Low Cycle Fatigue Analysis Research Articles

Effort of damage parameter in assessment of low cycle fatigue

A low-cycle fatigue (LCF) analysis is one of the main design stages for highly loaded structural elements used in various applications. For this analysis, it is necessary to determine the values of local stresses and deformations, taking into account both elastic and plastic regions in the zones of stress concentration. This study presents and assesses the engineering methods used for prediction of low-cycle fatigue in structural elements. For zones of stress (strain) concentration, the Neuber-Makhutov method for LCF, taking into account the type of material stress-strain diagrams, is employed. The concept of distributed damage, based on the main ideas of the continuum damage mechanics of Kachanov-Rabotnov, was used. An approach employing the damage parameter for assessment of damage accumulation in LCF in highly loaded areas of structural elements is presented.

Low cycle fatigue analysis of welded joints considering material cycling behavior

In order to describe the local cyclic behavior of welds during low cycle fatigue of welded structures, so as to effectively assess the low cycle fatigue of structures, a structural strain numerical method is proposed. Based on the plate and shell theory and elastic-plastic theory, the numerical method of equivalent structural strain parameter is derived, and the UMAT subroutine of ABAQUS software is used to solve the structural strain parameters directly. The fatigue life of the welded specimens of different materials was evaluated, and the predicted life was basically the same as the test life, among which the maximum error of the predicted life of structural steel was 12%, the maximum error of the predicted life of aluminum alloy was 15.7%, and the maximum error of the predicted life of titanium alloy was 7.3%. The fatigue data of different materials were analyzed by using the master E-N curve, and the results showed that all the evaluated fatigue data were distributed in the same narrow band of the master E-N curve, which proved the effectiveness of the equivalent structural strain range for the evaluation of low cycle fatigue of welded specimens.

Combined high and low cycle fatigue analysis of FGH96 alloy under high temperature conditions

The FGH96 alloy as a common material for preparing turbine discs, can withstand high- and low-cycle combined loading during service, which significantly affects its fatigue performance. In this work, the combined high and low cycle fatigue (CCF) behavior of the FGH96 alloy was explored at various low cycle stress amplitudes (600, 630, 650, 680, and 700 MPa) using a self-improved experimental platform at a high temperature of 650 °C. The results indicate that increasing the amplitude of the low-cycle stress decreases the service endurance of the FGH96 alloy at CCF condition. Under the same low cycle stress amplitude, its CCF life of alloy is lower than the pure low cycle fatigue life. A fatigue service life prediction model, grounded in the theory of crystal plasticity, has been developed to assess the combined fatigue service life across different low-cycle stress amplitudes. A comparison between the predicted and actual lifespan showed that the predicted lifetime fell within a range three times the scattering band.

A Combined Numerical–Analytical Study for Notched Fatigue Crack Initiation Assessment in TRIP Steel: A Local Strain and a Fracture Mechanics Approach

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies.

Numerical Analysis of Low-Cycle Fatigue Using the Direct Cyclic Method Considering Laser Welding Residual Stress

The sequential-coupled thermo-mechanical model and direct cyclic technique are employed to investigate laser welding and low-cycle fatigue residual stress, respectively. The effects of residual stress on fatigue properties and the relaxation behaviour are analyzed. The simulation results highlight the strong dependence of laser welding residual stress on constitutive models and how low-cycle fatigue residual stress is influenced by the initial laser welding residual stress. Furthermore, the simulation indicates that residual stress redistributes and relaxes in the weld and heat-affected zone after low-cycle fatigue. To validate the accuracy of the simulation, the neutron diffraction experiment is carried out, and the experimental data are consistent with the simulation results.

HCF and LCF Analysis of a Generic Full Admission Turbine Blade

A numerical turbine-blade fatigue-life analysis method is suggested. This method comprises a stationary thermal 3D finite element (FE) analysis of the hot run for the combined high-cycle fatigue (HCF) and creep analysis, and a follow-on (one-way coupled) quasi-stationary structural 3D FE analysis (including six load steps) of a single and two half turbine blades and the related disk and rotor section and a (modified Goodman equation based) post-processing fatigue life analysis for the highest HCF-loaded point of the turbine blade. For the low-cycle fatigue (LCF) analysis, this includes a transient thermal 3D FE analysis of two full loading cycles, a follow-on (one-way coupled) quasi-stationary structural 3D finite element analysis of a single and two half turbine blades and the related disk and rotor section and a (modified-Langer-equation-based) post-processing fatigue life analysis approach for the highest LCF-loaded point of the turbine blade. Finally, this approach is demonstrated by the numerical HCF, LCF and creep analysis of a generic turbine blade of the first rotor row of a full admission hydrogen turbo pump of a 1 MN thrust class gas generator LOX-LH2 liquid rocket engine (LRE). For this numerical example, the LCF loading turned out to be dominant. Creep turned out to be negligible.

Bayesian inference-based prognosis of fatigue damage for MPPO polymer using Zhurkov fatigue life model

In this study, the fatigue damage prognosis of a modified polyphenylene oxide (MPPO) polymer is performed using a Bayesian framework, and a Zhurkov model-based dynamic fatigue life model is employed to obtain the probabilistic stress–cycle (P-S-N) curve. Activation energy and tensile tests are performed to determine the aleatory uncertainty of the lethargy coefficient of the Zhurkov fatigue life model. This uncertainty is quantified by performing sequential statistical modeling using experimental data with embedded scattering. The P-S-N curve is estimated using these data, and the Zhurkov fatigue life model is validated via the fatigue test. Furthermore, damage data are obtained via a low-cycle fatigue analysis in conditions identical to those of the fatigue test conducted on the specimen scale. Based on computational damage data, the initial model parameters of the fatigue damage model are obtained using the least-squares method. These model parameters are estimated while considering scattering by applying the Markov Chain Monte Carlo and particle filter. Therefore, the remaining useful life (RUL) of the MPPO, which depends on the amplitude stress, is predicted under the tension–tension fatigue loading ( R = 0), and the prediction accuracy of the RUL is evaluated using prognostics metrics.

Experimental and computational investigation of fatigue crack propagation using the inelastic energy approach

This study presents the experimental and computational investigation of a damage evaluation of the ductile aluminium alloy AA 5083-H111. The Compact Tensile (CT) specimen was selected for experimental testing and computational analysis. The proposed computational model is based on the inelastic energy approach, including the damage initiation and damage evolution criteria. In the computational model, the direct cyclic algorithm in the framework of Abaqus software was used to obtain the stabilised cyclic response of the analysed specimen subjected to the cyclic loading. Furthermore, the research presented in this study was focused mainly on the damage evolution period and subsequent determination of the appropriate maximum damage variable Dmax, which defines the stage of the stiffness degradation in the analysed structure, and, therefore, influence significantly on the damage evolution process.The comparison between the experimentally and computationally obtained damage evolution period and also the damage path in the analysed CT specimen showed a reasonable agreement. Therefore, it can be concluded that the proposed computational model was validated, and could be used for the further low cycle fatigue analyses of structural components made of the aluminium alloy AA 5083-H111.

Sensitivity of Solid Element Spot Weld Modeling Methodology to Nugget Diameter

<div class="section abstract"><div class="htmlview paragraph">A vehicle’s structure has more than 4000 spot welds, which can be considered as one of the main design criteria that affects the general stiffness as well as the durability of an automobile. Considering the essential role of finite element-based simulations of the spot-welded structures on the design phases of vehicle development, accuracy of analyses can be considered as one of the main concerns to meet the design objectives. This study examines the sensitivity of solid element spot weld modeling technique to nugget diameter. Comparative stress and multi-axial low cycle fatigue analyses are conducted for the inspection of the effect of weld diameter on solid element spot weld simulations. Displacement contours as well as local stress profiles at the vicinity of the spot welds are examined to evaluate the effect of nugget diameter on general stiffness and safety. Multi-axial low cycle fatigue analyses are conducted for the specimens composed of U-profiles, and different thickness levels are simulated for a set of nugget diameters. Simulation results are compared with the low cycle fatigue tests’ life cycles to determine the accuracy of the solid element spot weld modeling methodology and its sensitivity to nugget diameter.</div></div>

Numerical study on thermal shock of central measuring shroud with fluid–solid thermal coupling in the “Monju” reactor

Thermal shock threatens the safe operation of the fast reactor and damages the structural integrity of the fast reactor. The stress of central measuring shroud under thermal shock accident in the “Monju” reactor is studied in this paper. The method of fluid–solid thermal coupling is adopted to solve the temperature field by fully coupling, while the stress field by one-way coupling. By comparing the temperature field with the experimental data, it is found that the temperature field is in good agreement with the error range within 3.44%. Under thermal shock condition, the core outlet temperature decreases with time, and the stress field shows a law consistent with the temperature field. The maximum stress position is located at the junction of the lower head and the sleeve. Based on the 1/3 model, thermal shock can be divided into three stages. At the initial stage of thermal shock, in the first 5 s after the thermal shock begins, the temperature gradient of the inner and outer walls increases with time, and the stress increases with time in the first 7 s after the thermal shock begins, reaching a maximum value of 345.67 MPa in 7 s. In the middle stage of thermal shock, between 5 and 15 s, the temperature gradient of the inner and outer walls decreases with time. Between 15 and 23 s, the temperature gradient of the inner and outer walls is small. The stress decreases with time between 7 and 24 s, reaching a minimum of 144.15 MPa in 24 s. At the end of thermal shock, after 23 s, the temperature gradient of the inner and outer walls increases with time. The stress increased with time after 24 s and reached a maximum of 339.62 MPa at 60 s. The maximum deformation is 23.38 mm in 8 s. Considering that the stress value is significant, further low-cycle fatigue analysis is recommended throughout the life cycle, which is very important.

Combined notch and size effect modeling of metallic materials for LCF using plasticity reformulated critical distance theory

Fatigue assessment of notched parts plays imperative performance in structural integrity and reliability of major equipment. The theory of critical distance (TCD) involves determining the fatigue strength of notched parts from the linear elastic stress field surrounding the notch apex. In addition, the fatigue strength of engineering part is also size dependent. Accordingly, by coupling a plasticity reformulated TCD with the weakest link theory, this study elaborates a novel scheme for probabilistic low cycle fatigue (LCF) analysis under notch and size effects. Results of fatigue tests and complementary FE analysis with the proposed model is utilized to compute highly stressed volume (HSV). Furthermore, test data of titanium alloys TC4 (Ti-6Al-4V) and TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) as well as aluminum alloy 7075-T6 are utilized for model verification. Results specify that the model predicted P–S–N curves are in good agreement with experimental data.

Constitutive and damage modelling of selective laser melted Ti-6Al-4V lattice structure subjected to low cycle fatigue

Based on low cycle fatigue (LCF) experiment data of SLM Ti-6Al-4V, a comprehensive finite element model of the combined isotropic and kinematic hardening and continuous damage for SLM Ti-6Al-4V is established. The simulation results of the stress-strain hysteresis loop and the LCF life are in good agreement with the experiment results under different strain amplitude levels. Based on the reliable achieved model of SLM Ti-6Al-4 V, the finite element model of the Gyroid and Diamond lattice structure is established for LCF analysis. It is found that the Diamond lattice structure has higher LCF resistance.

Analysis of biaxial proportional low-cycle fatigue and biaxial accumulative plasticity of hull inclined-crack plate

Fracture induced failure of plates in hull structures subjected to in-plane biaxial low-cycle fatigue loading are generally a combination of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of a hull structure that accounts for the accumulative plasticity effect can be suitable for the actual evaluation of the overall fracture induced failure of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for the J integral was presented for hull inclined-crack plate. The test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behaviour and the effects of the biaxiality and stress ratios were investigated by numerical analysis. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.

Computational model for low cycle fatigue analysis of lattice materials: Incorporating theory of critical distance with elastoplastic homogenization

A novel numerical framework for low cycle fatigue analysis of lattice materials is presented. The framework is based on computational elastoplastic homogenization equipped with the theory of critical distance to address the fatigue phenomenon. Explicit description of representative volume element and periodic boundary conditions are combined for computational efficiency and elimination of the boundary effects. The proposed method is generic and applicable to periodic micro-architectured materials. The method has been applied to 2-D auxetic and 3-D kelvin lattices. The classical Coffin-Manson and Morrow models are used to provide fatigue life predictions (strain-life curves). Predicted fatigue lives for the auxetic lattice are shown to provide good correspondence to experimentally found fatigue lives from the literature.

Finite element based Multi-Axial low cycle fatigue analyses of Spot-Welded components and correlation with tests

Spot welding is the most popular assembly method used in the automotive industry due to its simplicity, effectiveness, and low cost. Simulation techniques for spot welds play an essential role in today's aggressive vehicle development schedules, and the accuracy of the analysis impacts the quality of the design in terms of noise-vibration-harshness and durability. Using static and multiaxial low cycle fatigue analysis, this study focuses on three different spot weld modeling methods: Rigid spot weld, solid element spot weld, and umbrella spot weld. Spot weld modeling techniques were compared for specimens with different thickness levels and weld diameters, and under different loading conditions. In addition, the accuracy levels of spot weld modeling methods are presented by comparing the life found by multi-axial low cycle simulations with the fatigue tests.

Analysis of leaking ratio of sealing device for the intake port of EMC closed integral structure turbine disk through soft computing

The rotor disk produces an azimuthal velocity component in the space between the rotor and stator of a turbine disk, also known as the rim seal cavity. As part of an empirically study, the rim seal cavity is subjected to a test aimed to count the number of unstable structures and evaluate their rotational speed. An Electronic Control Unit (ECU) has a leak test aperture that is sealed by a sealing device that is selectively disposed in the leak test aperture. Designing pump and compressor machines and units requires the development of dependable seal assemblies that maintain tightness over a long period of time and in a wide variety of pressures and temperature. In the field of electrochemical machining (ECM), heat-resistant and high-strength materials may be machined into complicated forms using this well-known technology. ECM presents some issues as the electrochemical copying of grooves, insulating groove features, slots and mini-holes can cause water leakage due to the poor sealing device of the closed integral structure turbine disk. Sealing devices of rotor turbine disks are heavy components with low-cycle fatigue analysis to their life curves. However, there is rare analysis to detect their defects in various rotor regions (temperature, considering stress, mission profile). This study by use of hydro-thermal loading has attempted to focus on the mechanical seals rings and basic productive and operating requirements. Taking the damage and wear, the clearance has been altered that cause a raising in leakage. Generally, the leakage grows more rapidly than linearly with the after-damage clearance. Also, damage and wear were related to the labyrinth seal itself, resulting that the bending curvature and the percentage of bent tooth length were also relevant in defining the leakage in the case of bending damage.

Fatigue Life and Stress Analysis of the Crankshaft of a Single Cylinder Diesel Engine under Variable Forces and Speeds

Abstract In this paper, a single cylinder crankshaft manufactured with C45 steel was used to investigate variation in stress, deformation, and fatigue life and safety factor at critical locations of the crankshaft. For this purpose, numerical analyses using ANSYS/Workbench software were performed under different operating conditions. Additionally, low cycle fatigue analyses were conducted experimentally for the validation of the numerical results in terms of the failure characteristics of the crankshaft. It was concluded that chamfers at sharp corners of the crankshaft on the flywheel side showed critical regions, indicating that the experimental and numerical results were consistent. These results suggested that critical regions of the crankshaft could be optimized for the improvement of sustainability in long life service.

An Artificial-Intelligence and Machine-Learning-Based Methodology to Conduct Seemingly Strain-Controlled Fatigue Test in a Pressurized-Water-Reactor-Test-Loop-Autoclave, While Not Controlling the Strain

Abstract In general, low cycle fatigue analysis of pressurized water reactor (PWR) components, requires strain-controlled fatigue test data such as using strain versus life (ε–N) curves. Conducting strain-controlled fatigue tests under in-air conditions is not an issue. However, controlling strain in a PWR-test-loop-autoclave is a challenge, since an extensometer cannot be placed in a narrow autoclave (typically used in a high-temperature-pressure PWR-test-loop). This is due to lack of space inside an autoclave that houses the test specimen. In addition, installing a contact-type extensometer in the path of a high-pressure flow can be a challenge. These difficulties of using an extensometer inside an autoclave led us to use an outside-autoclave displacement sensor which measures the displacement of pull-rod-specimen assembly. However, in our study (based on in-air fatigue test data), we found that a pull-rod-controlled based fatigue test can lead to substantial cyclic hardening/softening resulting in substantially different cyclic strain amplitudes and their rates compared to the desired cyclic strain amplitudes and its rates. In this paper, we propose an Artificial-Intelligence and Machine-Learning based technique such as using k-means clustering technique to improve the pull-rod-control based fatigue test method, such that the gage-area strain amplitude and rates can reasonably be achieved. In support of this, we present the fatigue test results for both 316 SS base and 81/182 dissimilar-metal-weld specimens.

A computational framework for low-cycle fatigue in polycrystalline materials

A three-dimensional framework for low-cycle fatigue analysis of polycrystalline aggregates is proposed in this work. First, a cohesive law coupling plasticity and damage is developed for modelling cycle-by-cycle degradation of material interfaces up to complete de-cohesion and failure. The law may model both quasi-static degradation under increasing monotonic load and degradation under cyclic loading, through a coupled plasticity-damage model whose activation and flow rules are formulated in a thermodynamically consistent framework. The proposed interface laws have been then implemented and coupled with a multi-region boundary element formulation, with the aim of analysing low-cycle intergranular fatigue in polycrystalline aggregates. The boundary element formulation allows expressing the micro-mechanical problem in terms of grain-boundary displacements and tractions only, which are the quantities directly entering the cohesive laws, thus simplifying the coupling of the two tools. After assessing the response of an individual interface, to both quasi-static and cyclic loads, the coupled framework has been employed for the computational investigation of low-cycle degradation in fully-3D and pseudo-3D, or 2D columnar, polycrystalline aggregates, assuming that the degradation process remains confined in the intergranular regions. The discussed results show the potential of the developed formulation for multiscale materials modelling, which may find future application in the multiscale design of engineering structures subjected to complex loads and degradation processes, and for computational micromechanics, which may find direct application in the design and analysis of micro-electromechanical systems (MEMS).

Low cycle fatigue analysis of heat treated duplex stainless steel

ABSTRACT Duplex stainless steels (DSS) have become more attractive than single-phase austenitic and ferrite grades in industrial application due to their high strength and corrosion resistance in chloride-containing media. Due to low ductility region under hot working conditions these materials require special attention during processing. As heat treatments influence low cycle fatigue characterisation. The characteristics and properties of the industrially employed duplex grades has been discussed, moreover the presence and precipitation of sigma phases has been highlighted through ageing treatments. Based on up to date achieved outcomes, it appears that a quite homogeneous and good mechanical properties can be obtained by controlled composition and heat treatments .and it also enhances the wear properties. The most commonly formed and observed pernicious phase in DSS is sigma phase particles precipitation in a short period of time over a wide range of temperature. Hence an attempt has been made in our investigation to evaluate the low cycle fatigue changes when they are subjected to temperature 1100°C for a shorter duration for about 60 minutes. The low-cycle fatigue behaviour of a DSS was comparatively studied in standard heat-treated and aged at 800°C for 4 h.