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The development of high-temperature damage in Ni-alloy/steel dissimilar metal welds and its influence on creep-rupture (CR) times and creep-fatigue (CF) endurances is examined. The importance of a ...

Since the preliminary data suggest that Fe-25Ni-20Cr austenitic stainless steel (Alloy 709) is an excellent candidate as a structural material for high-temperature applications such as Sodium-coole...

This work focused on the effect of holding duration at maximum and minimum stress on creep fatigue (CF) interaction of P92 steel based on load-controlled CF tests and conventional creep tes...

Fatigue crack growth (FCG) and creep-fatigue crack growth (CFCG) in austenitic stainless-steel Fe-25Ni-20Cr (Alloy 709) were measured experimentally and simulated using the finite element m...

Creep-fatigue assessment procedures for the design of high-temperature components should ensure lifetime predictions which are safe but not excessively conservative. Adoption of more accurate asses...

The P91 steel is widely used in high temperature components of power plants, and it is a candidate material for Gen-IV reactors. The P91 steel has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water-steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.

It is shown that the assumption of unit (negative) slope in the well known Monkman–Grant plot of time to failure against minimum creep rate is too restrictive. By acknowledging observed slopes in the range 0.8–1, a ductility–strain-rate relation is deduced where ductility decreases with reducing strain rate. This in turn has implications for the ductility exhaustion method as applied during stress relaxation in the dwell period of low cycle fatigue tests of austenitic steels at elevated temperature. The simple method is used to calculate the cyclic creep damage in typical tests on austenitic steels in the region 550–650 °C and is compared to other calculations as employed in the R5 high temperature assessment procedure. The assumption of a uniform nucleation rate of grain boundary voids with creep strain goes some way to predicting the slope of the ductility–strain-rate relation. Both the ‘unconstrained’ and ‘constrained’ (lower shelf) regions of void growth are discussed.

This study reports the fatigue behaviour of 2.25Cr1MoV steel under low cycle fatigue (LCF) loading and creep-fatigue interaction (CFI) loading at 355, 455 and 555 °C. Various hold durations up to 600 s were introduced in the CFI tests at the peak/valley strain under strain or stress control. In LCF tests, the steel exhibited remarkable strengthening at 455 °C, which can be ascribed to the effect of dynamic strain aging. In CFI tests, tensile holds were found more damaging than compressive holds but considerably less harmful than the combined tensile-compressive holds. A modified plastic strain energy approach based on the damage mechanisms was proposed to predict fatigue life under LCF and CFI conditions. The predictions obtained compared very favourably with the experimental results.

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clea...

In this study, by conducting the creep–fatigue crack growth tests using circular notched round bar specimens for Alloy 617, the crack growth behaviour was characterized on the basis of crack growth data obtained from the direct current potential drop (DCPD) method, which results in the assurance of the high accuracy of crack length measurement. Furthermore, on the basis of catastrophe theory, an attempt is made to predict theoretically the threshold temperature of creep crack growth related to the diffusion creep mechanism.

In order to clarify the microdamage formation behaviour for high temperature brittle IN100 alloy, a polycrystalline nickel base superalloy, in situ observational test was carried out to observe creep damage formation behaviour of a notched IN100 specimen. Furthermore, the experimental findings of creep damage formation were evaluated by an original elastic–plastic creep finite element analysis. As a result, microcracks and damage forming areas were found to develop in the creep damage areas suggested by this analysis. This has resulted in the development of a non-destructive prediction method for creep damage accumulation and material’s remnant life. The application of this method to the cases under creep–fatigue conditions was addressed.

Given total endurances of low cycle fatigue specimens, several empirical formulae enable an estimate to be made of ‘initiation’ cycles to a specific crack depth in a zone ranging from 20 to 400 μm from the surface. For greater depths, the cyclic growth behaviour of cracks can be calculated directly from relations using established constants (including the effects of creep dwell) for a wide range of alloys. However, below a critical depth of 200 μm or thereabouts, these empirical growth laws, expressed in terms of total strain range, break down. Early crack growth can take place discontinuously, as the crack tip encounters grain boundaries or other obstacles. Such information arises from replica takings, or in situ microscopic observations of surfaces during continuous cycling and creep–fatigue tests. Striation spacings on the fracture surface and the first observed striation spacing provide valuable indications of early crack growth. In this region, the cyclic rate of growth depends on (i) crack s...

A new testing system has been developed to get a basic understanding on the thermo-mechanical creep-fatigue (TMCF) small crack propagation behaviour around the cooling holes in high temperature Ni base superalloy used for advanced gas turbine components. By means of the test system the TMCF tests under creep-fatigue condition were carried out for a directionally solidified Ni base superalloy hollow specimen with nine cooling holes which were machined in a 3×3 square array within the specimen gauge length, simulating the cooling system in air cooled gas turbine blades. Special attention was paid to the early growth of creep-fatigue crack(s) around the holes. Some fracture mechanics discussions are made to interpret the experimental results and the effect of cooling system on the TMCF failure, taking account of the roles of stress concentration and the local thermal stress induced under a significant temperature gradient.

The W-added 12%Cr ferritic heat resistant steel has been developed as a turbine rotor material which is used under high temperature creep-fatigue conditions. In this study, crack growth tests using C(T) specimen under the creep-fatigue interaction conditions were conducted under various conditions of stress holding time, applied stress and temperatures. As a result, the effect of load frequency on crack growth behaviour and its life for this steel was characterised by the three-dimensional curved surface based on the concept of non-equilibrium science. Furthermore, a predictive law of crack growth life under the creep-fatigue interaction conditions was derived based on the concept of Q* with the transition function of crack growth life from fatigue to creep.

The increasing need for flexible electricity generation means that most components must perform under cyclic operating conditions. The multiplicity of generating options also means that defining a ‘typical’ cycle is difficult. Thus, in addition to the traditional hot and cold starts and stops, there are requirements for rapid load changes on many occasions followed by operation at generation levels of 30% of rated capacity. These transients and operation at low loads result in complex changes in local pressure, temperature and flow at different parts of a boiler system. Indeed, these differences, in combination with the use of metallurgically complex steels, are such that the range of potential damage mechanisms has increased. Understanding these mechanisms is not straightforward as less post service investigations and root cause analyses are being carried out. EPRI seeks to facilitate expert evaluation of service experience and collaborates on establishing lessons learned to establish best practi...

Testing and modelling of creep and creep/fatigue in welded components are essential tools for improving safe lifing methodologies. Key factors which determine a successful methodology for modelling failure in engineering components are good testing data and the development of appropriate and accurate correlating parameters to treat the results in a the unified and verifiable manner. This work considers the first two aspects and identifies important issues and improvements in the development and standardisation in creep and creep/fatigue crack growth testing or welds. Under the auspices of the Versailles Agreement on Materials and Standards (VAMAS) committee a Code of Practice on creep crack growth of components has been developed. The procedure identifies methods of testing for non-standard, welded feature test components in VAMAS TWA31 Technical Working Area on Creep and Creep/Fatigue Crack Growth of weldments containing residual stress. The overview highlights the important elements in these pre...

This paper presents early results from an ongoing round robin study to verify the newly developed ASTM standard E-2760-10 on creep fatigue crack growth testing. This round robin is an international effort with 18 participants from 11 countries from Asia, North America and Europe. All participants are testing compact type, C(T) specimens according to the procedure described in the standard. The test material is P91 taken from a section of retired steam pipe that was heat treated to rejuvenate the microstructure. All background materials data such as tensile, creep deformation and creep fatigue crack formation data are already available on the test material. Creep fatigue crack growth testing was conducted as part of a pilot program at 625°C using C(T) specimens under constant load amplitude conditions at two load levels. The tests are conducted at hold times of 0, 60 and 600 s. Creep fatigue crack growth rates are analysed using the two methods included as part of the test standard. The data from t...

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.

This paper investigates the development of intergranular strains and stresses in AISI type 316H austenitic stainless steel during cyclic loading at high temperature. Isothermal cyclic creep tests at 650°C with 2 h displacement controlled creep dwells at maximum strain are conducted with in situ neutron diffraction monitoring at a time of flight facility. The evolution of intergranular strains in five independent {hkl} grain families were successfully measured during consecutive cycles. The grain family with {111} lattice planes normal to the loading direction exhibited linearly reversible behaviour with cyclic load whereas the {200} planes deformed in a non-linear manner forming a hysteresis loop. Intergranular strains during the first dwell remained unchanged with time, but relaxed with time during later dwells. The start of dwell intergranular strains increased from cycle 1 to cycle 2, but markedly less moving from cycle 2 to cycle 3.

In order to characterise the creep–fatigue crack growth behaviour of a directionally solidified Ni-base superalloy, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method using notched specimens has been carried out. The average misorientation in the vicinity of notches increases linearly up to the initiation of cracks with the increasing creep strains under creep or creep–fatigue conditions, whereas under fatigue conditions fatigue cracks grew without an increase in misorientation. The stress holding time clearly influenced the growth behaviour of creep–fatigue cracks and the appearance of misorientation development. However, it was shown that the relationship between the average misorientation and the relative notch opening displacement was independent of test conditions such as temperatures, stresses and stress wave forms.It is concluded that the misorientation analysis of damaged samples based on the EBSD method allows the prediction of the initiat...

316L(N) stainless steel (SS) containing 0·02–0·03 wt-% carbon and 0·06–0·08 wt-% nitrogen is the principal material for the high temperature structural components of the prototype fast breeder reactor in India. In order to increase the economic competitiveness of sodium cooled fast reactors (SFRs), there is a strong desire to increase the design life from the current level of 40 years to at least 60 years for the future reactors. As a part of the efforts to develop materials with superior mechanical properties suitable for longer design life, the influence of nitrogen at concentrations higher than 0·07 wt-%, on the high temperature mechanical properties of type 316L(N) SS is being studied. Four heats of 316L(N) SS, containing 0·07, 0·11, 0·14 and 0·22 wt-% nitrogen have been evaluated extensively in terms of their tensile, creep, low cycle fatigue and creep fatigue interaction properties. Based on these studies, the nitrogen content has been optimised at 0·14 wt-%. This nitrogen enhanced grade of ...

This paper attempts to link the energies to failure involved in low-cycle fatigue at elevated temperature with those involved in forward creep. These energy levels are connected by a suitable sliding strain-rate scale. The principle is then applied to (i) dwell tests on 1CrMoV, 304 and 20Cr25Ni/Nb steels at 550, 650 and 750°C respectively and (ii) service failure of a thick-walled ferritic steam vessel at 570°C. Calculated creep-fatigue endurance using the linear damage summation rule is then plotted on Damage Diagrams. Sensitivity analyses are carried out as regards the linking of energy levels in fatigue (upper-shelf) with those in creep (lower-shelf). These involve knowledge of creep laws or stress relaxation relations and the methods of connecting energy levels (power law or those giving an S-shaped curve). Within this paper, sufficient information has been provided for investigators to undertake their own analyses on relevant laboratory tests or service assessments.

This paper presents a creep-fatigue life evaluation for three kinds of solders of Sn–37Pb, Sn– 3.5Ag and Sn – 3.0Ag – 0.5Cu. Strain controlled push – pull creep-fatigue tests were performed for solid bar specimens under fast–fast, fast–slow, slow–fast and strain–hold strain waveforms and the effect of strain waveforms on creep-fatigue lives is discussed. Sn– 3.5Ag and Sn– 3.0Ag – 0.5Cu solders showed stronger creep-fatigue resistance compared to Sn –37Pb solder. The applicability of the linear damage rule, the strain range partitioning method and the grain boundary sliding model is discussed for predicting creep-fatigue lives of the three solders. The strain range partitioning method and the grain boundary sliding model satisfactorily predicted creep-fatigue lives within a small scatter.

Plant components often undergo loadings which have aspects of both fatigue and creep by experiencing repeated start-up/shut-downs between steady-state operation at high temperature. In such a case, structural materials show inelastic deformation different from that observed under pure fatigue or pure creep conditions. This finding prompted the development of “unified” constitutive models in which cyclic and creep deformations are treated in a unified way. The author’s group has been developing such a unified model for modified 9Cr–1Mo steel which is widely used in ultrasupercritical fossil power plants. Although the latest model has been demonstrated to be capable of describing deformation behaviour under various loading conditions, including cyclic and steady-state loadings, the model still has room for improvement, such as the need to represent accelerated deformation in the tertiary creep regime. In this study, an attempt is made to improve the capability of the model from various perspectives. A failu...

The consideration of a steel’s resistance to creep-fatigue crack initiation and development is an important part of the integrity assessment of high temperature power plant components. Defect-free or defect assessment procedures may be employed depending on the product form from which the component is manufactured, and the criteria adopted for design, inspection-management and remaining-life evaluations. Such assessment procedures require a comprehensive knowledge of the creep-fatigue properties of the component materials, and these are reviewed.

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...

Establishment of a creep-fatigue life prediction method is important for assessment of the remaining life of high temperature components. Creep-fatigue tests with strain hold duration including 100 hours per cycle have been carried out on high-temperature component materials in CRIEPI. The relationship between creep-fatigue life property and damage evolution during strain hold is discussed. It is suggested that creep damage during stress relaxation should be divided into the matrix creep damage and the grain boundary creep damage from the comparison between the experimental results and the creep deformation mechanism map. A nonlinear damage accumulation model (NLDA) is proposed based on experimental results and theoretical considerations. Creep-fatigue lives of high-temperature materials are predicted with high accuracy using the NLDA model.

Upper bound cyclic crack growth relations are employed for conservative estimates of the lifetime of components containing known or postulated defects. In the low-cycle fatigue (LCF, short crack) r...

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.

The deleterious effect of creep damage upon fatigue endurance in elevated temperature low cycle fatigue is well established and has become known as the ‘creep – fatigue interaction’. This has been identified as a narrow region where a transgranular crack becomes intergranular as creep damage develops. In other cases transgranular fatigue cracks and intergranular creep cavitation damage to some extent can take place separately (‘competitive’ mode) so that interaction is less pronounced. These phenomena have been incorporated into ‘damage interaction diagrams’ which feature in many high temperature Code cases (ASME, R5, RCC-MR etc.). Fractional creep and fatigue damage are considered separately, and failure is conceded when their sum is unity (‘additive’ mode). For some materials the sum is set to a value

This paper concerns the creep-fatigue lifetime assessment of a 1CrMoV steel. A procedure based on the calculation of fatigue and creep damage fractions and their summation according to a non-linear interaction rule is applied for the endurance evaluation of isothermal fatigue and service-like thermo-mechanical fatigue tests performed with plain (uniaxial) and notched testpieces. In order to avoid the uncertainties associated with heat-to-heat variability, testpieces were manufactured from a single heat, for which the fatigue and creep deformation/endurance properties were specifically characterised. With the availability of a reliable constitutive model, finite element calculations could be used to determine the local stress-strain response at the position of crack initiation in each test. The predictive capabilities of the proposed approach for lifetime calculation as well as the suitability of uniaxial fatigue and creep data for the damage assessment of notched components are evaluated.

In the present study five different widely used life models were modified to allow for life prediction under thermomechanical fatigue (TMF) loading conditions. In order to assess the predictive capabilities of the models, life prediction results were compared with TMF data obtained on a stainless steel and a SiC particulate reinforced aluminium alloy. The analysis of the data obtained demonstrated that TMF tests are not sufficient per se to evaluate a life model. It is emphasised that TMF tests chosen for validation of life models should be conducted under loading conditions such that coupling effects between the various damage mechanisms become apparent. The data analysed indicate that appropriate TMF test conditions can be estimated for the case of creep-affected damage evolution. By contrast, selection of test conditions that would clearly reveal other potential couplings, say between oxidation and plasticity, is less straightforward. Unless the tests used to validate a model are designed to ac...

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.

Comparisons have been made on evolutionary and stabilised cyclic stress-strain behaviour following different prior ageing treatments in a 316 steel. Prior stress-free ageing at a temperature higher than the test temperature reduces the ability of this steel to cyclically harden and consequently the material does not achieve maximum strength. An apparent saturation of cyclic hardening at room temperature can however, be continued by further cyclic hardening at 625°C. The rate of precipitation during low cycle fatigue can be two orders of magnitude greater than that occurring during stress-free ageing at 625°C. Quantitative measurements based on an extraction technique are presented and compared with particle size measurements in the literature. Prior high temperature low cycle fatigue exposure has been shown to reduce subsequent creep rates with a concomitant reduction in ductility: the same effect is found by prestrain at room temperature. Neither cyclic stress-strain properties nor creep behaviou...

In high temperature components creep-fatigue cracks are often associated with the heat affected zones of weldments. In the case of austenitic stainless steel welds, such as Type 316, these may ente...

Experimental data on fatigue or creep-fatigue crack growth on large components at high temperature are of primary importance to validate or develop further existing defect assessment procedures and...

‘Early’ creep-fatigue crack growth rates have been measured in complex-cycle large single edge notched bend feature-specimen tests on a 1¼ CrMoV turbine casting steel at 550°C. Crack propagation ra...

In this paper, a defect assessment according to the simplified methods proposed in the French A16 document is presented. These methods allow one to estimate fracture mechanic parameters such as δK or C* based on a reference stress approach. The overall objective of the study is to evaluate the ability of such methods to predict creep-fatigue crack growth obtained in a large component containing a semi- elliptical surface notch. The experiment consists of a wide austenitic stainless steel centre cracked plate heated up to a temperature of 650°C and subjected to more than 3000 bending cycles with a one hour holdtime at the maximum load of the cycle. The analysis according to the A16 document together with the material properties necessary for the analysis are detailed in the paper. The experimental crack growth rates are modelled both on the surface and through the thickness of the plate using a linear summation of cyclic and creep contributions. The applicability of two models, consisting for the first one...

The purpose of this work is to develop an approach in crack initiation assessment in components operating at high temperature. Components of fossil power plants, such as rotors, casings or headers, have to face the combination of highly damaging processes due to the operating temperatures and the high numbers of start-ups and shut-downs. The processes involved are indeed creep, thermal fatigue and creep-fatigue interaction. Our approach is based on viscoplastic constitutive equations coupled with a three dimensional law of continuum damage evolution. These laws use a formalism based on the thermodynamic of irreversible processes. Our goal is to represent the crack initiation of structures loaded with low cycles thermal loads more accurately. To identify the different constitutive coefficients, we have chosen to study a P22 (2,25CrMo) steel from a retired header of one of our coal fired plants. The methodology of identification is given and the influence of each variable is discussed. We focus our interest...