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Reliability assessment of solder joints under thermomechanical cycling is one of the long lasting concerns in the field of electronic materials. To pay attention to this issue, an artificial neural...

SS316Ti is one of the widely used materials for bellows in sodium cooled fast reactor systems. Conventional design codes for bellows do not address the high-temperature failure modes. In this investigation, a methodology has been proposed for the high-temperature design of the bellows. The bellows were preliminarily designed by standards of EJMA for room temperature application. Subsequently, the detailed visco-plastic finite element analysis of the bellows has been carried out to estimate the high-temperature low-cycle fatigue (LCF) and creep-fatigue (CFI) life. The material parameters required for the visco-plastic constitutive model were generated and validated for fatigue specimens. The LCF and CFI life of the bellows based on analysis were 83 and 74 cycles, respectively. The proposed methodology has been validated by high-temperature testing of the bellows. The LCF and CFI lives of the bellows obtained experimentally were found to be 281 and 237 cycles, respectively. The life arrived based on analysis was found to be conservative compared to corresponding life by testing. The safety margin available over failure based on testing of the bellows is 3.4 under LCF and 3.2 for CFI loading.

An experimental investigation and numerical modeling using multiphysics finite element method were performed to study the thermal failure mechanism of low-profile quad flat package solder joints of memory module due to low-cycle fatigue. The strain, stress, and number of cycles to failure have been calculated according to a strain life Coffin–Manson and energy-based Morrow fatigue models. Scanning electron microscopy imaging at the end of thermal cycle was used to evaluate the damage initiation and propagation. The effect of the solder volume on fatigue life of solder joints was discussed. Through analyses of theoretical results and experimental data on fatigue life, cracks initiation and propagation have been highlighted and their possible causes have been discussed.

In the design of components for service at elevated temperatures, fatigue and creep-fatigue due to cyclic loading are some of the most important failure modes, and the assessment of failure life at structural discontinuities is a key issue within the evaluation of the integrity of the components. Since several fatigue and creep-fatigue life evaluation methods have been proposed, this study compares and assesses these different methods by performing fatigue and creep-fatigue tests of perforated plate made of Mod.9Cr-1Mo steel. Multiperforated plate was subjected to mechanical cyclic loading at 550 °C, and crack initiation and propagation at the surfaces of the holes were observed. The stress distribution was varied by changing the hole arrangement and loading level. A series of finite element analyses (FEA) were carried out to predict the number of cycles to failure by the several failure life evaluation methods, and these predictions were then compared with the test results. Several types of evaluation methods that use the elastic FEA were applied, namely, the stress redistribution locus (SRL) method, simple elastic follow-up method, and the methods described in the design and construction code for fast reactors (FRs) published by the Japan Society of Mechanical Engineers (JSME FRs code). In addition to these, evaluation was also carried out using the results of inelastic FEA to compare these elastic FEA-based estimation methods. The comparisons indicate that, for all conditions tested, the SRL method provided a rational prediction of the fatigue and creep-fatigue life when κ = 1.6 was applied, where κ = 1.6 is the recommended reduction factor for this method in general use. A comparison of the SRL method and the results of the inelastic FEA indicated that the applicability of the value of factor κ in the SRL method depends on the elastic region remaining in the cross section including the evaluated point and the spread in the plastically deformed region in the specimen.

This paper develops a novel weight function modified critical distance method to estimate the creep-fatigue lives of geometrically complex structures. A geometrical feature factor was defined to account for the geometry-induced stress gradient, namely the average normalized stress gradient. The conventional weight function was modified using this concept to assess the weight effect of stress distribution on the low-cycle fatigue (LCF) and creep lives, respectively. The weight function modified critical distance methods were verified by the LCF and the creep test data of the specimens with significantly different geometries. Furthermore, creep-fatigue experiments on the full-scale hollow and solid turbine blades were conducted. The creep-fatigue lives of turbine blades were predicted using the systematical weight function modified methodology. The predicted results showed a good agreement with the experimental lives. Compared with other models, this novel method achieves a significantly better accuracy in the case of LCF, creep and creep-fatigue life predictions.

This paper presents the study of creep-fatigue interaction damage and creep rupture limit for 9–12% Cr steam turbine rotor under coupled cyclic thermal-mechanical loadings that alter in phase and out of phase. The investigation is implemented using the Linear Matching Method (LMM) and based on the newly developed creep-fatigue and creep rupture evaluation procedures. Latest experimental creep data of FB2 which belongs to 9–12% Cr heat-resistant steel family is employed to calculate creep-related damage and creep rupture bearing capacity of steam turbine rotor. Various factors that affect creep and fatigue damage of steam turbine rotor are analyzed and discussed, including dwell period and rotating speed of rotor. From the parametric studies, the damage locations and cycles to failure induced by creep and fatigue mechanism are presented respectively. Moreover, through creep rupture analyses for varying desired fracture time, the novel creep rupture curves under multi-type double-cyclic-loading conditions are given and further compared with cyclic plasticity failure curve. These surrogate-model analyses offer a deep understanding of structural responses of steam turbine rotor under long-term high temperature operation and provide critical loading conditions to be referenced for smooth running.

Creep design according to Annex B of EN13445-3, „Design by Analysis – Direct Route”, is based on nonlinear analyses with linear-elastic ideal-plastic material law. The Creep Rupture check, the Excessive Creep Strain check, the check for action cycles without plastification, and the applied method for creep fatigue damage accumulation are summarised. To get a better insight, the relation to different reference stress methods, on which the checks are based on, are explained. A simple, but realistic nozzle example illustrates the application of the method. The example includes different materials connected by welding. Handling of cycles without considerable creep, a shutdown cycle and a cold medium injection cycle, are shown. Advantages of the method, unclear formulations within the standard, and problems with missing material properties are discussed.

Aeroengine titanium alloys such as Ti-6Al-4V experience creep-fatigue loading conditions during the critical fighter aircraft operations. To ensure and enhance the reliable application of such alloys, mechanical behaviour across multiple length scales need renewed attention. Hence, in the present investigation, an attempt has been made to study creep-fatigue damage across multiple length scales (microstructure to component level) using damage mechanics methodology for Ti-6Al-4V alloy. This involves ambient creep, fatigue and creep-fatigue experimentation; identification of various parameters for creep and fatigue models; developing a creep-fatigue model and finally application of this model for damage simulation at specimen, component and microstructure levels. At specimen level, creep-fatigue damage has been obtained from simulations and compared with experimental values. A very good comparison has been observed for the applied maximum peak stress range of 925–975 MPa. The creep-fatigue model has also been successful in mapping damage and identifying critical damage locations in component as well as in the microstructure.

The purposes of the present work are to evaluate the prediction accuracy of characteristic cycle on creep-fatigue life prediction and to analyze the advantages and disadvantages of various models in tensile strain-dwell tests. The parameters of the first cycle, the 10% life, the half-life, and the characteristic cycle that we proposed (a turning point between the initial rapid softening and subsequent slight softening/hardening) are employed to linear-damage-summation (LDS) and energy-life prediction models based on the creep-fatigue data of directionally-solidified Nickel-based superalloy, DZ445, at 900 °C. It is found that the characteristic cycle parameters with clear physical significance have the highest life prediction accuracy. Moreover, the optimal critical value of fatigue and creep damage (the coordinate of the creep-fatigue envelope intersection) in the LDS is also determined. The prediction accuracy of creep damage based on the time-fraction, the simple ductility-exhaustion, and the strain-energy- density-exhaustion models is sequentially improved in the LDS rule. In the energy-life model, the life prediction accuracy based on damage mechanism and frequency correction is higher than the value without any correction. This investigation provides a new method of the parameter-selection for the creep-fatigue life prediction, which is accurate and convenient. It provides the theoretical-guidance for creep-fatigue life prediction in both laboratory experiment and actual components.

Several (accumulative) lifetime models were developed to assess the lifetime consumption of high-temperature components of steam and gas turbine power plants during flexible operation modes. These accumulative methods have several drawbacks, e.g. that measured loading profiles cannot be used within accumulative lifetime methods without manual corrections, and cannot be combined directly to sophisticated probabilistic methods. Although these methods are widely accepted and used for years, the accumulative lifetime prediction procedures need improvement regarding the lifetime consumption of thermal power plants during flexible operation modes. Furthermore, previous investigations show that the main influencing factor from the materials perspective, the critical damage threshold, cannot be statistically estimated from typical creep-fatigue experiments due to massive experimental effort and a low amount of available data. This paper seeks to investigate simple damage mechanics concepts applied to high-temperature components under creep-fatigue loading to demonstrate that these methods can overcome some drawbacks and use improvement potentials of traditional accumulative lifetime methods. Furthermore, damage mechanics models do not provide any reliability information, and the assessment of the resultant lifetime prediction is nearly impossible. At this point, probabilistic methods are used to quantify the missing information concerning failure probabilities and sensitivities and thus, the combination of both provides rigorous information for engineering judgment. Nearly 50 low cycle fatigue experiments of a high chromium cast steel, including dwell times and service-type cycles, are used to investigate the model properties of a simple damage evolution equation using the strain equivalence hypothesis. Furthermore, different temperatures from 300 °C to 625 °C and different strain ranges from 0.35% to 2% were applied during the experiments. The determination of the specimen stiffness allows a quantification of the damage evolution during the experiment. The model parameters are determined by Nelder-Mead optimization procedure, and the dependencies of the model parameters concerning to different temperatures and strain ranges are investigated. In this paper, polynomial chaos expansion (PCE) is used for uncertainty propagation of the model uncertainties while using non-intrusive methods (regression techniques). In a further post-processing step, the computed PCE coefficients of the damage variable are used to determine the probability of failure as a function of cycles and evolution of the probability density function (pdf). Except for the selected damage mechanics model which is considered simple, the advantages of using damage mechanics concepts combined with sophisticated probabilistic methods are presented in this paper.

This study aims to characterize nondestructively the creep-fatigue damage of tempered ferritic steel by dynamic coercivity measured by reversible permeability. The creep-fatigue test was achieved under load control with a hold time of 60 s and 600 s. The dynamic coercivity based on the reversible magnetic permeability profiles was successfully obtained on top of the test specimen. Based on these results, we attempted to evaluate the creep-fatigue of tempered ferritic steel with the support of microstructural evaluation such as dislocations and precipitates. The dynamic coercivity decreases with fatigue life fraction and closely related to the number of Cr23C6 precipitates with N0.8 and the dislocation density with ρ0.4 In addition, Vickers’s hardness continuously decreased up to the failure deducing the softening of mechanical strength. In conclusion, it can nondestructively characterize the influence of the precipitates and dislocation density on the dynamic coercivity of ferritic steel during creep-fatigue.

This paper highlights the wide range of applications for probabilistic analyses in structural integrity and design. Firstly, some background is given to introduce the basic principles which underpin a probabilistic approach and some aspects of its historical development. A discussion of the main benefits and attributesof probabilistic analyses follows, and a direct comparison with conventional deterministic design is made. Thereafter, five main modes of application are presented, and their individual objectives discussed. Through examining various case studies, the different utilities of a probabilistic approach are highlighted.Finally, some important considerations are discussed in terms of adopting probabilistic approaches for future implementation.

The low cycle fatigue-creep damage is the main parameter for the failures of gas turbine components under high temperature and cyclic loading. In this work, different combinations of Creep-Fatigue damage are introduced and checked for its impact on the Turbine life and its reliability. Linear and Non-Linear Creep-Fatigue combinations were considered as part of this work. The Turbine rotor under present study is made out of Nimonic-90 nickel base alloy forging. Weibull distribution method was used to study the reliability. It was found that, the reliability reduces from 99% to 25% when creep damage component is increased from 20% to 40% for a fixed Turbine life. The damage factor was found more in above linear Creep-Fatigue curve and less in below linear Creep-Fatigue curve.

For the design of future fusion reactors the ferritic-martensitic steel Eurofer97 is the main candidate for in-vessel structural application to be able to withstand harsh environment like irradiation and cyclic loading under elevated temperatures. In case of high temperatures, not only fatigue but also creep damage becomes significant and has to be considered. In the past a so called Creep-Fatigue Assessment (CFA) tool has been developed for Finite Element software ANSYS APDL as post-processor within the frame of Engineering Data & Design Integration (EDDI) in EUROfusion. This tool was initially based on the elastic Creep-Fatigue rules of ASME Boiler Pressure Vessel Code (BPVC) Section III Division 1 Subsection NH Appendix T.Nowadays the tool is able to automatically identify the critical region regarding Creep-Fatigue damage in ANSYS APDL and Workbench using local stress, maximum elastic strain range and temperature from elastic thermo-mechanical Finite Element analysis. The use of stress linearization in elastic analysis allows the calculation of a modified equivalent strain range including inelastic effects to finally determine the allowable number of cycles, creep and fatigue damage fraction. For Creep-Fatigue Assessment (CFA) post-processing design fatigue curves, creep stress versus time to rupture curves, monotonic and isochronous stress versus strain curves have been used in the combination with Creep-Fatigue damage interaction diagram to describe the Creep-Fatigue behavior of Eurofer97.As it is well known that Eurofer97 shows cyclic softening, a modified Creep-Fatigue rule has been implemented in CFA tool to improve the underestimation of creep damage. This modified rule uses for calculation of creep damage the stress versus strain and design creep curves of cyclic softened material for some fraction of lifetime and an improved Creep-Fatigue damage interaction diagram of Eurofer97.In cases where the elastic analysis of ASME BPVC is too conservative, inelastic analysis can be used to calculate total strain directly instead of the prediction used in the elastic analysis. This inelastic approach for Creep-Fatigue Assessment is less complicated compared to the elastic route because only few steps according to ASME BPVC are necessary. Nevertheless more efforts for Finite Element simulation including inelastic material behavior are required.In Summary, this CFA tool can be used for fast Creep-Fatigue evaluation. It simply allows the fast identification of Creep-Fatigue damage for a structural component which is made of Eurofer97. Keywords: Creep-Fatigue interaction, ASME BPVC, CFA tool, EUROFER97, Elastic route, Inelastic route

True stress and strain are necessary to estimate the rupture life under creep-fatigue conditions. Finite element analysis (FEA) is one of the most reliable method to calculate true stress and strain, but the accuracy of the obtained result is often greatly dependent on the constitutive model used. Non-unified constitutive model has been proposed, where inelastic strain is decomposed into creep strain and visco-plastic strain. In this paper, a cyclic hardening effect and a plastic strain range dependency were introduced into the non-unified constitutive model to predict the creep-fatigue damage of 316H stainless steel. This model was implemented in a finite element program and FEA were conducted to develop a life assessment method and calculate the creep-fatigue damage by modified ductile exhaustion method. As a consequence, it was revealed that the predicted creep-fatigue lives showed high correspondence with the experimental results by the modified ductile exhaustion method utilizing the non-unified constitutive model.

Physics-informed neural network has strong generalization ability for small dataset, due to the inclusion of underlying physical knowledge. Two strategies are enforced to incorporate physics constraints to a deep neural network in this work. One is to obtain extended features through physics-informed feature engineering, and the other is to incorporate physics-informed loss function into deep neural network as constraints. Conventional machine learning models, deep neural network and physics-informed neural network are applied to predict creep-fatigue life of 316 stainless steel. Results show that physics-informed neural network presents better prediction accuracy than deep neural network and conventional machine learning models.

The present paper characterized crack growth behavior and damage evolution in P92 steel under creep-fatigue interaction conditions. Creep-fatigue crack growth tests were conducted at a constant load amplitude with various hold times. To reveal the role of constraint in creep-fatigue regime, specimens with different crack depths and thicknesses were used. Combination with a non-linear creep-fatigue interaction damage constitutive model, the crack growth and damage evolution behaviors were simulated using finite element method. Under creep-fatigue condition, time dependent crack growth rate increased as the duration period was reduced. This was attributed to the role of enhanced fatigue damage with the decreasing of the dwell time. Moreover, the deviation of crack growth rate with different dwell times became small as crack grew. With the crack length increasing, the creep damage level was improved regardless the dwell time, which may be due to the fact that the creep damage dominated crack growth in this stage. Furthermore, the crack growth rate increased as the crack depth became deep and the specimen thickness became large, as a result of the increased constraint level. A load-independent constraint parameter Q * was introduced to correlate the crack growth rate, which provided a good prediction for specimens under different constraint conditions.

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...

This study discusses the creep-fatigue life prediction of eutectic Sn37Pb solder. Creep-fatigue lives were experimentally obtained at 313 K using fast-fast, slow-slow, slow-fast, fast-slow, and strain-hold waveforms. Conventional four creep-fatigue life prediction rules, time fraction rule, strain range partitioning method, frequency modified fatigue life, and ductility exhaustion model was used to predict the creep-fatigue lives. However, not all the rules were able to properly estimate them. Since the grain boundary sliding rates were closely related to creep-fatigue lives, a new life prediction method based on grain boundary sliding rates was proposed.

In this paper, a novel creep–fatigue interaction damage model was proposed to evaluate damage evolution and crack growth behavior in creep–fatigue regime. The model incorporated the maximum stress effect into the low cycle fatigue damage accumulation and utilized the non-linear summation approach to consider the creep–fatigue interacted effect. The model could predict the relation between (da/dt) avg against ( C t ) avg in creep–fatigue interacted environment and demonstrate the crack growth behavior under pure fatigue condition, which respectively matched well the corresponding experimental results. In addition, the crack growth rate was greatly dependent on the dwell time in creep–fatigue regime. The crack growth rate (d a /d t ) avg was increased with decreasing the hold time at the same ( C t ) avg values. During crack growth process, the creep damage accounted for a larger portion compared with the fatigue damage. Furthermore, in creep–fatigue regime, the effect of the initial crack depth on crack growth behavior was dependent on the hold time while the applied initial stress factor range had slight effect on the crack growth behavior.

The evolution of the energy production market is responsible for more demanding conditions in terms of thermo-mechanical fatigue (TMF) for GT critical components. In fact, daily cycling is generally conducted by plant owners to feed the market when the request is higher or when renewable energy sources are absent or weak. TMF benchmark tests have been exploited to assess and validate a creep-fatigue lifing procedure developed for two Ni based superalloys applied on the most critical GT blade and vane of an F-class power plant. The TMF cycles have been defined on the basis of start-up/shut-down transient FE simulations for three case studies. For one of the studied components, the effect of the metallic coating on life reduction has been evaluated. The TMF benchmark tests results have been compared to field feedback from real components in order to further validate the whole creep-fatigue life procedure.

This study investigates the mechanistic approach to evaluate total stored energy consisting of damage energy (due to voids and microcracks) and deformation energy (due to dislocations). Thermal data obtained by online IRT technique have been employed for this evaluation. The deformation energy is estimated through EBSD analysis. Subsequently, the damage and deformation induced by creep and fatigue have been modeled as functions of applied stress and cumulative plastic strain, individually. By using the data of experimental creep stress and fatigue strain for each creep-fatigue cycle, the contributions of creep and fatigue damage energies have been estimated for creep-fatigue interaction. New fatigue and creep damage parameters have been proposed based on the damage mechanics concepts. Different regimes have been identified and explained with creep and fatigue damage evolution curves. The present approach has also been extended to characterize incremental creep-fatigue interactions.

A creep-fatigue crack growth model considering creep-fatigue interactions based on continuum damage mechanics was proposed in this paper. Numerical analyses of creep-fatigue crack growth of P91 steel at 625 °C using compact specimens were conducted. The results agreed well with the experiment which indicated its good capability in predicting creep-fatigue crack growth behavior. The effects of initial crack depth, specimen dimension and hold time on crack growth behavior were investigated using the model. The results indicated that the increasing initial crack depth and specimen dimension promoted the crack growth rate, while the decreasing hold time accelerated the crack growth.

The purposes of the present work are to extend the previous energy-based model on the basis of strain energy density exhaustion and to estimate creep-fatigue endurance and accumulated damage using cycle-by-cycle concept in tension-hold-only tests. The nickel-based GH4169 superalloy at 650 °C is employed to fit material constants and to verify the prediction capacity of the present model under various loading conditions. The present model exhibits a higher accuracy than some existing models, especially for certain loading waveforms, where the half-life cycle cannot be considered as a representative one. In addition, the trajectory for time-dependent accumulated damage during the operating period can be monitored in the creep-fatigue interaction diagram.

In this paper, the extended Direct Steady Cyclic Analysis method (eDSCA) within the Linear Matching Method Framework (LMMF) is combined with the Stress Modified Ductility Exhaustion method and the modified Cavity Growth Factor (CGF) for the first time. This new procedure is used to systematically investigate the effect of several load parameters including load level, load type and creep dwell duration on the creep–fatigue crack initiation process in a notched specimen. The results obtained are verified through a direct comparison with experimental results available in the literature demonstrating great accuracy in predicting the crack initiation life and the driving mechanisms. Furthermore, this extensive numerical study highlighted the possible detrimental effect of the creep–ratchetting mechanism on the crack growth process. This work has a significant impact on structural integrity assessments of complex industrial components and for the better understanding of creep–fatigue lab scale tests.

Alloy 617 is a primary candidate material to be used in the next generation of nuclear power plants. Structural materials for these plants are expected to undergo creep and fatigue at temperatures as high as 950 °C. This study uses a hybrid-control creep-fatigue loading profile, as opposed to the traditional strain-controlled loading, to generate creep dominated failure. Qualitative and quantitative image analysis through SEM, EDS, and EBSD, is used to show that hybrid control testing is capable of producing creep dominated failure and that time fraction approach is not a valid indicator of creep or fatigue dominated damage. The focus of image analysis is on surface fatigue cracks and internal creep voids. A creep-fatigue damage interaction diagram based on these micro-scale features is plotted. It is shown that the classical time fraction approach suggested by the ASME code does not agree with the experimental findings and has a poor correlation with observed microscale damage features. A new definition of creep damage fraction based on an effective hold time is found to correlate well with the micro-scale image analysis.

In the present study, to develop an evaluation procedure and design rules for Mod.9Cr-1Mo steel weld joints, a method for evaluating the creep-fatigue life of Mod.9Cr-1Mo steel weld joints was proposed based on finite element analysis (FEA) and a series of cyclic plate bending tests of longitudinal and horizontal seamed plates. The strain concentration and redistribution behaviors were evaluated and the failure cycles were estimated using FEA by considering the test conditions and metallurgical discontinuities in the weld joints. Inelastic FEA models consisting of the base metal, heat-affected zone and weld metal were employed to estimate the elastic follow-up behavior caused by the metallurgical discontinuities. The elastic follow-up factors determined by comparing the elastic and inelastic FEA results were determined to be less than 1.5. Based on the estimated elastic follow-up factors obtained via inelastic FEA, a simplified technique using elastic FEA was proposed for evaluating the creep-fatigue life in Mod.9Cr-1Mo steel weld joints. The creep-fatigue life obtained using the plate bending test was compared to those estimated from the results of inelastic FEA and by a simplified evaluation method.

The Simplified Model Test (SMT) approach is an alternative creep-fatigue evaluation method that no longer requires the use of the damage interaction diagram, or D-diagram. The reason is that the combined effects of creep and fatigue are accounted for in the test data by means of a SMT specimen that is designed to replicate or bound the stress and strain redistribution that occurs in actual components when loaded in the creep regime. However, creep-fatigue experiments on SMT key feature articles are specialized and difficult to perform by the general research community. In this paper, two innovative SMT based creep-fatigue experimental methods are developed and implemented. These newly-developed SMT test methods have resolved all the critical challenges in the SMT key feature article testing and enable the potential of further development of the SMT based creep-fatigue evaluation method into a standard testing method. Scoping test results on Alloy 617 and SS 316H using the newly developed SMT methods are summarized and discussed. The concepts of the SMT methodology for creep-fatigue evaluation are explained.

The possibility to use real operational data as an input for lifetime assessment methods is a key requirement in terms of both service applications as well as within the design of components by underlying specific service relevant scenarios. To address this, so called “Constitutive Viscoplastic Material Models” have been developed which represent a more generalized alternative for assessing turbo machinery components which undergo an irregular creep-fatigue loading. Based on several experimental and theory related national research programs, performed within the German working group W10 in the last years, the current status of the model development and the performance potentials are summarized in this paper. Within the first part, the general and developed model structure of one candidate material model is introduced by discussing different aspects of the equation system together with the specific practical related aspects. Secondly, the validation of this constitutive material model is shown by comparing the model results with a set of conducted complex experiments, like ansiothermal service like experiments performed on smooth, notched and biaxially loaded cruciform test samples. As the third focus, the applicability and the potential of using such a model for assessing real components will be discussed e.g. by introducing extrapolation or cycle jump concepts which allows to majorly reduce the calculation time without decreasing the result accuracy significantly. Finally, future potentials will be introduced with the goal to use such sophisticated models to train meta-models and finally allow for a machine-learning based on-site and service related on-line component assessment.

Low cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings are the main factors resulting in the failure of many critical components in the infrastructure of power plants and aeronautics. Accurate prediction of life spans under specified loading conditions is significant for the design and maintenance of components. In the present study, various LCF and CFI tests are conducted to investigate the effects of temperature, strain amplitude, hold time and hold direction on the fatigue life of P92 steel. To predict fatigue life under different experimental conditions, various conventional life prediction models are evaluated and discussed. Moreover, a new empirical life prediction model is proposed based on the conventional Manson-Coffin-Basquin (MCB) model. The newly proposed model is able to simultaneously consider the effects of temperature, strain amplitude, hold time and hold direction on predicted life. The main advantage is that only the known input experimental parameters are required to perform the prediction. In addition to the validation made through the experimental data of P92 steel conducted in the present paper, the model is also verified through numerous experimental data reported in the literature for various 9&ndash, 12% Cr steels.

A numerical process based on crystal plasticity finite element (CPFE) was implemented to predict creep-fatigue crack initiation life. CPFE-based model can describe the macroscopic cyclic deformation and reveal grain-level damage mechanism. A new life prediction approach was then constructed by introducing fatigue and creep indicator parameters. Furthermore, a series of strain-controlled creep-fatigue tests in GH4169 superalloy at 650℃ were used to validate predicted accuracy of this model, where most of the data points lied within ±1.5 error band. Finally, a potential methodology for conservative creep-fatigue life evaluation was provided by flexible creep-fatigue damage summation rule in engineering applications.

T Multiaxial creep-fatigue tests under non-proportional loading conditions with various strain rates were carried out using a hollow cylinder specimen of a high chromium ferritic steel at 823K in air to discuss the influence of non-proportional loading on failure life. Strain paths employed were a push-pull loading and a circle loading. The push-pull loading test is proportional strain path test. The circle loading test is non-proportional strain path test in which sinusoidal waveforms of axial and shear strains have 90 degree phase difference. The failure life is affected largely by the strain rate and the non-proportional loading. This paper presents a modified strain range for life evaluation considering the strain rate based on a non-proportional strain parameter proposed by authors. The strain range is a suitable parameter for life evaluation of tested material under non-proportional loading at high temperature. KEYWORDS. Life evaluation; Creep-fatigue; Multiaxial strain; Nonproportional loading; High chromium ferritic steel.

A novel creep-fatigue life prediction model was deduced based on an expression of the strain energy density in this study. In order to obtain the expression of the strain energy density, the load-controlled creep-fatigue (CF) tests of P92 steel at 873 K were carried out. Cyclic strain of P92 steel under CF load was divided into elastic strain, applying and unloading plastic strain, creep strain, and anelastic strain. Analysis of cyclic strain indicates that the damage process of P92 steel under CF load consists of three stages, similar to pure creep. According to the characteristics of the strains above, an expression was defined to describe the strain energy density for each cycle. The strain energy density at stable stage is inversely proportional to the total strain energy density dissipated by P92 steel. However, the total strain energy densities under different test conditions are proportional to the fatigue life. Therefore, the expression of the strain energy density at stable stage was chosen to predict the fatigue life. The CF experimental data on P92 steel were employed to verify the rationality of the novel model. The model obtained from the load-controlled CF test of P92 steel with short holding time could predict the fatigue life of P92 steel with long holding time.

The accumulated creep–fatigue damage is expected to be an important failure mechanism for lots of high-temperature components. The aim of this paper is to propose a modified strain energy density exhaustion model to predict the tension-hold-only creep–fatigue life. This model exhibits high accuracy due to the reasonable evaluation of creep damage. The proposed model elaborates the determinations of mean stress, stress relaxation rate and creep damage. A few existing experimental data sets of Grade 91 steel, Alloy 617 and 304 stainless steel are used to verify the prediction capacity of the present model under different temperatures and loading conditions. Results show that most of the experimental data falls into a range within a scatter band of ±1.5 on life.

The goal of creep fatigue modelling is the compounding of the damage caused by creep and fatigue mechanisms. The different approaches for compounding these damage mechanisms have led to several different creep fatigue models: (i) ignore fatigue damage — the creep ductility (energy density) exhaustion models; (ii) lumping plastic and creep strain (energy) into inelastic strain (energy) — the model of Dauvearx's crack initiation and propagation; (iii) linearly sum fatigue and creep damage — the model of linear damage summation; (iv) model creep and fatigue damage using a common parameter — the models of fracture mechanics; (v) partition damage into fatigue, cyclic creep, and cyclic creep-fatigue interaction — strain range/energy partitioning models; (vi) model creep and fatigue damage using a common parameter at rates that are dependent on the current state of damage — the model unified damage; (vii) model creep and fatigue damage using separate damage parameters — the mechanism based model; and (viii) integrate creep damage into the fatigue equation — creep modified strain-life equations. The rigour of the approaches increases from (i) to (vii). The creep modified strain-life equation requires no evaluation of creep strain and facilitates design analysis and evaluation of acceleration factors; however, its rigour depends on the choice of the creep functions. The unified equation is capable of covering the full spectrum of creep-fatigue from pure fatigue to pure creep rupture.

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.

In this paper a thermodynamically consistent formulation for creep and creep-damage modelling is given. The model is developed for isotropic solids by using proper expressions for the Helmholtz free energy and the complementary form of the dissipation potential, and can be proven to fulfill the dissipation inequality. Also the coupled energy equation is derived. Continuum damage model with scalar damage variable is used to facilitate simulations with tertiary creep phase. The complementary dissipation potential is written in terms of the thermodynamic forces dual to the dissipative variables of creep strain-rate and damage-rate. The model accounts for the multiaxial stress state and the difference in creep rupture time in shear and axial loading as well as in tensile and compressive axial stress. In addition, the model is simple and only four to eight material model parameters are required in addition to the elasticity parameters. A specific version of the proposed model is obtained when constrained to obey the Monkman-Grant relationship between the minimum creep strain-rate and the creep rupture time. The applicability of the Monkman-Grant hypothesis in the model development is discussed. The proposed 3D-model is implemented in the ANSYS finite element software by the USERMAT subroutine. Material parameters have been estimated for the 7CrMoVTiB10-10 steel (T24) for temperatures ranging from 500 to 600 degrees of celcius. Some test cases with cyclic thermal fatigue analysis are presented.