Creep-fatigue Life Prediction Research Articles

Creep-Fatigue Interaction Life Prediction for P91 Steel Based on Improved Elman Neural Network Model

A creep-fatigue life prediction model for P91 steel using Improved Elman Neural Network is founded based on the creep-fatigue experiment datas in this paper. The load keeping time is selected as the model input and the creep life or fatigue life as the output in the model. The prediction ability is validated from training the sample data. The results show the model has a very high prediction accuracy for training samples. But the model generalization ability is relatively weak, so the forecasting accuracy for test sample is lower than the training samples. Through increasing test sample number and decreasing the uncertain factors during the experiment in the follow-on working, the prediction accuracy and the model generalization ability will be increased.

Material Modeling of Elasto-Plastic Creep Behavior for Gray Cast Iron Based on Damage Mechanics and Its Applications to Creep Fatigue Life Prediction

The elasto-plastic constitutive equation based on the concept of continuum damage mechanics is extended to cover the creep behavior for the gray cast iron as used in diesel engine parts. It employs the creep strain given by Perrin and Hayhurst and the creep damage given by Dunne and Hayhurst. Lots of material parameters in the constitutive equation are determined by using the uniaxial tensile/compressive, the creep, the relaxation, and the fatigue test results at 400°C. The identified model is used to predict the creep fatigue life with 600 sec hold-time at the maximum compressive strain. The validity of the proposed model is illustrated by comparing the calculated results with the experimental results. It is expected that the developed computational modeling can be used effectively to predict the creep fatigue life of gray cast iron machine parts.

Experimental and Mechanical Characterizations of a Lead Free Solder Alloy for Electronic Devices

Electronic power modules devices are paramount components in the aeronautical, automotive and military applications. The solder layers are the most critical parts of the module and are usually subjected in their whole life to complex loading conditions. To improve the design task, realistic thermoelastoviscoplastic and lifetime prediction models which can describe efficiently the deformation-damage of the electrical device must be chosen carefully. Some of the most common behavior models are based on the separation between creep and plasticity deformations such as power law, Garofalo, Darveaux… So, to take into account the creep-plasticity interaction, the thermal cycling as well as the hardening-softening effects, unified viscoplastic models are increasingly being used to describe more efficiently the physical state of the material. We propose in this framework a survey of some unified viscoplastic models used in the electronic applications for the viscoplastic modeling of the solder as well as creep-fatigue life prediction rules. The models are used for the characterization of a SnAgCu solder and are briefly compared within tensile, creep data and stabilized responses.

Analysis of creep–fatigue life prediction models for nickel-based super alloys

Creep fatigue is the main damage form of engine parts operating under both cyclic mechanical and thermal loading conditions. In the present work time-dependent creep and cycle-dependent fatigue of nickel-based super alloys is discussed. Three different kinds of creep–fatigue life models are investigated for the nickel-based super alloys. Effects of temperature, loading ratio and loading hold time to material failure are considered. Comparison with experiments reveals that the linear accumulation concept based on Forman model provides reasonable results on the creep–fatigue interaction if the life is dominated by mechanical fatigue damage. For higher loading ratios, however, deviations become significant. In the lower frequency region the modified FCE models and Saxena models give more reasonable predictions, but show large discrepancy to tests with hold time. The modified Saxena model suggested in the present work contains a separate term for hold time effects and agrees with the experimental observations more accurately.

A numerical investigation of creep-fatigue life prediction utilizing hysteresis energy as a damage parameter

This paper explores the hypothesis that there exists an intrinsic material property, hysteresis damage energy at failure, which could be used as a creep-fatigue life prediction parameter. The connection between hysteresis energy and fatigue damage was introduced in the 1920’s by Inglis, but the use of hysteresis energy as a measure of damage was first presented by Morrow and Halford. Hysteresis energy shows promise in bridging the gaps associated with life prediction when the combination of both creep and fatigue scenarios are present. Numerical simulations which replicate experimental test configurations with 9Cr–1Mo steel were performed from which the hysteresis energy failure density (HEFD) could be calculated for each experiment. Taking the average of the HEFD values calculated for all of the experimental data as the parameter for failure (E Intrinsic), creep-fatigue life predictions were made using a simplistic hysteresis energy based method as well as the time fraction/cycle fraction method endorsed by ASME Code and compared to experimental results. A good correlation with experimental results was obtained for life predictions using hysteresis energy density as a damage parameter. An investigation of the interaction between creep damage and fatigue damage based on the hysteresis energy method was also performed and compared with the damage interaction diagram utilized by the ASME and RCC-MR design codes. The hysteresis energy based method proved easy to implement and gave improved accuracy over the time fraction/cycle fraction method for low cycle creep-fatigue loading.

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.

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.

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.

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.

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.

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.

Study on creep-fatigue evaluation procedures for high chromium steels—Part II: Sensitivity to calculated deformation

High-chromium steels containing 9–12% chromium such as ASME SA-213 Grade 91 or Grade 122 are widely used in conventional and combined cycle fossil power plants and are also regarded as candidate structural materials for future nuclear power plants aiming at operation in the creep range. Evaluation of failure life under creep-fatigue conditions constitutes an important part of assessing the structural integrity of these plants. The author has been conducting a series of creep-fatigue tests for three types of high-chromium steels and the validity of life prediction methods has been evaluated using the measured deformation data. Here an additional exercise was carried out in order to evaluate the adequacy of the total life prediction procedure, including the process of predicting stress and strain. After making comparisons of the test data with various existing equations developed for describing deformation and failure behaviour of Grade 91 and Grade 122, prediction of creep-fatigue life was attempted using deformation behaviour analytically estimated by these equations. Many calculations revealed that failure lives predicted by the time fraction approach showed a strong dependency on stress relaxation behaviour, whereas those based on the modified ductility exhaustion method showed a much smaller sensitivity, and therefore some error or uncertainty in stress and strain would be tolerated.

Multiaxial creep–fatigue life prediction for cruciform specimen

This paper describes the high temperature multiaxial creep–fatigue life prediction for type 304 stainless steel. Finite element analyses were performed for determining the stress–strain state in the gage part of a cruciform specimen subjected to creep–fatigue loading under four strain waves at three principal strain ratios. Creep–fatigue lives of cruciform specimens were discussed in relation to the principal stress amplitude calculated by finite element analysis. Creep–fatigue damage was evaluated by linear damage rule and the suitability of three low cycle fatigue and three creep damage parameters was discussed.

Normalized creep–fatigue life prediction model based on the energy dissipation during hold time

A new life function based on a model for the creep–fatigue life prediction in terms of stress relaxation during hold time has been proposed. From the concept that the reduction of fatigue life with hold time is due to the creep effect of stress relaxation that results in the additional energy dissipation in the hysteresis loop, the relaxed stress range is suggested to be a creep–fatigue damage function. The plastic strain range (Δ ɛ p) exhibiting a functional correlation of both strain and time requires experimental efforts following Coffin–Manson relationship [L.F. Coffin Jr., Trans. ASME 76 (1954) 931–950; S.S. Manson, NASA Technical Note 2933, NASA Research Center, Cleveland, OH, 1954] upon applied creep–fatigue conditions. After the new concept was applied to the relationship, all the Coffin–Manson plots at the various levels of hold time under strain-controlled fatigue tests were identified as a linear relationship with relaxed stress range (Δ σ) and fatigue life ( N f). The normalized creep–fatigue life prediction model, which can be applied to high temperature materials regardless of the fracture morphology is presented in view of a real situation.

Total strain version of strain energy partitioning

A total strain version of strain energy partitioning (TS-SEP) is formulated based on strain energy partitioning(SEP), utilizing the method of HALFORD and SALTSMAN for developing TS-SRP. Like TS-SRP, TS-SEP characterizes the behavior of alloys in two aspects: Failure behavior and flow behavior. Elastic strainrange-life relations are established by using the empirical equations of flow behavior of alloys. Total strainrange-life relations are given by summing elastic strain-range-life relations and inelastic strainrange-life relations. TS-SEP can be used for creep-fatigue life prediction without partitioning the inelastic strainrange, so it is applicable in the low-strain, long-life regime where SEP is difficult to use. Then, TS-SEP is evaluated and verified by using two advanced nickel-base superalloys, Rene95 at 650℃and AF2-1DA at 760℃. The results indicate that TS-SEP can predict the creep-fatigue life of Rene95 at 650℃and AF2-1DA at 760℃more reliable than SEP. The agreement of the predictions with the observed lives of the baseline tests is quite good for TS-SEP, with 97.5% percent of the Rene95 baseline data being predicted to within factors of two, and 100% of AF2-1DA, and the scatter band of TS-SEP for verification tests is lower than all other three methods (SEP, TS-SRP, SRP).

Study on creep-fatigue life prediction methods for low-carbon nitrogen-controlled 316 stainless steel (316FR)

Low-carbon, nitrogen-controlled 316 stainless steel called 316FR was developed and is regarded as a principal candidate for a main structural material of liquid metal-cooled fast breeder reactor plants in Japan. To develop a creep-fatigue evaluation method suitable for this steel, a number of uniaxial creep-fatigue tests have been conducted for three products of this steel. Long-term data up to about 35,000 h were obtained and applicability of failure life prediction methods was studied based upon their results. Cruciform shaped specimens were also tested under biaxial loading conditions to examine the effect of stress multiaxiality on failure life under creep-fatigue condition.

Finite element-based life prediction for high-temperature cyclic loading of a large superplastic forming die

The thermomechanical, time-dependent, elastic-plastic creep behaviour of a representative tool in a high-temperature superplastic forming press has been simulated using a sequential finite element-based heat transfer and mechanical stress analysis approach. The heating and cooling aspects of the heat transfer model have been previously validated against laboratory measurements. This paper focuses on the prediction of die life based on this model. The determination and modelling of the high-temperature mechanical properties of the materials and die are described for both so-called (a) major cycles, relating to die heat-up and cool-down from and to ambient temperature, and (b) minor cycles, relating to die heat-up and cool-down associated with part removal and blank insertion. A creep-fatigue life prediction method using non-linear damage summation is developed and applied to the die-life problem. Isothermal fatigue and creep-fatigue interaction tests at 700 and 900 °C on 40%Ni-20%Cr die material XN40F are carried out to estimate the required strain-life relationships.

Creep-fatigue life prediction under fully-reversed multiaxial loading at high temperatures

A multiaxial fatigue damage parameter based on the critical plane approach was proposed to calculate the pure fatigue damage under uniaxial/multiaxial loading at constant high temperatures. For the fully-reversed low-cycle fatigue loading under low frequency at high temperature, one-half of the maximum equivalent stress response value at cyclic stabilization is used as the creep stress to evaluate the multiaxial creep damage. The linear damage accumulation rule is used to predict the multiaxial creep-fatigue life at high temperature. The creep-fatigue experimental data of thin tubular specimens with GH4169 superalloy and 2.25Cr–1Mo steel were used to verify the proposed creep-fatigue life prediction model. The results showed that the proposed creep-fatigue damage calculation model can be used under either uniaxial or multiaxial nonproportional loading at high temperature. The proposed model is used to predict multiaxial creep-fatigue life, and a good agreement is demonstrated with experimental data.

Power-exponent function model for low-cycle fatigue life prediction and its applications – Part I: Models and validations

Based on power transformation method, this paper advances a power-exponent function model for low-cycle fatigue (LCF) life prediction and then introduces it to creep–fatigue life prediction methods – the strain range partitioning (SRP) method and the linear damage summation (LDS) method for comprehensive assessment of power-exponent function model. Power transformation exponents ( p) are given for some typical materials. Through LCF and creep–fatigue life predictions for these materials, we illustrate the validity of power-exponent function model and its applications. The comparisons between different models show that the power-exponent function model has better performances for LCF life prediction. In nature, Manson–Coffin law is the first order Taylor expansion approximation of power-exponent function model in log–log scale coordinates, corresponding to p = 1. The enhancement of nonlinearity makes the power-exponent function model and its applications to creep–fatigue life prediction methods more precise than the corresponding unmodified methods, whereas these models still use linear parameter regression method and remain simple for engineering applications. An example of these models’ applications to life prediction of turbine blades under creep–fatigue interaction is given in the accompanying paper.

A Strain Rate Ratio Approach for Assessing Creep-Fatigue Life of 63Sn-37Pb Solder Under Shear Loading

This paper studies creep-fatigue life prediction under shear loading by making extensive torsion creep-fatigue experiments using four kinds of strain waves. The linear damage rule, strain range partitioning method, frequency modified fatigue life, and ductility exhaustion model were applied to the experimental data, but no methods accurately predicted the creep-fatigue life. A new method based on the strain rate ratio, which predicted the creep-fatigue life within a factor of 4 scatter band, was developed.