature

The paper is concerned with the modeling of the behaviour of welds when subjected to severe thermal and mechanical loads where the maximum temperature during dwell periods lies in the creep range. The methodology of the life assessment method R5 is applied where the detailed calculations are carried out using the linear matching method (LMM), with the objective of generating an analytic model. The linear matching method has been developed to allow accurate predictions using the methodology of R5, the UK life assessment method. The method is here applied to a set of weld endurance tests, where reverse bending is interrupted by creep dwell periods. The weld and parent material are both Type 316L(N) material, and data were available for fatigue tests and tests with 1 and 5h dwell periods to failure. The elastic, plastic, and creep behavior of the weld geometry is predicted with the LMM using the best available understanding of the properties of the weld and parent material. The numerical results are translated into a semi-analytic model. Using the R5 standard creep/fatigue model, the predicted life of the experimental welds specimens are compared with experimental data. The analysis shows that the most severe conditions occur at the weld/parent material interface, with fatigue damage concentrated predominantly in the parent material, whereas the creep damage occurs predominantly in the weld material. Hence, creep and fatigue damage proceed relatively independently. The predictions of the model are good, except that the reduction in fatigue life due to the presence of the weld is underestimated. This is attributed to the lack of separate fatigue date for the weld and parent material and the lack of information concerning the heat affected zone. With an adjustment of a single factor in the model, the predictions are very good. The analysis in this paper demonstrates that the primary properties of weld structures may be understood through a number of structural parameters, defined by cyclic analysis using the linear matching method and through the choice of appropriate material data. The physical assumptions adopted conform to those of the R5 life assessment procedure. The resulting semi-analytic model provides a more secure method for extrapolation of experimental data than previously available.

Integrity assessment of a 3D tubeplate using the linear matching method. Part 2: Creep relaxation and reverse plasticity

In an accompanying paper [Int J Pres Ves Pip, 2004], a 3D tubeplate in a typical AGR superheater header was analysed for the shakedown limit, reverse plasticity and ratchet limit based on the application of the linear matching method (LMM). Both the perfectly plastic model and the cyclic hardening model were adopted for the evaluation of the plastic strain range. In the present paper, based on the creep-reverse plasticity model, the accumulated creep strain during a creep dwell at the steady cyclic state is assessed by the LMM. The accumulated creep strains, the creep flow stresses as well as the elastic follow-up factors with differing dwell times are evaluated in detail. The total inelastic strain over the cycle at the steady cyclic state is calculated. By comparing these results with ABAQUS step-by-step inelastic analyses, the applicability of the methods is demonstrated.

A simplified creep-reverse plasticity solution method for bodies subjected to cyclic loading

An extension of the upper bound shakedown theorem to load histories in excess of shakedown has been applied recently to the evaluation of a ratchet limit and the varying plastic strain magnitudes associated with a varying residual stress field. Solutions were obtained by the Linear Matching Method. In the present paper, this technique is extended to the evaluation of creep-reverse plasticity mechanism for bodies subjected to thermal cyclic loading including creep effects. The accumulated creep strain, the varying flow stress and the corresponding varying residual stress field during a creep dwell time are evaluated as well as the elastic follow-up factor. Three alternative computational strategies are discussed with differing but related assumptions. The problem of a plate with a central circular hole is discussed, subjected to cyclic thermal load. All three methods provide similar values for the elastic follow-up factor, indicating that the result is insensitive to the range of assumptions made. The simplest method, Method 1, is suggested as the basis of a general purpose method for use in life assessment.

A simplified creep-reverse plasticity solution method for bodies subjected to cyclic loading

An extension of the upper bound shakedown theorem to load histories in excess of shakedown has been applied recently to the evaluation of a ratchet limit and the varying plastic strain magnitudes associated with a varying residual stress field. Solutions were obtained by the Linear Matching Method. In the present paper, this technique is extended to the evaluation of creep-reverse plasticity mechanism for bodies subjected to thermal cyclic loading including creep effects. The accumulated creep strain, the varying flow stress and the corresponding varying residual stress field during a creep dwell time are evaluated as well as the elastic follow-up factor. Three alternative computational strategies are discussed with differing but related assumptions. The problem of a plate with a central circular hole is discussed, subjected to cyclic thermal load. All three methods provide similar values for the elastic follow-up factor, indicating that the result is insensitive to the range of assumptions made. The simplest method, Method 1, is suggested as the basis of a general purpose method for use in life assessment.

Flex testing of natural and butyl rubber

Abstract Interspersed fatigue (pulsating flexing) and creep (dwell time) cycles were applied to samples of natural and butyl rubber at room temperature, lab-air conditions. In this experimental program the natural rubber was found to be sensitive to the application of dwell times under peak load conditions. Cavities and cracking were observed as a result of dwell time lowering its resistance to interspersed fatigue and creep cycles. On the other hand, flexing resistance of butyl rubber was found to be inferior to natural rubber. These results are presented in this technical note.

Cyclic plasticity, creep and fracture in a Nimonic alloy

Deformation and fracture are examined for a cyclic loading condition in which a 24 h period of forward creep in IN 597 at 850°C is reversed (a) by a similar period of creep and (b) by fast monotonic strain-limited plasticity with and without a subsequent 1 h period of relaxation. When ratchetting is evident, it is shown that a life fraction rule based upon the Monkman—Grant law describes the observed behaviour and that, for a given creep stress, the accumulated creep strain—time plot correlates with a simple creep curve. At lower-magnitude creep dwell stresses a cyclic equilibrium state is achieved. The absence of ratchetting during equilibrium results in a net increase in rupture life and fracture strain when compared with those for simple creep under the same stress. Time-limited cyclic behaviour is compared with further cyclic tests in which the reversal occurs at fixed limits of total strain. These did not show an equilibrium condition but a progressive cyclic softening in which the creep time and strain successively diminished. Predictions to individual creep curves are made from the mechanical equation of state and the time hardening law. However, empiricism is preferred to describe the softening behaviour observed for the alloy more precisely. A theoretical assessment is made of the effect of imposing a period of relaxation at peak reversed strain upon cyclic creep deformation and life.

INTERACTIONS BETWEEN TIME AND CYCLIC DEFORMATION PROCESSES IN A NIMONIC ALLOY

Interactive creep–fatigue behaviour of a nickel‐base superalloy (IN 597) has been examined at 850 °C under various strain‐limited, cyclic torsional loading conditions. In one test, forward creep deformation was reversed by creep under equal magnitude stress levels and strain limits. In other tests, forward creep strain was reversed by fast monotonic plasticity with and without a subsequent period of relaxation. These cycles were repeated within each test until fracture. This paper examines empirically the influence of a number of test variables upon cyclic creep curves, and demonstrates the usefulness of predictions based upon continuous low cycle fatigue and simple creep data when used in conjunction with a mechanical equation of state. A cyclic equilibrium condition was not achieved from these tests. Instead, a progressive softening occurred giving reductions to the amount of creep strain, creep time interval and reversed peak stress with each new cycle. Such reductions are expressed from derived formulae that embrace the range of inelastic strain, cycle number, creep dwell stress, reversed peak stress, and times expended in creep and relaxation.Observations made on accumulated creep strain reveal the contribution to a creep–fatigue fracture from cyclic creep. This has led to a modified form of the linear damage rule which can provide conservative life predictions for components operating in service under similar cyclic conditions.

Inelastic behaviour of rectangular beams subjected to steady axial load and cyclic bending

Analytical and numerical solutions are given for the inelastic behaviour of a rectangular cross-section beam, subjected to a steady axial load and cyclic, fully reversed load-controlled bending. Elastic-perfectly plastic, linear isotropic and kinematic materials are examined and solutions for the strain accumulation and cyclic reverse plastic strains obtained. Also the effect of introducing a creep dwell period into the cycle is considered.

The Accumulation of Damage in Plates Subjected to Steady Mechanical Load and Cyclic Thermal Downshocks on One Surface

Computer calculations, incorporating analysis of elastic, plastic and creep effects, are performed on a Bree plate subjected to steady mechanical load and cyclic thermal downshocks on one surface. Particular attention is given to the accumulation of creep and fatigue damage which is calculated from the stress-strain distributions using linear damage summation rules. The computations examine the dependence of the damage accumulation on such factors as the applied loads, the degree of hardening in the material, the length of the creep dwell period and the severity of the transient thermal loadings as measured by the Fourier and Biot numbers. This indicates which considerations are likely to be important in the design of fast reactor components which experience loadings of this type.