Creep-fatigue Life Research Articles

Evaluation of Extrapolation for Creep-Fatigue Life by Hysteresis Energy

The creep-fatigue life has been evaluated by the hysteresis energy in 316FR stainless steel with low carbon and medium nitrogen, which is a candidate for structural material in Fast Breeder Reactor (FBR) plant with the design life of 60 years. The creep-fatigue is a main damage mode to prevent. The hysteresis energy rate is considered as the parameter to predict the life time. It is clear that the relationship between this parameter and the time to failure can be approximately expressed by the power-law function. The function depends on the ratio of plastic strain to total strain. Total fracture energy for creep-fatigue loading intends to be independent of the ratio of plastic strain to total strain in long-term test condition. The value is related to grain boundary strength for creep-fatigue loading because fracture mode in long-term test condition is intergranular fracture. The life could be predicted by the function in the case of no significant change of fracture energy. Coarse precipitation, for example sigma phase, might be considered as a factor to change the fracture energy. It is important to predict the precipitation formation. The result of life prediction by the hysteresis energy rate is compared with that of the time fraction rule based on “Demonstration Reactor Design Standard (Draft)”. The predicted lives by both methods for long-term region are comparable and independent of the ratio of plastic strain to total strain.

Creep-Fatigue Life Evaluation for Materials Subjected to Nonproportionally Combined Tension and Torsion

The author and his colleagues have proposed in their recent works the creep-fatigue life prediction method based on the strain range partitioning concept for nonproportionally combined tension and torsion using new inelastic strain range parameters and have been studying their applicability to the materials subjected to nonproportional axial-torsional loading at elevated temperatures. In the present paper are summarized the results of in-phase and 90 deg out-of-phase axial-torsional straining tests conducted on thin-walled tubular specimens of SUS304, 316LC and 2.25Cr-1Mo steels.

Effect of Thermal Cycling on Properties and Microstructure of SnAgCuCe Soldered Joints in QFP Devices

Increasing global concern about the environment is bringing regulatory (European directives) and consumer ("green products") pressure on the electronics industry in Europe and Japan to reduce or completely eliminate the use of lead (Pb) in products. Among all lead-free solder alloys, SnAgCu solder system, which has better thermo-mechanical properties compared with those of SnPb solder, is proven to be one of the promising candidates for electronic assembly. Previous work also revealed that adding a small amount of rare earth Ce into SnAgCu solder can visibly improve the properties and inhibit the excessive growth of the intermetallic compound layer. Thermal fatigue properties of SnAgCuCe soldered joints in QFP devices under thermal conditions have been investigated by finite element method and experiments. Based on creep model of low stress and high stress, corresponding creep subroutine was established for simulating the stress and strain response of SnAgCuCe soldered joint from -55 ℃ to 125 ℃, and fatigue life was calculated using creep fatigue life prediction equation. Moreover, thermal cycling experiments were conducted, the experimental results were found to be close to the simulated results. In addition, the tensile force of SnAgCuCe soldered joints decreased with increasing number of thermal cycles, and the fracture mechanism transformed from toughness fracture to brittle intergranular fracture. Moreover the tensile force changes and fracture microstructure evolution could benefit the quantitative evaluations of the mechanical performances of lead-free soldered joints under thermal cycling loadings.

OS0703 非比例多軸負荷におけるフェライト系高クロム鋼F82Hのクリープ疲労寿命(OS7-1 高温・寿命,OS-7 多軸応力下における疲労損傷および疲労き裂進展1)

This study discusses creep-fatigue life of type F82H steel used in a blanket structure of a DEMO fusion reactor under uniaxial and multiaxial loadings at 823K. Strain controlled creep-fatigue tests were carried out using a hollow cylinder specimen. Three kinds of loading paths were employed; they were push-pull straining, reversed torsion straining and circular straining. The former two were proportional loadings and the latter one was a non-proportional loading where principal direction of stress and strain was changed in a cycle. Four kinds of waveforms (PP, PC, CP and CC) were employed in order to evaluate an effect of strain rate on the lives. F82H showed that failure life was affected by multiaxial strain path and reduction in the life due to non-proportional straining could be seen. However, the effect of strain rate on the lives was small.

Fatigue-creep life analysis for powder metallurgy material with inclusion

To predict the life of the life limit parts is an important work for the airworthiness compliance. The compliance work of life limit parts involves typical flight profile, heat transfer analysis, stress analysis, materials data, and life predict method. In this study, fatigue-creep interaction behaviors of powder metallurgy materials with inclusion have been performed with the Kachanov-Rabotnov damage law to show the life compliance. On the constant fatigue-creep loading, different types of inclusion are investigated in the work. The research shows that the type of inclusion has strong influence on the damage of matrix. Generally, the elliptical inclusion has much more damage than the round inclusion. Furthermore, the fatigue-creep life of powder metallurgy materials is calculated by the damage model, which can be considered for the design of the engine life limit parts.

Microstructure optimization of austenitic Alloy 800H (Fe–21Cr–32Ni)

The microstructural evolution, specifically of grain boundaries, precipitates, and dislocations in thermomechanically processed (TMP) Alloy 800H samples was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The TMP not only significantly increased the fraction of low-Σ coincidence site lattice boundaries, but also introduced nanoscale precipitates in the matrix and altered the distribution of dislocations. Statistical analysis indicates that the morphology and distribution of grain boundary precipitates were dependent on grain boundary types. The microstructure optimization played a synergistic effect on the significantly increased strength with comparable ductility and enhanced intergranular corrosion resistance and creep-fatigue life compared to the as-received samples.

A generalized frequency separation–strain energy damage function model for low cycle fatigue–creep life prediction

ABSTRACTFatigue–creep interaction is a key factor for the failures of many engineering components and structures under high temperature and cyclic loading. These fatigue–creep life prediction issues are significant in selection, design and safety assessments of those components. Based on the frequency‐modified Manson–Coffin equation and Ostergren's model, a new model for high temperature low cycle fatigue (HTLCF), a generalized frequency separation–strain energy damage function model is developed. The approach used in this model to reflect the effects of time‐dependent damaging mechanisms on HTLCF life is different from those used in all the earlier models. A new strain energy damage function is used to reduce the difference between the approximate strain energy and real strain energy absorbed during the damage process. This proposed model can describe the effects of different time‐dependent damaging mechanisms on HTLCF life more accurately than others. Comparing traditional frequency separation technique (FS) and strain energy frequency‐modified approach (SEFS), the proposed model is widely applicable and more precise in predicting the life of fatigue–creep interaction. Experimental data from existing literature are used to demonstrate the feasibility and applicability of the proposed model. A good agreement is found between the predicted results and experimental data.

Creep-fatigue damage and life prediction of alloy steels

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.

Damage assessment in structural metallic materials for advanced nuclear plants

Future advanced nuclear plants are considered to operate as cogeneration plants for electricity and heat. Metals and alloys will be the main portion of structural materials employed (including fuel claddings). Due to the operating conditions these materials are exposed to damaging conditions like creep, fatigue, irradiation and its combinations. The paper uses the most important alloys: ferritic-martensitic steels, superalloys, oxide dispersion strengthened steels and to some extent titanium aluminides to discuss its responses to these exposure conditions. Extrapolation of stress rupture data, creep strain, swelling, irradiation creep and creep–fatigue interactions are considered. Although the stress rupture- and the creep behavior seem to meet expectations, the long design lives of 60 years are really challenging for extrapolations and particularly questions like negligible creep or occurrence of diffusion creep need special attention. Ferritic matrices (including oxide dispersion strengthened (ODS), steels) have better irradiation swelling behavior than austenites. Presence and size of dispersoids having a strong influence on high-temperature strength bring only insignificant improvements in irradiation creep. A strain-range-separation based approach for creep–fatigue interactions is presented which allows a real prediction of creep–fatigue lives. An assessment of capabilities and limitations of advanced materials modeling tools with respect to damage development is given.

Creep-fatigue damage and life prediction of alloy steels

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.

T0101-1-6 高温環境下におけるステンレス極薄板ベルトのクリープ疲労特性([T0101-1]マイクロ・ナノ材料システムの力学と強度・機能評価(1):ナノインプリント・シートの力学特性評価)

In order to investigate the creep deformation behavior of stainless steel thin sheet belt under high temperature, creep fatigue tests were carried out at high temperature using specimens of SUS305, SUS430 and SUS632. As the results, creep fatigue lives of SUS305 and SUS430 decreased with increasing temperature, and creep fatigue life of SUS632 did not decreasing with increasing temperature. Creep strain of SUS305 was almost equal from 100℃ to 300℃, and increased over 300℃. Creep strain of SUS430 increased with increasing temperature. Creep strain of SUS430 increased from 100℃ to 200℃, and decreased from 200℃ to 400℃.

高温ガス炉中間熱交換器用プレートフィン型熱交換器の開発 ハステロイＸ製プレートフィン構造の製作性，高温強度特性およびクリープ疲労寿命評価

The helium/helium heat exchanger (i.e., intermediate heat exchanger: IHX) of a high-temperature gas-cooled reactor (HTGR) system with nuclear heat applications is installed between a primary system and a secondary system. IHX is operated at the highest temperature of 950°C and has a high capacity of up to 600 MWt. A plate-fin-type heat exchanger is the most suitable for IHX to improve construction cost. The purpose of this study is to develop an ultrafine plate-fin-type heat exchanger with a finer pitch fin than a conventional technology. In the first step, fabrication conditions of the ultrafine plate fin were optimized by press tests. In the second step, a brazing material was selected from several candidates through brazing tests of rods, and brazing conditions were optimized for plate-fin structures. In the third step, tensile strength, creep rupture, fatigue, and creep-fatigue tests were performed as typical strength tests for plate-fin structures. The obtained data were compared with those of the base metal and plate-fin element fabricated from SUS316. Finally, the accuracy of the creep-fatigue life prediction using both the linear cumulative damage rule and the equivalent homogeneous solid method was confirmed through the evaluation of creep-fatigue test results of plate-fin structures.

A strain energy density method for the prediction of creep–fatigue damage in high temperature components

The accumulation of creep–fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature equipment. A model has been developed that assumes on a macroscopic level that the energy dissipated in the material may be taken as a measure of the creep damage induced in the material and hence the creep damage is directly proportional to absorbed internal energy density. The model developed is derived from considerations of mechanistic cavity growth and is based on rupture elongation to failure data using true strain. The predictions of the energy density exhaustion approach are compared with the results of creep–fatigue tests on low alloy ferritic steels. The predicted results of the energy density model are found to have good correlation with the measured creep–fatigue lives.

Experimental study on creep–fatigue interaction behavior of GH4133B superalloy

The creep–fatigue tests have been conducted with nickel-based superalloy GH4133B at 600 °C in three cases of type loading to study the creep–fatigue behavior of the alloy and the loading history effect on the creep–fatigue damage. Since the conventional linear cumulative damage rule failed in evaluating the creep–fatigue life based on experimental data, a continuous non-linear model proposed by Mao et al. was employed to describe the creep–fatigue interaction. The creep–fatigue damage in the cases of continuous cyclic creep loading (CF) and prior fatigue followed by creep loading (F + C) was larger than unity and smaller than unity when the type loading was prior creep followed by fatigue loading (C + F). Scanning electron microscope (SEM) analyses of the fracture surface showed that the cracks initiated from the specimen surface and the fracture modes in different loading history were different. The crack mode at CF loading depended on the cyclic period. In the case of F + C loading, the primary fracture mode was transgranular, and in the condition where the type of waveform was C + F, the fracture mode was of mixed transgranular and intergranular type. In addition, the origin of the history effect on creep–fatigue interaction was explained by the SEM observations.

Creep-Fatigue Life Assessment for Sn-3.0Ag-0.5Cu Solder

This paper describes the creep-fatigue life assessment of Sn-3.0Ag-0.5Cu solder under pull-push loading using fast-fast, fast-slow, slow-fast and strain-hold strain waveforms. Strain controlled creep-fatigue tests were carried out using solid bar specimen, and the effect of strain waveform on the creep-fatigue life was discussed. Creep-fatigue damage was evaluated by the linear damage rule, the frequency modified fatigue life, the ductility exhaustion model, the strain range partitioning method and the grain boundary sliding model. The strain range partitioning method and the grain boundary sliding model only predicted the creep-fatigue lives within a small scatter.

OS0732 高速炉原子炉容器の高温構造設計評価技術の開発 : その4:ラチェットひずみの316FRクリープ疲労強度への影響(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

The effect of ratcheting strain on creep-fatigue strength was investigated in order to adjust the strain limit. Ratcheting creep-fatigue tests were conducted at 600℃ with lhr hold time. Creep fatigue lives did not decrease when the accumulated strain was superimposed within 5%. Therefore the effect of ratcheting on creep-fatigue strength is negligible when accumulated strain exceeds 2%.

Creep-Fatigue Evaluation by Hysteresis Energy in Modified 9Cr-1Mo Steel

Researchers proposed the methods of creep-fatigue evaluation, such as time fraction rules or ductility exhaustion rules. However, the microstructure change during creep-fatigue should not be directly considered in these methods. The hysteresis energy contributes to the microstructure change before the crack initiation and the crack initiation and propagation. The creep-fatigue has evaluated by the hysteresis energy in modified 9Cr-1Mo steel which is a candidate for structural material in Fast Breeder Reactor (FBR) plant. Creep-fatigue and fatigue tests were carried out at 723-873K in air. The hysteresis energy per hour at the middle of life (Nf / 2, Nf is the number of cycles to failure) has been evaluated. It is clear that the relationship between this parameter and the time to failure can be expressed by the power-law function. The creep-fatigue life can be evaluated based on the hysteresis energy an hour at Nf / 2 using this relation.

Creep-fatigue Life Prediction Methods of Pressure Vessel Typical Steels under Complicated Loading Conditions

With emphasis on complicated loading conditions,i.e.multi-axial loading condition and multi-step loading condition,creep-fatigue behavior and life prediction methods are investigated for typical steels of pressure vessels.As to multi-axial loading condition,the interaction behavior between high temperature fatigue and cyclic creep is discussed for 16MnR notched specimens and a mean displacement rate life prediction method is proposed on the basis of ductility exhaustion theory and damage mechanics.By this method,high temperature fatigue lives are well predicted for specimens with different notch radiuses.As to multi-step loading condition,cyclic deformation behavior is investigated for 316L steel and creep-fatigue interaction coupled with dynamic strain aging effect is also discussed.Based on the ductility exhaustion theory,a nonlinear damage evolution model is developed.Moreover,a new damage cumulated rule is proposed with the load history effect taken into account.By using this model,2-step creep-fatigue lives are well predicted.

21316 円孔板に対する荷重制御型高温低サイクル疲労・クリープ疲労寿命予測(材料強度と構造設(2),OS.5 材料強度と構造設計)

The purpose of this study is to obtain the fundamental feature of perforated plate under the load-controlled low-cycle fatigue and creep-fatigue condition. The hysteresis-loop and the crack propagation were measured during the test to examine the relation between crack propagation and the history of strain. And the Stress Redistribution Locus (SRL) method is applied for estimation of stress concentration in order to predict fatigue and creep-fatigue life for the specimens. It was found that the strain varies with increasing number of cycle. In the latter of cycle the strain increased sharply. It was shown that the life evaluation can be more accurate using the strain obtained by applying SRL method. Additionally, it was shown that the life evaluation using crack initiation cycle is more accurate than that using failure cycle.

OS0738 316LC鋼の1023Kにおける軸ねじりPCおよびCP試験寿命におよぼす位相角の影響(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

OS0738 316LC鋼の1023Kにおける軸ねじりPCおよびCP試験寿命におよぼす位相角の影響(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)