Fatigue Life Of Components Research Articles

Prediction of residual stress fields after shot-peening of TRIP780 steel with second-order and artificial neural network models based on multi-impact finite element simulations

Shot-peening is a mechanical surface treatment widely employed to enhance the fatigue life of metallic components by generating compressive residual stress fields below the surface. These fields are mainly impacted by the selection of the process parameters. The aim of this work is to propose a hybrid approach to conduct two predictive models: second-order model and feed-forward artificial neural network model. For this purpose, a 3D multiple-impact finite element model coupled to a central composite design of experiments was employed. A parametric analysis was also conducted to investigate the effect of the shot diameter, the shot velocity, the coverage, and the impact angle on the induced residual stress profile within a TRIP780 steel. It was found that both models predict with good agreement, the residual stress profile as a function of the process parameters and can be used in shot-peening optimization due to their responsiveness.

Evaluation method of fatigue life for weld joints with defects in the horizontal restraint components of main steam pipelines

Horizontal restraint components are mainly used in nuclear power plants to limit the horizontal displacement of the “super pipelines”, such as the main steam pipelines and main water supply pipelines. Horizontal restraint components are very important for ensuring the safety of the second loop of nuclear power plants under unexpected conditions. However, because the size of the horizontal restraint components are quite large, and the pipelines are complicated, defects or cracks easily occur in the weld joints connected with the components and pipelines. The fatigue crack growth in the weld joints is one of the important reasons of the fatigue failure for the horizontal restraint components. Therefore, investigating the fatigue crack growth of the weld joints is significant to accurately predict the service life of the horizontal restraint components. In this paper, multiple cracks were found in the weld joints of horizontal restraint components in a nuclear power plant. In order to evaluate the fatigue life of the weld joints, fatigue crack growth tests were carried out under different temperature. The material parameters, C and n, of the Paris Equation for the weld joints were obtained and compared with those for the base metal (ferrite steel) in the ASME code. Results showed that the results show that the fatigue crack growth rate of the weld joints in the room temperature is lower first and then higher than that of the material in the high temperature weld joints and the base metal at room temperature, but the fatigue crack growth rate of the weld joints in the high temperature is always higher than that of the base metal at room temperature. The fatigue life of the horizontal restraint components calculated by the Paris Equation from the experiments is quite different compared with the fatigue life calculated from the base metal (ferrite steel) in the ASME code at room temperature.

3D numerical modelling of turning-induced residual stresses – A two-scale approach based on equivalent thermo-mechanical loadings

Nowadays, fatigue life of engineering components is a major topic. In the literature, extensive works have been reported in order to assess the machining-induced surface integrity which is known to be a first-order parameter for the fatigue resistance. This paper lies in the framework of turning-induced residual stress prediction by presenting an up-dated method based on a two-scale approach. Previous works aimed at building a 3D hybrid method consist of modelling the turning consequences through equivalent thermo-mechanical loadings combining a finite element model with experimental tests. The new proposed global hybrid strategy keeps the same concept but has the advantage of defining the thermo-mechanical shapes more accurately thanks to local 2D orthogonal cutting models. Moreover, the calibration of the equivalent thermo-mechanical loadings is easier as only turning forces have to be measured, which makes it more accessible to end-users. Finally, the model provides results considering the effective machined surface topography instead of considering a basic flat surface. After carefully detailing each step of the numerical method, a validation case study concerning a longitudinal turning operation on a 15-5PH martensitic stainless steel is proposed. By comparing numerical results with experimental residual stresses coming from X-Ray diffraction, the new numerical method shows its superiority to predict, within few hours, much more accuratly the residual stress state induced by a real longitudinal turning operation.

A novel optimised nacelle to alleviate wind turbine unsteady loads

A comparative study of the effectiveness of an innovative optimised nacelle on fatigue loads alleviation and performance improvement in both upwind and downwind configurations has been conducted. The idea is to optimise wind turbine nacelles using flaps to push the flow towards outer spans, which leads to improved turbine performance, as well as mitigates unsteady loads, therefore, extending the fatigue life of wind turbine's components. Theoretical explanations indicate that in the case of upwind configuration, increasing downstream velocity leads to a reduction in turbine power. However, lower wind speeds seen by inner spans lead to mitigate thrust load generated by inner spans. In the case of downwind configuration, increasing upstream velocity leads to improve turbine power based on the turbine Euler equation, and pushing flow towards outer spans with higher lift/drag ratios leads to mitigate unsteady loads generated by inner spans. Experimental results show that using flaps to optimise the nacelle shape not only mitigates unsteady loads and therefore extends the fatigue lifetime of blades but also improves turbine power and yaw stability. Furthermore, using flaps in downwind configuration can compensate for the adverse effect of wind turbine yaw misalignment on the wind turbine performance and unsteady loads.

Concentrated Chloride Effects on Chromate Inhibition of Al Alloy-316 Stainless Steel Galvanic Couple-Induced Corrosion

Concentrated Chloride Effects on Chromate Inhibition of Al Alloy-316 Stainless Steel Galvanic Couple-Induced Corrosion

Surface integrity evolution of a Ni-based single crystal superalloy by laser shock peening

The single crystal superalloy SRR99 experimental samples were single point treated by laser shock peening (LSP) with laser pulse energy of 5–7 J. The surface morphology, micro-hardness, surface residual stress and surface microstructure of samples prior and after LSP were determined by corresponding characterization instrument. From the surface morphology, a circular pit was formed in surface, which due to the severe plastic deformation induced by LSP, and the diameter & maximum relative height of circular pit were increase with laser pulse energy. From the micro-hardness and residual stress test results of samples, these were increased after LSP treatment. In detail, the initial microhardness and residual stress were 398 HV and -498 MPa, with laser pulse energy of 5 J, the microhardness and residual stress were increased to 455 HV and -843 MPa, when the laser pulse energy was increased to 7 J, the microhardness and residual stress were increased to 512 HV and -942 MPa. In addition, the surface roughness of samples was increased too, which reflect the severe plastic deformation. By observing the microscopic morphology of the bottom of the pit, it is found that there are convex structure and crater shape defect structures on the bottom surface, and the formation of these structures may affect the surface roughness of the shock area. Although LSP caused certain degree of twist deformation of the shape of the γ' phase, the original strengthening method characteristics of the matrix phase and strengthening phase was not destroyed, which still maintained the creep strength and thermal fatigue properties of superalloy. The experiment show that LSP can cause significant work hardening effect on the surface of superalloy, and the generated high residual compressive stress can effectively delay the initiation and propagation of cracks. Therefore, LSP technology can effectively improve the fatigue life of single crystal blade components.

Assessing the magnetic flux leakage contraction parameters for the fatigue life prediction of SAE1045 steel specimens

The aim of this paper is to assess the stress concentration zone (SCZ) of a circular smooth specimen at an early stage. Following the detection of the stress concentration zone, the specimen residual life was calculated and compared to the experimental result. Uniaxial loading on the servo-hydraulic testing machine was used for the laboratory testing. The stress concentration tester was used to detect the SCZ on the specimen, based on spontaneous surface micro magnetic signals on the ferromagnetic surface. In addition, cyclic testing for fatigue characteristics was conducted at 70% and 75% of ultimate tensile stress (UTS). The results indicated that the estimated residual life for the 70% UTS was nearly identical to the experimental value, based on two magnetic parameters, i.e, Kmax and the contraction value. Meanwhile, the estimated residual life for the 75% UTS was equivalent to the experimental findings. The minimum and maximum error values for life estimation using both parameters were 0.47% and 24.35%, respectively, for the contraction value. The correlation value, R2, between the estimated life using MMM and experimental life was 0.95, showing good estimation. All the estimated life data using this method was distributed within the range of a 95% to 90% confidence interval. Hence, the proposed method can be used to predict the fatigue life of ferromagnetic components through magnetic flux leakage signals, especially in the stress concentration region.

A numerical and experimental investigation on the evolution of three-dimensional surface topography of 12Cr2Ni4A steel in shot peening

Shot peening is widely used as a surface strengthening technology as it can enhance fatigue life of mechanical components by introducing reasonably distributed compressive residual stress. However, improper selection of processing parameters would inevitably lead to deterioration of the surface quality, and negatively affect on the fatigue performance. In this paper, a random multiple-shot model was proposed to accurately calculate the evolution of 3D surface topography 12Cr2Ni4A steels during shot peening. The simulated results are in good agreement with the experimental results, with an error of less than 10%. It is found that surface roughness parameter Sa first increases significantly and then decreases slowly with the increase of peening time. Moreover, the surface height after shot peening obeys a normal distribution and the 3D surface roughness is positively correlated with the initial roughness.

Novel Fatigue-Damage Sensor for Prediction of Remaining Fatigue Lifetime of Mechanical Components and Structures

In this paper, a novel Internet of Things (IoT) fatigue damage sensor is presented to predict the fatigue damage accumulation level and residual fatigue life of critical mechanical and structural components. The intelligent radio-frequency identification (RFID)-integrated fatigue sensor system has special designed breakable mini beams with multiple parallel oriented geometry and symmetric U-type stress concentrated notches. The fatigue sensing beams with different stress and fatigue lifetime levels are designed to estimate the fatigue damage accumulation and remaining fatigue life of unidirectional and multidirectional structural or mechanical elements, including composite structures. The fatigue damage sensor attached on a real structure goes through the same fatigue life experience of critical structural elements or mechanical components from the beginning of service life or any point in the service life to the end of service life. A fatigue sensor with five parallel symmetric U notched mini beams with different fatigue lifetimes (10% N, 25% N, 50% N, 70% N, 85%–90% N) normalized to the total life (N) of a structure are considered to predict the remaining fatigue lifetime of a real structure using the cumulative damage Miner’s rule. The mini fatigue sensor beams attached to the surface of a structure are designed to fail, depending on their fatigue life, earlier than the total fatigue lifetime of the structure, like a fatigue fuse system. The fatigue damage sensing beams fail gradually while passing through the same fatigue cycles as a structural member. When the service lifetime of a real monitored structure is very close to the total fatigue life, the critical maximum lifetime (80%–90% N) notched mini sensor beam breaks and issues an alarm that the service life of the real structural part is approaching its fatigue strength limit and the end of its service life. The smart fatigue damage sensor is wireless-enabled, which makes it possible to check the health status of the sensor over a sensor network as often as one likes. Since the wireless distributed fatigue sensor network system monitors the fatigue health conditions of structures periodically or on demand, the data collected on the sensor network system can be used not only for condition-based fatigue life prediction but also for sensor-based predictive fatigue maintenance and the development of new fatigue design tools for fatigue-sensitive complex and large engineering structures or mechanical systems.

Determining the endurance limit of AISI 4340 steels in terms of different statistical approaches

In engineering applications, fatigue phenomenon is a key issue and needs to be analyzed in the beginning of design phase in case of any component exposed to alternating loading on operation otherwise catastrophic fatigue failure may cause. Component can be designed with safe-life, fail-safe, and damage tolerant approach based on whether redundant load path and damage sensitive. Before starting analyzing the structure, material allowable data needs to be presented in a reliable way to predict fatigue life of components. SN curves with presented confidence levels are the robust approach to make a prediction on safe life of a structure in terms of fatigue. In this point, there are so many approaches to determine fatigue limit of materials and issue shall be handled by statistical manner. In literature, different staircase and curve fitting methods were presented to estimate endurance limit of materials and some reliability manuscript published. In this paper, fatigue limit of AISI 4340 steel will be investigated through most convinced staircase and curve fitting approaches and their reliability will be queried.

Probabilistic fatigue modeling of notched components under size effect using modified energy field intensity approach

Fatigue assessment in the presence of intricate geometries is critical for ensuring structural integrity and reliability of engineering components. Based on the energy field intensity (EFI) approach, this study establishes a probabilistic framework to predict fatigue life of notch components considering size effect. Specially, a new concept of effective damage zone (EDZ) is introduced based on characteristic total strain energy density (TSED), and the relation between EDZ and fatigue life under size effect was explored and quantified. Through combining the Weibull distribution, an EFI-based probabilistic assessment is conducted for fatigue failure probability analysis of notch components. Experimental data of Ni-based superalloy GH4169 and Al 2024-T351 alloy with different scales were utilized for model validation and comparison. Results indicate that the proposed framework yields good consistency with experimental lives.

Management of Airframe In-Service Pitting Corrosion by Decoupling Fatigue and Environment

Corrosion-induced maintenance is a significant cost driver and availability degrader for aircraft structures. Although well-established analyses enable assessing the corrosion impact on structural integrity, this is not the case for fatigue nucleation and crack growth. This forces fleet managers to directly address detected corrosion to maintain flight safety. Corrosion damage occurs despite protection systems, which inevitably degrade. In particular, pitting corrosion is a common potential source of fatigue. Corrosion pits are discontinuities whose metrics can be used to predict the impact on the fatigue lives of structural components. However, a damage tolerance (DT) approach would be more useful and flexible. A potential hindrance to DT has been the assumption that corrosion-induced fatigue nucleation transitions to corrosion fatigue, about which little is known for service environments. Fortunately, several sources indicate that corrosion fatigue is rare for aircraft, and corrosion is largely confined to ground situations because aircraft generally fly at altitudes with low temperature and humidity Thus, it is reasonable to propose the decoupling of corrosion from the in-flight dynamic (fatigue) loading. This paper presents information to support this proposition, and provides an example of how a DT approach can allow deferring corrosion maintenance to a more opportune time.

Correlations between crack initiation and crack propagation lives of notched specimens under constant and variable amplitude loading

AbstractThis paper starts with an overview of the application of the three guidelines (GL) of the German Research Association of Mechanical Engineers (FKM). Each of these provides algorithms for calculating fatigue lives of components under constant or variable amplitude loading, however, with underlying different failure criteria, that is, technical crack initiation life (GL‐nonlinear), fatigue crack growth life (GL‐fracture mechanics), and total fracture life (GL‐linear). Since the GL‐linear only considers linear‐elastic material behavior in the fracture fatigue life calculation, the guideline is not capable for the LCF and is not able to take sequence effects into account. For a better approximation of fracture fatigue lives, the elastic–plastic material behavior has to be taken into account. This paper introduces the U‐Concept which has been evaluated from a large structural durability database from the literature with more than 1,500 experiments for constant and more than 700 experiments for variable amplitude loading. The U‐Concept is a small add‐on to the local strain approach (LSA) which is the backbone of the GL‐nonlinear. It enables (1) to directly calculate the fatigue life to total fracture based on elastic–plastic material behavior according to the LSA or (2) to estimate the remaining fatigue life from crack initiation to fracture without a crack growth simulation. Furthermore, the U‐Concept needs no experimental determined crack growth parameters, the only necessary material parameter is the ultimate tensile strength Rm. The U‐Concept is therefore easy applicable for industrial users and increases the accuracy of the approximated fatigue lives by considering the elastic–plastic material behavior.

Comparative study: Impact of Ball burnishing and shot peening Process on Aluminium 2024 alloy

Compressive residual stress is the major aspect in the extension of the fatigue life of aeroengine components. In this study, a modified burnishing surface treatment and conventional shot peening process was used was proposed to improve surface integrity characteristics such as surface finish, hardness, and stable, advantageous compressive residual stress in turned Cylindrical Aluminum 2024Specimen. In burnishing process, a rolling rigid spherical HSS ball is pressed across an Aluminum 2024Specimen under definite fluid pressure generated by the hydraulic unit and also shot peening was carried out at a shot velocity of 300 m/s. This research examined the effect of burnishing treatment and shot peening process on beneficial compressive residual stresses.

Fatigue life prediction of nuclear reactor main bolt based on temperature effect and size effect

The temperature effect and size effect on the fatigue life of nuclear reactor main bolt material were studied, and the high temperature fatigue limit of actual bolt thread was evaluated based on the temperature effect and exponential model of size effect. The results showed that the fatigue life of bolt material decreased with the increase of temperature, which exhibited obvious temperature effect. On the other hand, the size effect mainly depended on the loading stress levels because of different effects of stress level on fatigue crack initiation. When the stress level was higher than 340 MPa, fatigue crack initiation was less dependent on defects and the size effect was small, which the fatigue life of large size component could be approximately evaluated by smaller size specimen; by comparison, when the stress level was lower than 300 MPa, fatigue crack initiation was strongly dependent on defects and the size effect became obvious, and at that time the fatigue limit of different size specimen was consistent with exponential model. The fatigue limit of large size bolt under actual service temperature could be obtained by the product of size effect and temperature effect. By combining the theory of critical distance (TCD) with a series of small size fatigue test, the fatigue life of large size components could be efficiently evaluated under the premise of greatly reducing small size fatigue tests.

Semi-Empirical Prediction of Turned Surface Residual Stress for Inconel 718 Grounded in Experiments and Finite Element Simulations.

The surface residual stress after machining, especially for finishing, has a vital influence on the shape stability and fatigue life of components. The current study focuses on proposing an original empirical equation to predict turned surface residual stress for Inconel 718 material, taking tool parameters into consideration. The tool cutting-edge angle, rake angle, and inclination angle are introduced for the first time in the equation based on the Inconel 718 material turning experiments and finite element simulations. In this study, the reliability of simulation parameters’ setting is firstly calibrated by comparing the residual stresses and chips of the experiments and simulations. The changing trends of turned forces, temperatures of lathe tool nose, and surface residual stress with turning parameters are analyzed. Then, the predictive equation of surface residual stress is proposed considering relationships between the back-rake angle, the side-rake angle, and the tool cutting-edge angle, rake angle, and inclination angle. Moreover, the genetic algorithm optimizes the objective function to obtain the undetermined coefficients in the prediction equation. Finally, the predicted accuracy of the surface residual stress is proven by comparing the experimental, simulation, and prediction values. The results indicate that the predictive equation of surface residual stress has a good accuracy in predicting turned surface residual stress for Inconel 718 materials. The correlation coefficient, R, and absolute average error between the predicted and the simulated value are 0.9624 and 13.40%, respectively. In the range of cutting parameters studied and experimental errors, this study provides an accurate predictive equation of turned surface residual stress for Inconel 718 materials.

Estimation of elastic–plastic notch strains and stresses using artificial neural networks

AbstractThe prediction of fatigue lives of metallic components using the notch strain approach is based on local stresses and strains. These can be determined either by analytical approximation formulas such as Neuber's rule based on linear elastic stresses or by finite element analysis with elastic–plastic material behavior. The latter has the disadvantage of longer computation times.This paper discusses a draft for application of artificial neural networks as an alternative approach for estimating uniaxial and multiaxial proportional elastic–plastic stresses and strains in notch roots based on linear elastic stresses. These networks are trained on results from elastic–plastic finite element analyses on flat bars notched on both sides and circumferentially notched shafts to predict local stresses. The resulting accuracy is compared to the analytical approximation formulas.The research findings confirm that neural networks can estimate local stresses with similar scatter and better mean value compared to analytical formulas.

Multi-microgrid energy management through tertiary-level control: Structure and case study

The widespread emergence of renewable energy resources has made microgrids neighbors. These microgrids share their power, and multi-microgrids are formed, which is more stable and flexible. However, the main problem of these smart grids is their complex control. To this end, tertiary-level control can play a significant role. This paper sought to cite the existing control models and strategic plans for microgrid operation, review the literature and provide a comprehensive picture of microgrid and multi-microgrids energy management at the tertiary-level control. Then, by introducing the component fatigue life as a new factor, an optimization method for energy management of multi-microgrids at the tertiary level is developed. For solving the problem, MATLAB and DigSilent software are used simultaneously. Different connection models and secondary level controller settings are determined using the binary grey wolf optimization in MATLAB. This data is provided to the DigSilent to carry out the optimal dispatch of generation units. The iterations are repeated until the desired operating point is reached. Several case studies are implemented to evaluate the performance of the proposed method. The results show that operating costs can be reduced up to 62.91% under the proposed method compared with the island mode.

Finite Element Simulation Technique for Evaluation of Opening Stresses Under High Plasticity

Abstract The crack closure phenomenon is important to study as it estimates the fatigue life of the components. It becomes even more complex under low-cycle fatigue (LCF) since under LCF high amount of plasticity is induced within the material near notches or defects. As a result, the assumptions used by the linear elastic fracture mechanics (LEFM) approach become invalid. However, several experimental techniques are reported on the topic, the utilization of numerical tools can provide substantial cost and time-saving. In this study, the authors present a finite element simulation technique to evaluate the opening stress levels for two structural steels (25CrMo4 and 30NiCrMoV12) under low-cycle fatigue conditions. The LCF experimental results were used to obtain kinematic hardening parameters through the Chaboche model. The finite element analysis (FEA) model was designed and validated, following the fatigue crack propagation simulation under high plasticity conditions using abaqus. Crack opening displacement versus stress data were exported from abaqus, and 1.5% offset method was employed to define opening stress levels. Numerical simulation results were compared with the experimental results obtained earlier through the digital image correlation (DIC) technique. To conclude, FEA could be a valuable tool to predict crack closure phenomena and, ultimately, the fatigue life of components. However, analysis of opening stresses using crystal plasticity models or extended finite element method (XFEM) tools should be explored for a better approximation in future studies.

Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.