Low Cycle Fatigue Test Data Research Articles

Fatigue limit prediction of 7050 aluminium alloy based on experimental and shallow + deep hybrid neural network

7050 aluminium alloy, as an important lightweight high-strength structural material because of its excellent characteristics, has increasing applications in aviation, aerospace, and launch vehicle manufacturing. Fatigue failure accounts for approximately 80 % of the structural failures in the engineering field, which leads to numerous losses and is highly uncertain and sudden. Therefore, fatigue life prediction of 7050 aluminium alloy has become a prominent research field. In this study, considering the two variables of stress ratio and stress concentration coefficient, fatigue tests of 7050 aluminium alloy under five different working conditions were conducted to evaluate the effect of these factors on the durability of 7050 aluminium alloy. A novel hybrid neural network model was proposed to solve the time-consuming problem of obtaining a large amount of S-N curve data through traditional fatigue tests. The model uses relatively low cycle fatigue test data for training, realizes data derivation, accurately predicts the fatigue limit of materials, and realizes the purpose of quickly evaluating the fatigue properties of 7050 aluminium alloy materials under different working conditions.

A numerical model simulating cyclic behavior of high-strength steel

High-strength steel (HSS) can effectively and economically design structural members that withstand large forces induced by extreme events such as earthquakes. To evaluate the seismic performance of HSS members and structures using nonlinear finite element (FE) analyses, using an accurate material model is important, which can simulate the inelastic cyclic behavior of the HSS. The peculiar material behavior of HSS needs to be considered in the model. This study used a combined hardening model to construct the material model. The configuration of the model and the constituent model parameter values are determined to precisely simulate the low-cycle fatigue (LCF) behavior of HSS. An efficient particle swarm optimization (PSO) algorithm is used to determine parameter values with LCF test data of 54 individual HSS coupons. In additions, to conveniently determine the model parameter values with only monotonic tensile test data instead of conducting expensive LCT tests, empirical equations are proposed. It is shown that the LCF curves of HSS can be accurately simulated using the constructed material model with the proposed equations.

Low-cycle fatigue mechanical behavior of 30CrMo steel under hydrogen environment and numerical verification of chaboche model

In order to investigate the fatigue behavior of the hydrogen storage material, 30CrMo steel, in a hydrogen environment, an electrochemical hydrogen charging method was employed. Low-cycle fatigue experiments were conducted on the material to obtain half-life stress–strain hysteresis curves, cyclic response characteristics, and strain-life relationships under different hydrogen charging durations. The results indicate that the material exhibited an overall cyclic softening behavior, transitioning from ductile fracture to brittle fracture after hydrogen charging, resulting in a significant reduction in fatigue life. The Manson-Coffin formula was fitted based on material cyclic response characteristics and strain-life relationship curves. Additionally, fatigue toughness and Chaboche kinematic hardening models were fitted based on low-cycle fatigue test data. Finite element analysis was used to validate the accuracy and reliability of the Chaboche kinematic hardening model. The Chaboche kinematic hardening model showed minimal error compared to experimental data and accurately described the influence of hydrogen on the low-cycle fatigue mechanical behavior of 30CrMo steel.

A peridynamic model for rail crack initiation with cavity defect

Cavity defects can aggravate the rail crack initiation. To study their effect on rail fatigue crack initiation behaviour, a peridynamic model of rail crack initiation considering the characteristics of the cavity defect was proposed. First, based on the low-cycle fatigue test data, the bond failure criterion of the rail material was derived and verified to characterize the fatigue damage performance. Then, mathematical expressions for circular, triangular and rectangular defect characteristics were proposed, establishing a prefabricated method for rail cavity defects. Based on the ordinary state-based peridynamic fatigue theory, a rail crack initiation model with the cavity defect was constructed. Finally, the behaviour of rail crack initiation with the cavity defect was analysed.

Experimental investigation and constitutive material modelling of low cycle fatigue of EUROFER97 for fusion applications

EUROFER97 is one of the candidate materials for structural components of fusion nuclear reactors (i.e. ITER and DEMO). Due to the plasma instabilities and cyclic operating conditions, these structural components are expected to face asymmetric thermomechanical fatigue loading. In this work, existing models based on the Chaboche viscoplasticity model have been modified in order to represent the fatigue behaviour of EUROFER97 under different strain ratios and ranges. The modified model has been parameterized based on experimental data obtained from fatigue tests of EUROFER97, in which the strain range varied from 0.6 % to 1.5 % and the strain ratio was either -1 or 0 at room temperature or 350 °C. The incorporation of the strain memory effect into the isotropic hardening due to the observed non-Masing behaviour of EUROFER97 enabled the use of unified parameters for every temperature that could cover all the different testing conditions. In order to validate the current model, the simulated results are compared to strain controlled low cycle fatigue test data performed at room temperature and 350 °C, which were not used for the parameterization. A good match between the simulation and experimental results proves the predictive capability of the proposed modified model.

Experimental validation of ASME strain-based seismic assessment methods using piping elbow test data

To quantify the conservatism of existing ASME strain-based evaluation methods for seismic loading, this paper presents very low cycle fatigue test data of elbows under various cyclic loading conditions and comparison of evaluation results with experimental failure cycles. For strain-based evaluation methods, the method presented in ASME BPVC CC N-900 and Sec. VIII are used. Predicted failure cycles are compared with experimental failure cycle to quantify the conservatism of evaluation methods. All methods give very conservative failure cycles. The CC N-900 method is the most conservative and prediction results are only ∼0.5% of experimental data. For Sec. VIII method, the use of the option using code tensile properties gives ∼3% of experimental data, and the use of the material-specific reduction of area can reduce conservatism but still gives ∼15% of experimental data.

Plastic-Creep Fatigue Damage Rule for Aluminum Alloys Identified by Particle Swarm Optimization Method

Automotive engine components made of aluminum alloys are often subjected to cyclic loading in a high-temperature range that exceeds 1/2 of the melting temperature Tm of aluminum alloys. The fatigue life of an aluminum alloy subjected to such loading should be evaluated by a fatigue damage law considering not only plastic damage but also creep damage. Then, in this study, we attempted to derive the fatigue damage law for an Al-Si-Cu-Ni-Mg alloy by conducting plastic-creep separation strain analyses using low cycle fatigue (LCF) test data. The material parameters used in the damage law were estimated by employing the particle swarm optimization (PSO) method. The derived fatigue damage law was applied to the fatigue life evaluation for the thermo-mechanical fatigue (TMF) tests which closely relate to the strength reliability evaluation for the engine components in actual use. The results show that the fatigue damage law with the material parameters estimated by the PSO method can well evaluate the fatigue life of the TMF test even though it was derived by using the LCF test data.

Research on low cycle fatigue life prediction considering average strain

To address the difficult problems in the study of the effect of average strain on fatigue life under low-cycle fatigue loads, the effect of average strain on the low-cycle fatigue life of materials under different strain cycle ratios was discussed based on the framework of damage mechanics and its irreversible thermodynamics. By introducing the Ramberg-Osgood cyclic constitutive equation, a new low-cycle fatigue life prediction method based on the intrinsic damage dissipation theory considering average strain was proposed, which revealed the correlation between low-cycle fatigue strain life , material properties, and average strain. Through the analysis of the low-cycle fatigue test data of five different metal materials, the model parameters of the corresponding materials were obtained. The calculation results indicate that the proposed life prediction method is in good agreement with the test, and a reasonable characterization of the low-cycle fatigue life under the influence of average strain is realized. Comparing calculations with three typical low-cycle fatigue life prediction models, the new method is within two times the error band, and the prediction effect is significantly better than the existing models, which is more suitable for low-cycle fatigue life prediction. The low-cycle fatigue life prediction of different cyclic strain ratios based on the critical region intrinsic damage dissipation power method provides a new idea for the research of low-cycle fatigue life prediction of metallic materials.

Low-cycle fatigue behavior of 7075-T6 aluminum alloy at different strain amplitudes

Low-cycle fatigue (LCF) life and failure mechanism of 7075-T6 high strength aluminum alloys were investigated under MTS 809. The cycling stress response and the cyclic stress—strain relationships under different strain amplitudes were investigated. Using Manson-Coffin law, the three parameter exponential function equation and the damage function model of tensile hysteresis energy (Ostergren equation), the regression analysis of LCF test data was carried out, it was found that the fatigue life prediction results of the three parameter exponential function equation were better than the other two life prediction methods in terms of statistical analysis methods of standard deviation and dispersion band. Analysis of microstructure and fatigue failure fracture revealed that fatigue crack initiated at the interface of precipitations and α-phase aluminium substrate in surface or near surface of the sample.

Life evaluation of a combustion chamber by thermomechanical fatigue panel tests based on a creep fatigue and ductile damage model

The inner liner of a combustion chamber of a cryogenic liquid rocket engine is exposed to a high load induced by the high temperature of the hot gas and the low temperature of the coolant. The high load causes some inelastic strain that accumulates with each operational cycle until the fracture or rupture of the inner liner. A model that can reproduce the propagation of damage under a thermally cycled load is essential for precisely predicting the chamber life. However, the damage propagation phenomenon or the quantitative value of the damage was so far not fully discussed using the damage data obtained from basic testing of a rocket chamber material. The purpose of the present study was to investigate a precise prediction model based on damage mechanics for simulating the damage propagation of a rocket chamber material. In this study, low cycle fatigue test data at a high temperature (900 K) were analyzed, and damage models that could reproduce the damage propagation under cyclic load conditions were investigated. Then the parameters were identified to reproduce uniaxial test data. These damage models were also subject to a finite element method analysis of a thermomechanical fatigue panel test in order to quantitatively evaluate the deformation, damage propagation, and life of a chamber wall. The analysis of low cycle fatigue test data at 900 K suggested a specific model that could precisely reproduce the damage propagation phenomenon and the basic material test data. From the results, it was confirmed that the model could predict the location of crack initiation.

Low cycle fatigue life prediction for GH4133 at 550°C based on power-exponent function

The low cycle fatigue tests of GH4133 superalloy are carried out under total axial strain at 550℃. The relationships of cyclic stress-strain and strain-life are analyzed. The low cycle fatigue test data of GH4133 superalloy are processed by Manson-Coffin method. When experimental data is processed, the relationship between plastic strain and failure reverse number presents nonlinear in the double logarithmic coordinates. Therefore, there must be some prediction error by Manson-Coffin method. In order to solve the problem, a new method based on power-exponent function is developed. The results of life prediction are given by using new method at the same time. The results showed that two methods chosen were able to give a reasonable life prediction result for GH4133 superalloy. Prediction accuracy within ± 1.5 times scatter band. In order to verify the applicability of power-exponent function model, the investigation gives the model performances contrast results of several materials. The results of life prediction showed new method gave better results with the smaller scatter band and standard deviation than Manson-Coffin method．The life prediction capability of new methods proves more effective and accurate than Manson-Coffin method.

A study of high temperature low cycle fatigue life prediction for two superalloys

The high temperature low cycle fatigue tests of DZ22 superalloy and K403 superalloyare carried out under strain controlled. The low cycle fatigue test data of DZ22superalloy at 850℃ and K403 superalloy at 750℃ are processed by Coffin-Mansonmethod. When experimental data is processed, negative or small plastic strain valuesoften appear and the results of life prediction fail to meet the needs of the designer. Inorder to solve the problem, a new method based on stress fatigue and Coffin-Mansonmethod is developed, the results of life prediction are given by using new method atthe same time. The results showed that new method is more accurate than the Coffin-Manson method. The life prediction capability of two methods for DZ22 superalloyproves more effective than K403 superalloy.

A structural strain method for low-cycle fatigue evaluation of welded components

In this paper, a new structural strain method is presented to extend the early structural stress based master S–N curve method to low cycle fatigue regime in which plastic deformation can be significant while an elastic core is still present. The method is formulated by taking advantage of elastically calculated mesh-insensitive structural stresses based on nodal forces available from finite element solutions. The structural strain definition is consistent with classical plate and shell theory in which a linear through-thickness deformation field is assumed a priori in both elastic or elastic–plastic regimes. With considerations of both yield and equilibrium conditions, the resulting structural strains are analytically solved if assuming elastic and perfectly plastic material behavior. The formulation can be readily extended to strain-hardening materials for which structural strains can be numerically calculated with ease. The method is shown effective in correlating low-cycle fatigue test data of various sources documented in the literature into a single narrow scatter band which is remarkable consistent with the scatter band of the existing master S–N curve adopted ASME B&PV Code since 2007.With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure in 2007 ASME Div 2 Code can now be eliminated, such as its use of Neuber's rule in approximating structural strain beyond yield. More importantly, both low cycle and high cycle fatigue behaviors can now be treated in a unified manner. The earlier mesh-insensitive structural stress based master S–N curve method can now be viewed as an application of the structural strain method in high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke's law. In low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. The use of a pseudo-elastic structural stress definition is not fundamental, but merely a means to put low-cycle and high-cycle fatigue test data in a conventional stress-based S–N data representation which is typically preferred in engineering practice, than a strain-based representation.

A new method to predict fatigue crack growth rate of materials based on average cyclic plasticity strain damage accumulation

By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren (HRR) field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cyclic plastic deformation region is defined due to Manson-Coffin law. Then, according to the linear damage accumulation theory—Miner law, a new model for predicting the fatigue crack growth (FCG) of the opening mode crack based on the low cycle fatigue (LCF) damage is set up. The step length of crack propagation is assumed to be the size of cyclic plastic zone. It is clear that every parameter of the new model has clearly physical meaning which does not need any human debugging. Based on the LCF test data, the FCG predictions given by the new model are consistent with the FCG test results of Cr2Ni2MoV and X12CrMoWVNbN 10-1-1. What’s more, referring to the relative researches, the good predictability of the new model is also proved on six kinds of materials.

Analysis of Multiaxial Low Cycle Fatigue Life for Single Crystal Ni-Based Superalloy Based on Two-Phase Unit Cell Model

Based on micro structure of Ni-based single crystal superalloy, a γ/γ’ two-phase unit cell finite element model was established, and its cyclic stress-strain was simulated under tension/torsion cyclic loading. A low cycle fatigue (LCF) life prediction model of single crystal superalloy was proposed by using cyclic plasticity strain energy as a parameter based on energy dissipation theory. Calculation results of macro finite element model and γ/γ’ two-phase unit cell micro finite element model, and multiaxial LCF test data of CMSX-2 Ni-based single crystal superalloy along [001] orientation were applied to fit the LCF life model by multiple linear regression. The results show that the unit cell model not only reflects the microstructure characteristics of single crystal Ni-based superalloy, but also is better than the macro model in accuracy of analysis, and greatly improve the accuracy of fatigue life prediction. Almost test data fall into the factor of 2.0 scatter band.

Evaluation method of multiaxial low cycle fatigue life for face-centered cubic crystal single crystal material

By taking into account the coupling effect of normal stress and shear stress on orthotropic material when the applied loading deflects from the principal axes directions of material coordinate system, formulas of equivalent strain and stress for cubical single crystal material are used together with a variable k, which is introduced to express the effect under asymmetrical cyclic loading on fatigue life, to put forward a low cycle fatigue (LCF) life prediction model for face-centered cubic crystal (FCC) single crystal in multiaxial stress states. Based on the yield criterion and constitutive model of cubical single crystal material, a subroutine to calculate the thermo elastic-plastic stress-strain of the material on ANSYS platform is developed. The cyclic stress-strain of DD3 notched specimens under asymmetrical loading at 680 ℃ is analyzed. Different failure parameters of the model are analyzed with multiple regression correlation analysis. The equivalent strain and stress for cubical single crystal material as failure parameters have the largest correlation coefficient. A power law exists between k and the failure cycle. The model is validated with LCF test data of CMSX-2 and DD3 single crystal nickel-based superalloys. All the test data fall into the factor of 2.5 for CMSX-2 hollow cylinder specimens and 2.0 scatter band for DD3 notched specimens, respectively.

Evaluation method of multiaxial low cycle fatigue life for cubic single crystal material

The coupling effect of normal stress and shear stress on orthotropic materials happens when applied loading deflects from the directions of the principal axes of the material coordinate system. By taking account of the coupling effects, formulas of equivalent stress and strain for cubic single crystal materials are cited. Using the equivalent strain and equivalent stress for such material and a variable k, which is introduced to express the effect of asymmetrical cyclic loading on fatigue life, a low cycle fatigue (LCF) life prediction model for such material in multiaxial stress starts is proposed. On the basis of the yield criterion and constitutive model of cubic single crystal materials, a subroutine to calculate the thermo elastic-plastic stress-strain of the material on an ANSYS platform was developed. The cyclic stress-strain of DD3 notched specimens under asymmetrical loading at 680°C was analyzed. Low cycle fatigue test data of the single crystal nickel-based superalloy are used to fit the different parameters of the power law with multiple linear regression analysis. The equivalent stress and strain for a cubic single crystal material as failure parameters have the largest correlation coefficient. A power law exists between k and the failure cycle. The model was validated with LCF test data of CMSX-2 and DD3 single crystal nickel-based superalloys. All the test data fall into the factor of 2.5 for CMSX-2 hollow cylinder specimens and 2.0 scatter band for DD3 notched specimens, respectively.

High-Temperature Fatigue Properties of Single Crystal Superalloys in Air and Hydrogen

Hot section components in high-performance aircraft and rocket engines are increasingly being made of single crystal nickel superalloys such as PWA1480, PWA1484, CMSX-4, and Rene N-4 as these materials provide superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over their polycrystalline counterparts. Fatigue failures in PWA1480 single crystal nickel-base superalloy turbine blades used in the space shuttle main engine fuel turbopump are discussed. During testing many turbine blades experienced stage II noncrystallographic fatigue cracks with multiple origins at the core leading edge radius and extending down the airfoil span along the core surface. The longer cracks transitioned from stage II fatigue to crystallographic stage I fatigue propagation, on octahedral planes. An investigation of crack depths on the population of blades as a function of secondary crystallographic orientation (β) revealed that for β=45+/−15 deg tip cracks arrested after some growth or did not initiate at all. Finite element analysis of stress response at the blade tip, as a function of primary and secondary crystal orientation, revealed that there are preferential β orientations for which crack growth is minimized at the blade tip. To assess blade fatigue life and durability extensive testing of uniaxial single crystal specimens with different orientations has been tested over a wide temperature range in air and hydrogen. A detailed analysis of the experimentally determined low cycle fatigue properties for PWA1480 and SC 7-14-6 single crystal materials as a function of specimen crystallographic orientation is presented at high temperature (75°F–1800°F) in high-pressure hydrogen and air. Fatigue failure parameters are investigated for low cycle fatigue data of single crystal material based on the shear stress amplitudes on the 24 octahedral and 6 cube slip systems for FCC single crystals. The max shear stress amplitude [Δτmax] on the slip planes reduces the scatter in the low cycle fatigue data and is found to be a good fatigue damage parameter, especially at elevated temperatures. The parameter Δτmax did not characterize the room temperature low cycle fatigue data in high-pressure hydrogen well because of the noncrystallographic eutectic failure mechanism activated by hydrogen at room temperature. Fatigue life equations are developed for various temperature ranges and environmental conditions based on power-law curve fits of the failure parameter with low cycle fatigue test data. These curve fits can be used for assessing blade fatigue life.

A unified approach for the design of steel structures under low and/or high cycle fatigue

In this paper, a method is presented trying to unify both design and damage assessment methods for high and low cycle fatigue. In particular it is shown that, by interpreting the stress range Δω as the ideal stress range associated to the real strain range Δϵ in an ideal perfectly elastic material, high and low cycle fatigue test data can be interpreted by the same Wöhler (S-N) lines usually given in recommendations for (high cycle) fatigue design of steel structures. Furthermore, local buckling can be regarded as a notch effect, an effect which is intrinsic to the various shapes, because it is strictly correlated to their geometrical properties (in particular of the slenderness ratios b/t and h/t w of the flanges and the web). It is also shown that, in the case of variable amplitude loading histories reprocessed by the rainflow cycle counting method, a linear damage cumulation rule together with the previously defined S-N curves (a procedure usually adopted in high cycle fatigue) can lead to a reliable collapse criterium for low cycle fatigue also.

A PROBABILISTIC MODEL FOR PREDICTION OF LCF SURFACE CRACK INITIATION IN PM ALLOYS

AbstractThe Low Cycle Fatigue (LCF) life of PM Ni‐base superalloys is commonly reduced by surface crack initiation at ceramic inclusions. For this reason, a probabilistic model has been developed that predicts the size of surface crack initiation sites from the inclusion size distribution. For the experimental correlation of the model two sets of alloys were examined: a “standard” (i.e. as‐received) alloy, and a second material of identical composition to which a known distribution of ceramic inclusions was incorporated (or seeded). Model predictions were found to be in excellent agreement with the results obtained from the seeded materials in which the defect size distribution is larger and better characterized, and were satisfactory for the unseeded material in which two types of surface defects (pores and ceramic inclusions) initiate LCF cracks. The results of these experiments were employed in LCF simulations of both test specimens and full scale components. These indicated that differences exist between the site preference for LCF crack initiation in small test specimens and large scale components due to a scale effect. Such results demonstrate the utility of seeding experiments for generation of LCF test data used in component design.