Creep-fatigue Tests Research Articles

10802 円孔板の比較的長時間保持を有するクリープ疲労き裂進展評価

The object of study is to evaluate crack growth in creep fatigue test. In this study, fatigue test and creep-fatigue test with relatively long hold time (30 minutes, 60 minutes, and 150 minutes) were conducted under strain-controlled. The specimen is perforated plate that made of SUS304 stainless steel. We measured crack length by using CCD video camera at a constant frequency and compared it with the expression of relations of crack propagation rate and J-integral given in a precedent study. It was found that strain hold time is longer in creep-fatigue test, crack propagation rate per cycle is faster. And the expression in a precedent study is effective in the SUS304 stainless steel.

904 ステンレス鋼の比較的長時間保持を有するクリープ疲労試験におけるき裂進展速度評価(OS9 オーガナイズドセッション≪発電機器の材料・構造解析と評価≫)

The object of study is to evaluate crack growth rate in creep-fatigue test. In this study, fatigue test and creep-fatigue test with relatively long hold time (up to 150 minutes) were conducted under strain-controlled. The specimen is perforated plate that made of SUS304 stainless steel and SUS310S stainless steel. Then we measured the crack length by using photographs with the CCD video camera at constant frequency. It was found that strain hold time is longer in creep-fatigue test, the number of failure cycles is more decrease and crack growth rate per cycle is faster. The formula of the previous study that conducted creep-fatigue tests of SUS316FR stainless steel is effective in SUS304 stainless steel. But the formula is not effective in SUS310S stainless steel. It is caused by embrittlement.

損傷力学によるニオブ合金材の高温機械特性モデリングとクリープ疲労解析

The elasto-plastic constitutive equation based on the concept of continuum damage mechanics was formulated for the niobium alloy (C103) which is used as a chamber material in a satellite's thruster. The existing theory was extended to deal with the creep fatigue behavior at the high temperature of 1500°C. Creep, tensile and creep fatigue tests (stress-controlled) were conducted with plate bar specimens made of the niobium alloy. The tests were conducted at 1500°C and obtained data were used to determine the material parameters employed in the constitutive equation. Time-strain curves and damage-evolution curves were also simulated and the validity of material parameters was confirmed by comparing calculated values with test results. Creep fatigue life analyses were also conducted and the results were compared with the experimental values. From the results, it is expected that the used constitutive equations and determined material parameters can predict the mechanical features and creep fatigue life for the niobium alloy at the high temperature.

ICONE23-1559 CREEP-FATIGUE TESTS OF DOUBLE-END NOTCHED BAR MADE OF MOD.9CR-1MO STEEL

This paper describes experimental and analytical works to confirm that the design standard for Sodium cooled Fast Reactor (SFR) components sufficiently covers possible failure mechanisms. Present design standards for SFR components have been proposed by Japan Society of Mechanical Engineers (JSME). For economical reasons, Mod.9Cr-1Mo ferritic steel, equivalent to ASME Gr.91, will be widely adopted to coolant systems in future SFRs. Creep-fatigue damage evaluation method in JSME design standard for SFR components has been constructed based on experiments and/or numerical analyses of conventional austenitic stainless steels, such as 304SS. Since the material characteristics of Mod.9Cr-1Mo steel are substantially different from those of austenitic stainless steels, it is required to verify the applicability of the design standards to the SFR components made of Mod.9Cr-1Mo steel. However, few structural test data exist to verify the applicability of the conventional design standards. Therefore, to investigate creep-fatigue failure aspect of the several types of structural discontinuities, a series of uni-axial creep-fatigue tests were conducted using double-ended notch bar specimens made of Mod.9Cr-1Mo steel under displacement controlled condition with 30 minute holding. In these tests, imposed axial displacement was controlled to result in 0.7% axial strain range at the minimum section area. The curvature radii of the specimens were 1.6 mm, 11.2 mm and 40.0 mm. The specimen having 1.6 mm notch and 11.2 mm notch failed from outer surface but the specimen having 40.0 mm notch showed obvious internal crack nucleation. In addition, though total duration time of the creep-fatigue test was only 2,000 hours, a lot of creep voids and inter granular crack growth were observed. To clarify the cause of such peculiar failure, some additional experiments were performed, as well as some numerical analyses. We could point out that such a peculiar failure aspect might result from corresponding stress distribution in the cross section. Since radial temperature distribution resulted from high-frequency induction heating and local multiaxiality also might produce such peculiar failure, analytical investigations were carried out. As a result of a series of investigations, possible causes of such peculiar failure could be narrowed down. A future investigation plan was proposed to clarify the most significant cause.

Low Cycle Fatigue Crack Growth Behavior of Modified Rotor Steel under High-Temperature

Many components in power plants are subjected to low cycle loading under high-temperature during continuing service. Therefore, creep-fatigue damage has been become a significant concern in the design and operation of these components. This damage mechanism initiates cracks at critical locations and subsequently creates a potential risk to safe and reliable operation. Creep-fatigue crack growth rate data are necessary inputs for assessing the structural integrity of high temperature components in power plants. Creep-fatigue crack growth tests were conducted for a modified X12Cr rotor steel at 600°C under trapezoidal wave shape load amplitude conditions with various hold times using C(T) specimens. A direct current electrical potential-drop was used to monitor crack growth length during the tests.It is shown that crack growth behavior under creep-fatigue conditions is characterized better by the (Ct)avg parameter, and the formula of assessment low cycle fatigue crack growth under high-temperature was fitted by use of ΔK and (Ct)avg parameter.

Effect of Pipe Bend Shape Imperfections on Creep-fatigue Crack Growth

This study is concerned with assessing the structural integrity of power plant pipes which operate under creep and fatigue conditions. In the manufacturing process of pipe bends it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross section of the bend also becomes non circular due to bending process; this tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of pipe bend on stress-strain redistributions and creep-fatigue crack growth rate was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bend was performed for different stage of lifetime under considering loading conditions. As a result it was found that the effect of ovality and thinning cross sections interaction is significant and is a function of creep time. The tensile tests were performed for determination the main mechanical properties of pipe bend's material after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross sections as well as operating time. The creep-fatigue crack growth rate tests were performed on special designed program test-cycle. Comparison between new and old materials shows tensile properties of 12Cr1MoV steel have degraded by long-term high-temperature service; while comparison between the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature is used in the form of creep stress intensity factor.

Microstructure Characteristics of 12Cr1MoV Heat Pipes with the Combined Effects of Compression Creep and Molten Alkali Salts Corrosion

12Cr1MoV is one of the low alloy steels which is widely applied in heat pipes of biomass fired boilers. But with the combined effects of molten alkali salts corrosion caused from combustion of biomass and creep deformation which is resulted by the high temperature stress as well as steam pressure, they bring negative influence on the mechanical properties. Based on the fact, the present work mainly focuses on studying the microstructure deformation of samples after compression creep in different molten alkali salts. 12Cr1MoV steels chosen as candidate materials are first cut by EDM. Molten salts conditions are described as NaCl-40%KCl, NaCl-70%KCl and NaCl-40%Na2SO4. Creep properties of 12Cr1MoV in molten salts were obtained by the creep test at 150MPa. Experiments were tested on RDL05 electronic creep fatigue test machine after reformation to create a condition of hot corrosion. The results showed that different creep deformation was generated by different salt mixtures. When the temperature was set at 650°C, sample in molten NaCl-70%KCl showed the severest destruction than other samples on surfaces and cross sections. In these areas, the applied stress played a synergic role with corrosion leading to the progressive loss of material load-bearing capacity. On the substrate, spheroidization of cementites and precipitation of carbides along grain boundaries were clearly detected, which is also the reason to lower the strength of 12Cr1MoV. It is interesting to note that creep deformation of microstructures in NaCl-40%Na2SO4 was the weakest, indicating that sulfates can suppress corrosion effects of molten chlorides.

A generalized hysteresis energy method for fatigue and creep-fatigue life prediction of 316L(N)

Abstract Creep-fatigue interaction is extremely important for the life prediction of structural components subjected to high temperature conditions. The widely used life prediction methods such as time fraction rule and ductility exhaustion method are found to be insufficient in long time regimes and cannot give a clear physical explanation for the material failure. In this paper, a generalized life prediction model is proposed on the basis of the hysteresis energy and law of energy conservation. The influence of hold time on the cyclic stress–strain behavior is described based on isochronous stress–strain curves and stress relaxation during hold time is evaluated with the time hardening rule. An analytical approach is set up to calculate the hysteresis energy for the creep-fatigue tests. The generalized hysteresis energy is used as a model parameter to predict the fatigue and creep-fatigue life of 316L(N) and it is found to have a power law relationship with the number of cycles to failure. The fatigue and creep-fatigue life predicted with the new energy-based method shows a good correlation with the experimental results from different researchers.

Cyclic softening as a parameter for prediction of remnant creep rupture life of a Indian reduced activation ferritic–martensitic (IN-RAFM) steel subjected to fatigue exposures

Sequential fatigue-creep tests were conducted on Indian reduced activation ferritic–martensitic steel at 823K leading to sharp decrease in residual creep life with increase in prior fatigue exposures. Extensive recovery of martensitic-lath structure taking place during fatigue deformation, manifested as cyclic softening in the cyclic stress response, shortens the residual creep life. Based on the experimental results, cyclic softening occurring during fatigue stage can be correlated with residual creep life, evolving in an empirical model which predicts residual creep life as a function of cyclic softening. Predicted creep lives for specimens pre-cycled at various strain amplitudes are explained on the basis of mechanism of cyclic softening.

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.

Creep–fatigue crack development from short crack starters

A series of isothermal strain controlled creep–fatigue tests on fully instrumented cylindrical specimens with shallow chordal crack starters has been conducted for an advanced 9%Cr turbine rotor steel at 600 and 625°C. Cyclic/hold wave shapes involving a dwell period at peak strain in tension or compression were also performed with crack development being monitored by means of electrical potential drop instrumentation. It is found that temperature, total strain range and hold period are the most influential factors on short creep–fatigue crack propagation rates and specimen life. In order to establish a reliable relationship to represent subcritical crack development for high temperature component integrity assessment, the effectiveness of candidate correlating parameters such as cyclic strain range, cyclic J integral and strain energy density factor have been evaluated. Their application to circumstances involving short crack development due to fatigue, and interacting and non-interacting creep loading are evaluated with reference to the evidence determined from post-test metallurgical examination.

Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

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.

Creep-Fatigue Life Prediction and Reliability Analysis of P91 Steel Based on Applied Mechanical Work Density

A creep-fatigue (CF) life prediction model and its simplified expression were developed based on the applied mechanical work density (AMWD). The foundation of this model was an integration of N-S curve. Comparisons of the model predicted fatigue lifetimes with the experimental data of load-controlled CF tests on P91 base metal and welded metal at 848 K from the reference were made and apparently illustrated that the model predictions were in a good agreement with the experimental fatigue lifetimes. In addition, the curve of the numbers of cycles to failure versus AMWD at the associated probability was deduced. A reliability model was constructed by combining the curve and the simplified life prediction model.

A Recent Development in Creep-Fatigue Testing

Abstract Procedures for the assessment of components subject to cyclic loading at high temperatures require material property input data that characterize the creep-fatigue deformation response and resistance to cracking. For many years, there was no testing standard or code of practice to ensure that such information was generated in a uniform way. This was mainly because the creep-fatigue test data requirements for organizations in various industrial sectors appeared to be so different that the need for standardization was questioned. In the mid-2000s, it was recognized that even though it would make no sense to be prescriptive about such details as cycle shape, there were many aspects of creep-fatigue testing for which guidance would be beneficial to ensure acceptable uniformity in deformation and endurance data generation. In response to this realization, the state of the art relating to creep-fatigue interaction was extensively reviewed by an international group of specialists, and the generated knowledge base was used to underpin a new ASTM testing standard, ASTM E2714-09. The gathered knowledge is reviewed. There is a requirement for all ASTM standards to include a precision and bias statement, and an international interlaboratory creep-fatigue test comparison activity was facilitated to form the basis of this section of ASTM E2714-09. An integral part of the guidance given in the new standard is the recommendation for post-test metallurgical inspection and the way in which this information can be used to give added value to creep-fatigue crack initiation endurance results. The evidence gathered from this study is also examined.

Short Creep-Fatigue Crack Growth in an Advanced 9 %Cr Steel

Abstract While the development of short cracks due to cyclic elastic loading has been relatively widely studied, in particular at room temperature, their consideration for cyclic inelastic loading at high temperatures and lower frequencies is not so common. Short creep-fatigue crack growth rates may be correlated in terms of cyclic strain range, cyclic J-integral or strain energy density factor, with appropriate allowance for associated creep damage accumulation. Candidate approaches are evaluated with reference to test results generated for an advanced 9 %Cr turbine rotor steel. This paper promotes the use of cyclic strain range and strain energy density factor relative to other candidate correlating parameters in relation to the results of a series of 30 min hold time creep-fatigue tests performed using fully instrumented uniaxial specimens with short crack starters. The focus of the testing campaign is an advanced 9 %Cr turbine rotor steel at temperatures of 600 and 625°C.

Creep-Fatigue Interaction in Eurofer 3 Electron Beam Welds

In future nuclear fusion reactors, structural materials will undergo a large thermal cycling due to pulsed operation and the occurrence of several maintenance periods. Therefore, the investigation of the combined role of creep and fatigue loading is of major importance. In this study, we focused on Eurofer 3 electron beam welds. Two different post-welding heat treatments were carried out: a two-step heat treatment (30 minutes at 980°C followed by 2 hours at 750°C) and a one-step heat treatment (2 hours at 750°C). Fatigue, creep and creep-fatigue tests were performed. A 550°C test temperature was chosen, corresponding to the upper operation temperature currently foreseen for this material. Creep-fatigue experiments were achieved by interrupting a fatigue test and then applying a creep loading until the fracture of the specimen. Several fatigue pre-stress conditions were studied. The post-weld heat treatment influence was analyzed. The damage contributions of fatigue and creep were studied using electron microscopy. The results were compared to previous results obtained on base material.

A unified viscoplastic constitutive model with damage for multi-axial creep–fatigue loading

A unified viscoplastic constitutive model with damage is presented to describe creep–fatigue deformation behavior. The primary improvement on the existing models is to introduce creep and fatigue damage variables into the constitutive equations of adapted complexity. The material model, which is developed from the thermodynamic framework, combines nonlinear isotropic and kinematic hardening variables, and is implemented using a user-defined subroutine. A standard specimen and a hollow cylinder are used to carry out uniaxial and multi-axial creep–fatigue tests to validate the material model. Good agreement is obtained between the simulation and the measurement under various thermo-mechanical loading paths.

Comparison and Assessment of the Creep-Fatigue Evaluation Methods With Notched Specimen Made of Mod.9Cr-1Mo Steel

In components design at elevated temperature, creep-fatigue is one of the most important failure modes, and assessment of creep-fatigue life in structural discontinuities is an important issue in evaluating the integrity of components. Therefore, a lot of creep-fatigue life evaluation methods were proposed until now. To compare and assess the evaluation methods, a series of creep-fatigue test was carried out with notched specimens. All the specimens were made of Mod.9Cr-1Mo steel, which is a candidate material for primary and secondary heat transport system components of the Japan sodium-cooled fast reactor (JSFR). Mechanical creep-fatigue tests and thermal creep-fatigue test were performed by using a conventional uni-axial push–pull fatigue test machine and a thermal gradient generating system with an induction heating. The stress concentration levels were adjusted by varying the notch radius in the each test. The creep-fatigue lives, crack initiation, and propagation processes were monitored by a digital microscope and the replica method. A series of finite element analysis (FEA) was carried out to predict the number of cycles to failure by the several creep-fatigue life evaluation methods. Then, these predictions were compared with the test results. Several types of evaluation methods such are stress redistribution locus (SRL) method, simple elastic follow-up method and the methods described in the design and constriction code for fast reactor (FR) published by the Japan Society of Mechanical Engineers (JSME FRs code) were applied. Through the comparisons, it was appeared that SRL method gave rational conservative prediction of the creep-fatigue life when the factor of κ = 1.6 was applied for all conditions tested in this study. A comparison of SRL method and simple elastic follow-up method indicated that SRL method applied factor of κ = 1.6 gave the smallest creep-fatigue life in practicable stress range level. The JSME FRs code gave an evaluation 70–100 times conservative lives comparing with the test results.

High Temperature Multiaxial Creep-Fatigue Life Prediction for YH61 Nickel-Base Single Crystal Superalloy

This paper discusses an evaluation method of creep-fatigue lives of YH61 single crystal superalloy under multiaxial loading at high temperature. Three types of creep-fatigue tests were performed using three types of the single crystalsuperalloy specimens at 1173K. They were push-pull tests using solid bar specimens, tension-torsion tests using hollow cylinder specimens and biaxial tension-compression tests using cruciform specimens. Anisotropic strain and Mises stress in combination with frequency modified fatigue equation were applied for evaluating the creep-fatigue lives in the three types of tests. The former parameter gave a relatively large scatter but the latter parameter a small scatter in the correlation.

A constitutive model for inelastic behavior of casting materials under thermo-mechanical loading

High-temperature components, for example turbochargers, are often subject to complex thermal and mechanical loading paths. Non-uniform temperature distribution and constraints by neighboring components result in complex timely varying stress and strain states during operation. The aim of this paper is to analyze inelastic behavior of a casting material Ni-resist D-5S in a wide stress, strain rate and temperature ranges. The material model including a constitutive equation for the inelastic strain rate tensor and a non-linear kinematic hardening rule is discussed. To calibrate the model, experimental databases from creep and low cycle fatigue tests are generated. They include creep curves for temperatures within the range 600–800 °C and stress levels from 10 to 150 MPa. The low cycle fatigue data collect a family of hysteresis loops for the strain rate of 10−3 1/s, the strain amplitude from 0.4% to 2% and temperature levels within the range 200–800 °C. For the verification of the model, simulations of the material behavior under uniaxial thermo-mechanical fatigue loading conditions are performed. The results for the stress response are compared with experimental data.