Low Cycle Fatigue Life Prediction Research Articles

Strain-controlled fatigue characteristics of a cast Mg–Nd–Zn under peak-aged and over-aged conditions

Strain-controlled fatigue characteristics of peak-aged and over-aged Mg96.47Nd2.9Zn0.21 magnesium alloys containing 0.42Zr, including stress response, strain resistance, hysteresis loops, strain–life and corresponding low-cycle fatigue life prediction model, were studied. In the peak-aged state (T61: 540 °C × 8 h + 200 °C × 14 h), the alloy shows higher cyclic stress response, but lower ductility than the alloy in the over-aged state (T62: 540 °C × 8 h + 200 °C × 400 h). The yield strength and ultimate tensile strength of the alloy under T61- and T62-treated conditions are close. Compared with T61-treated alloy, the steady stress amplitude occurred in T62-treated alloy is due to higher ductility and more homogenous deformation. In T61 state, the fatigue cracks in the alloy first initiate along the cracked persistent slip bands and then propagate in the trans-granular mode, while in the T62 state, the fatigue cracks initiate along grain boundaries and then propagate in the inter-granular mode.

High-Temperature Low Cycle Fatigue Life Prediction and Experimental Research of Pre-Tightened Bolts

Bolted connections are widely used in various mechanical structures due to their superior fastening properties. However, vibration and fatigue loads in the structure are likely to cause fatigue failure of the bolted joints, especially those under high temperature, such as in aero-engines. This paper mainly studies the low-cycle fatigue life of the pre-tightened bolts working at a high temperature. A novel test fixture is designed for fatigue tests, and low cycle fatigue tests of pre-tightened bolts are conducted at the temperatures of 550 °C and 650 °C, respectively. Furthermore, a new low cycle fatigue model that is based on the Von Mises equivalent stress/strain criterion is proposed. Meanwhile, the proposed model is used to predict the high-temperature low cycle fatigue life of pre-tightened bolts according to the stress/strain results obtained by finite element analysis. There is good agreement between the experimental results and those obtained by theoretical prediction, which validates the accuracy of the proposed fatigue model. Research results will provide a theoretical basis for the low cycle fatigue life prediction of pre-tightened bolts.

Grain size based low cycle fatigue life prediction model for nickel-based superalloy

Abstract Nickel-based superalloys are easy to produce low cycle fatigue (LCF) damage when they are subjected to high temperature and mechanical stresses. Fatigue life prediction of nickel-based superalloys is of great importance for their reliable practical application. To investigate the effects of total strain and grain size on LCF behavior, the high temperature LCF tests were carried out for a nickel-based superalloy. The results show that the fatigue lives decreased with the increase of strain amplitude and grain size. A new LCF life prediction model was established considering the effect of grain size on fatigue life. Error analyses indicate that the prediction accuracy of the new LCF life model is higher than those of Manson-Coffin relationship and Ostergren energy method.

Energy-based low cycle fatigue analysis of low yield point steels

As examples of the new advanced high-performance structural steels, low yield point steels have attracted increasing interest owing to their excellent properties in the technology of energy dissipation and seismic design. Because dynamic cyclic loading is inevitable during service life in engineering applications, it is critical to develop in-depth understanding of the fatigue behavior of this material. Here, the low cycle fatigue behavior of low yield point steels produced in China, namely LY100, LY160, and LY225, is investigated using an energy-based approach. Axial steel coupons are tested by fully reversed and push-pull cyclic loading with a nominal strain ratio R = −1 at a constant strain rate of 0.1% S−1. The strain amplitudes range from 0.5% to 6.0% in 0.5% increments. First, experimental details and results of fatigue life are introduced. Subsequently, using an energy-based approach, the cyclic plastic strain energy, cyclic hysteresis loop properties, and fatigue life prediction are thoroughly analyzed. Finally, a simplified method for fatigue life prediction is proposed. The results show that plastic strain energy density is an important parameter for predicting the low cycle fatigue life of low yield point steels with an acceptable degree of accuracy. The proposed simplified method can provide an effective and reliable alternative for low cycle fatigue life prediction of low yield point steels.

Applicability of Notch Stress-Strain Correction Methods to Low-Cycle Fatigue Life Prediction of Turbine Rotors Subjected to Thermomechanical Loads

Abstract The paper analyses the possibility of using analytical methods of notch stress-strain correction in low-cycle fatigue life predictions of steam turbine rotors operating under non-isothermal conditions. The assessment was performed by comparing strain amplitudes calculated using the Neuber and Glinka-Molski methods and those predicted by the finite element analysis (FEA) employing elastic-plastic material model. The results of investigations reveal that the Neuber method provides an upper bound limit, while the Glinka-Molski method results in a lower bound limit of strain amplitude. In the case of rotor heat grooves, both methods provide equally accurate results of notch strain amplitude and are suited to estimating lower and upper bound limits of low-cycle fatigue life under non-isothermal conditions.

Substructure-based distributed collaborative probabilistic analysis method for low-cycle fatigue damage assessment of turbine blade–disk

A numerical simulation-based probabilistic analysis methodology, substructure-based distributed collaborative probabilistic analysis method (SDCPAM), is proposed for accurate and efficient fatigue prognosis based on distributed collaborative response surface method (DCRSM) and substructure analysis method. This paper focuses on the low-cycle fatigue (LCF) damage assessment for turbine blade–disk system. Based on the established probabilistic strain–life models and fatigue reliability model for tandem system, the LCF damage principle of turbine blade–disk system is proposed and integrated with SDCPAM. Following that, the LCF life prediction of the turbine blade–disk is completed, and probabilistic sensitivity analyses of blade and disk to the LCF life of the turbine blade–disk system are achieved. According to the above efforts, the feasibility and effectiveness of SDCPAM is verified. Finally, the LCF damage assessment for the blade–disk system is accomplished, and the influences of applied cycle and reliability level on the LCF damage are investigated. Through the comparisons of the proposed with traditional fatigue reliability model, it is illustrated that the proposed fatigue reliability model is reasonable. The results show that blade and disk almost have the same great influence on the blade–disk LCF life. In addition, applied cycles under normal and lognormal distributions produce the same LCF damage reliability that decreases with increasing applied cycle and reliability level. The efforts of this study indicate the reasonability of the proposed method and models in describing the LCF damage reliability of the blade–disk system, and enrich the reliability theory and method for the complex structure with multi-component and multi-failure mode.

Strain energy gradient-based LCF life prediction of turbine discs using critical distance concept

For aeroengine components with geometrical discontinuities like notches, low cycle fatigue (LCF) life prediction is critical for ensuring the engine structural integrity. In this regard, a concept of strain energy gradient is elaborated and a general procedure combining this concept with theory of critical distance is established for LCF life prediction of notch components, which relates its life with the strain energy distributed within the effective damage zone. For GH4169 and TC4 notched specimens as well as a case study of a high pressure turbine disc, the proposed procedure provides better life correlations than models of Fatemi-Socie and Smith-Watson-Topper.

Life prediction for high temperature low cycle fatigue of two kinds of titanium alloys based on exponential function

The high temperature low cycle fatigue tests of TC4 titanium alloy and TC11 titanium alloy are carried out under strain controlled. The relationships between cyclic stress-life and strain-life are analyzed. The high temperature low cycle fatigue life prediction model of two kinds of titanium alloys is established by using Manson-Coffin method. The relationship between failure inverse number and plastic strain range presents nonlinear in the double logarithmic coordinates. Manson-Coffin method assumes that they have linear relation. Therefore, there is bound to be a certain prediction error by using the Manson-Coffin method. In order to solve this problem, a new method based on exponential function is proposed. The results show that the fatigue life of the two kinds of titanium alloys can be predicted accurately and effectively by using these two methods. Prediction accuracy is within ±1.83 times scatter zone. The life prediction capability of new methods based on exponential function proves more effective and accurate than Manson-Coffin method for two kinds of titanium alloys. The new method based on exponential function can give better fatigue life prediction results with the smaller standard deviation and scatter zone than Manson-Coffin method. The life prediction results of two methods for TC4 titanium alloy prove better than TC11 titanium alloy.

Thermo-mechanical fatigue behavior and life prediction of the Al-Si piston alloy

The thermo-mechanical fatigue (TMF) behaviors and corresponding damage mechanisms of Al-Si piston alloy were investigated in the temperature ranges of 120–350°C and 120–425°C in this study. For TMF cyclic stress response behavior, the rapid cyclic softening occurs in the initial stage and then the cyclic stress maintains stable at lower strain amplitudes; but the cyclic stress displays gradual decrease up to the final failure at higher strain amplitudes. For TMF damage behavior, the cracks mainly initiate from the broken primary silicon in the temperature of 120–350°C range, and commonly nucleate from the boundary between primary Si and matrix in the temperature of 120–425°C range. For both cases, creep may have obvious influence and result in the formation of many micro-voids, but the oxidation may only have a little effect. A new energy-based model for low-cycle fatigue (LCF) and TMF life prediction was proposed based on the hysteresis energy with strain rate modification, considering both fatigue and creep damages. The predicted results agree well with the experimental ones for the Al-Si piston alloy.

Zone-based reliability analysis on fatigue life of GH720Li turbine disk concerning uncertainty quantification

Probabilistic-based design for turbine disk can quantify risk and thus improve the reliability of the component. This paper presents a zone-based reliability framework in combination with uncertainty quantification for low cycle fatigue (LCF) life prediction of a turbine disk. In this investigation, a zone-based modeling methodology is proposed instead of the traditional hot spot method, which accounts for all of the potential regions influencing the turbine disk's failure probability. The LCF life reliability of the whole turbine disk is achieved by jointing failure probability at each zone. The reliability framework developed involves the uncertainties from physical inputs and model parameters, in which geometry and model parameters are quantified using statistical method and Bayesian inference respectively, while the discretization error induced by finite element analysis is corrected using Richardson extrapolation method. Finally, the reliability analysis on the LCF life of a GH720Li turbine disk is performed considering the uncertainty quantification. It is demonstrated that the proposed zone-based reliability method in this paper is more accurate compared with the traditional hot spot method.

Low-cycle fatigue properties and life prediction of Al-Si piston alloy at elevated temperature

The influences of temperature on the microstructure evolution, tensile properties, especially low-cycle fatigue (LCF) behaviors and damage mechanisms of Al-Si piston alloy have been investigated in this paper. The results show that the alloy exhibits cyclic softening at high-temperature. Fatigue cracks usually initiate from primary silicon phase and preferentially grow along particles in a slightly zigzag path at relatively low temperature. With temperature increasing, however, the ductile tearing fracture through micro-cracks can be found. In order to evaluate the fatigue life, considering the temperature and loading conditions, a comprehensive 3-parameter model based on hysteresis energy has been proposed; at a constant temperature the fatigue life can be controlled by two parameters, i.e., the intrinsic fatigue toughness W0 (the resistance to crack propagation) and the fatigue cracking exponent β (the resistance to fatigue cracking), which dominate the LCF damage mechanisms (from fatigue-induced particle cracking to rapid fatigue crack growth). For the current Al-Si alloy, the combined effect of W0, β and temperature T can lead to an optimal fatigue life at a critical temperature. This model provides a new clue for optimizing and designing the high-temperature materials.

Predicting Low Cycle Fatigue Life through Simulation of Crack in Cover Plate Welded Beam to Column Connections

This paper presents a low cycle fatigue life curve by simulating a crack in a cover plate welded moment connection. Initiation of ductile fracture in steel is controlled by growth and coalescence of micro-voids. This research used a numerical method using finite element modeling and simulation of ductile crack initiation by a micromechanical model. Therefore, a finite element model of a cover plate welded moment connection was developed in ABAQUS software, and a FORTRAN subroutine was used in order to simulate cracking in the connection model. Thus, each crack location and the number of cycles to initiate the crack were detected. Utilizing cyclic void micromechanical model of growth analysis, which is a technique to predict fracture in a ductile material, six different cover plate connections (divided in three categories) were modeled in the steel moment frame, and then their critical points to trigger the crack were identified. Finally, for the cover plate moment connection, considering the constant amplitude of loading curves data and in order to present the low cycle fatigue life prediction, displacement versus the number of half cycles diagram is produced.

Low-cycle multiaxial fatigue behaviour and fatigue life prediction for CuZn37 brass using the stress-strain models

The low-cycle, strain-controlled fatigue tests have been conducted on CuZn37 brass (ASTM: C27200, EN: CW508L). Machined specimens have been subjected to uniaxial, proportional and non-proportional axial-torsional loadings. The monotonic and cyclic material properties have been determined for the loadings used. The obtained fatigue lives have been analysed in reference to the total Huber-von Mises equivalent strain ranges, the hysteresis loops energy in the loading cycle and the total plastic strain energy in the fatigue test. The relations between the plastic strain energy and the total as well as the plastic strain have been described. On the basis of the obtained experimental results, the selected stress-strain multiaxial fatigue criteria have been evaluated. In the conclusion it was found that fatigue lives predicted using the Fatemi-Socie and Garud criteria are closest to the experimental ones.

Proposal of a stress-based isothermal LCF life model for Aluminium alloy cylinder heads

The paper presents and discusses the calibration procedure and the results related to a new easy-to-use low cycle fatigue (LCF) life prediction empirical stress-based model. The model was applied to a commercial Aluminium alloy diesel engine cylinder head. The material characterization was carried out on commercial cylinder heads made by primary AlSi9Cu1, investigating the mechanical properties on sets of specimens obtained from layers positioned at different distances from the gas face of the cylinder heads. The results of mechanical characterization and LCF model calibration parameters are presented for each layer. The material characterization was carried out at room temperature to assess the procedure and validate the model.The life assessment performance of the model was compared with the corresponding Basquin-Manson-Coffin model. The model prediction fitted the experimental data trend with a determination coefficient ranging from 0.75 to 0.98, which is globally higher with respect to the parameter fitting obtained with the Basquin-Manson-Coffin calibration. Furthermore, all life forecasts are close to the experimental results with a variance lower than 55%. A future development of the research work with further material characterization at different temperature will allow to validate and discuss the temperature dependence of the model parameters and to investigate its thermo-Mechanical Fatigue (TMF) life assessment performance.

A new energy gradient-based model for LCF life prediction of turbine discs

With continuous raising of thrust-weight ratio, low cycle fatigue (LCF) at high temperature is one of main failure modes for engine hot section components. Accurate life prediction of turbine discs has been critical for ensuring the engine integrity. According to this, a new LCF model through combining the energy gradient concept with critical distance theory is proposed for fatigue life prediction of turbine discs. In this paper, assuming that the processes of crack initiation and propagation in a LCF regime can be described by the cumulative strain energy. A relationship between the total strain energy in the fatigue process zone and the LCF life is explored. In particular, the energy parameters are weighted based on the energy gradient in the fatigue process zone. Using experimental data of GH4169 alloy at 650°C, a good agreement was achieved between model predictions and experimental results.

Low-cycle fatigue performance of solid cylindrical steel components subjected to torsion at very large strains

In this research, low-cycle fatigue performance of cylindrical specimens produced from different ductile steel grades subjected to torsion is investigated. The primary motivation behind this research is the need for data on low-cycle fatigue in solid cylindrical specimens of S355 structural steel to use in a new steel Multi-Directional Torsional Hysteretic Damper (MTHD). This steel damper is intended to be used for seismic protection of buildings and bridges. In the newly developed steel damper the energy dissipaters are in the shape of a cylinder with enlarged ends and can undergo cyclic shear strain as large as 13%. Such large strains are not encountered under service conditions in structural or mechanical components but are common in earthquake engineering in those components which are designed to plasticize and dissipate energy, e.g., plastic hinge regions, and in the case of the specific application presented in this paper, in the energy dissipaters of the steel damper. A total of 61 solid cylindrical specimens made of S355J2+AR and C45E steel grades are tested using a special test setup capable of twisting the specimens up to ±50°. The steel specimens are tested under different levels of torsional shear strain ranging between 0.044 and 0.130 and the data is used to calibrate the Coffin-Manson model for low-cycle fatigue life prediction of specimens. Also, the impact of various other relevant factors on low-cycle fatigue behavior of specimens are looked into, including loading pattern and sequence, low temperature, surface finishing and aspect ratio (height/diameter).

Low cycle fatigue life prediction in shot‐peened components of different geometries—part I: residual stress relaxation

AbstractIn this study, the residual stress relaxation behaviour occurring during low‐cycle fatigue in shot‐peened specimens with either a flat or a notched geometry has been studied. A representative low‐pressure steam turbine material, FV448, was used. The residual stress and strain hardening profiles caused by shot peening were measured experimentally and were then incorporated into a finite element model. By allowing for both effects of shot peening, the residual stress relaxation behaviour was successfully simulated using this model and correlated well with the experimental data. Although more modelling work may be required to simulate the interaction between shot peening effects and external loads in a range of notched geometries, the model predictions are consistent with the specimens tested in the current study. The novelty of this study lies in the development of such a modelling approach which can be used to effectively simulate the complex interaction between shot peening effects and external loads in notched regions. Compared with the un‐notched geometry, the notched geometry was found to be more effective in retaining the improvement in fatigue life resulting from shot peening, by restricting the compressive residual stress relaxation during fatigue loading.

Isothermal low-cycle fatigue and fatigue-creep of Haynes 230

• A broad set of isothermal experimental responses of Haynes 230 is presented. • Loading included temperatures 24–981°C and strain rates 2.6×10 −5 -1.1×10 −2 /s. • Effect of temperature and strain rate on fatigue response and life is demonstrated. • Effect of dynamic strain aging on cyclic hardening-softening is demonstrated. • Challenges in constitutive model development are discussed. Service temperature of airplane gas turbine engine combustors fluctuates between ambient to as high as 982 °C, during which structural constraints induce cyclic stresses and strains resulting in thermo-mechanical fatigue damage accumulation in the combustor liner. In order to substantially improve the current design methodologies or low-cycle fatigue (LCF) life predictions of such high-temperature components, it is essential to develop an experimentally validated advanced constitutive model. This requires a broad set of fatigue data of the combustor liner material, Haynes 230 (HA 230) – a nickel-based superalloy, to characterize its fatigue failure responses. Hence, a systematic set of isothermal experiments are conducted prescribing uniaxial strain-controlled loading cycles, with and without a compression peak strain-dwell, with and without a mean strain, at seven different temperatures in the range of 24–982 °C and at three strain rates. The experimental responses are critically examined to explore various fatigue failure responses of HA230, which is a complex material showing unique fatigue-creep, strain rate sensitivity, strain range dependence, temperature dependence and dynamic strain aging (DSA) properties. DSA is found to occur in the temperature domain 427–760 °C. Isothermal experimental responses at different strain rates show that HA 230 can be considered rate-independent at and below 760 °C. However, stress relaxation is observed at lower temperatures up to 649 °C during the peak strain-dwell period. Finally, fatigue lives of HA 230 from the isothermal experiments are found to decrease with increase in temperature. These experimental responses are presented and challenges in constitutive model development are discussed.

On the prediction of low-cycle fatigue in steel welded beam-to-column joints

This paper presents a phenomenological criterion for crack initiation based failure prediction of steel structural components exposed to low-cycle fatigue loading. The criterion was established on the basis of available effective damage concept and associated two-parameter criterion. First, experimental cyclic test results on rib stiffened and cover plate stiffened beam-to-column joints are presented, which were used subsequently to verify the numerical model employed for the development of the proposed crack initiation criterion. The two-parameter criterion in which total accumulated plastic strain and stress triaxiality h were adopted as mechanical parameters that control the LCF cracking was applied to define a new damage curve. Several validation examples are presented to demonstrate the capability and accuracy of the proposed methodology for low-cycle fatigue life prediction. The applicability aspect of the proposed cracking criterion is further presented in terms of systematically defined complementary numerical analysis on welded stiffened beam-to-column joints focused on exploring any potential adverse beam member type and size effects on the cyclic response of the full-strength joint configurations. To this aim a set of eight practically applicable I and H European beam profiles was considered for the joints. All the analysed stiffened joints subjected to cyclic loading simulations possessed sufficient degree of overstrength to allow for the development of the full beam plastic rotation capacity. However, from the subsequent analysis important difference in fatigue behaviour between RS and CP joints was found.

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.