Fatigue Life Of Turbine Blades Research Articles

Fatigue life estimation of nickel-based single crystal superalloy with different inclined film cooling holes: Initial damage quantification and coupling of damage-fracture mechanics models

Quantitative assessment of the initial damage state of a structure and fatigue life prediction on this basis, especially when it is sensitive to manufacturing quality, is crucial for both engineering application and material science. Traditional solutions based on fracture or damage mechanics either require precise constitutive relationships and cumbersome physical mechanism models or simply ignore the initial damage state. This study developed a novel equivalent initial flaw size (EIFS) quantitative evaluation method for film cooling holes in Nickel-based single crystal superalloy turbine blades to address the strong correlation between the initial damage in and fatigue life of the material. The temperature and stress field differences at the edges of holes with different inclination angles were simulated using a solid-liquid-gas three-phase level-set model and considered to represent the same initial damage. Next, the coupled damage–fracture mechanics model was used to achieve equivalent crack insertion, crack propagation, and fatigue life prediction based on the EIFS. The results indicate that this method can provide a highly robust EIFS distribution interval and that the crack geometry correction factor and EIFS distribution range are weakly correlated with the loading conditions (with an error within 2 %). The EIFS-based fatigue life predictions demonstrated a notable degree of accuracy, with the majority of the predicted and experimental fatigue lives falling within a three-fold dispersion band and all predictions remaining confined within a five-fold dispersion band of the actual lifetime. These results represent a substantial advancement in accuracy compared to traditional damage mechanics predictions. Therefore, this study provides a powerful approach for evaluating the fatigue life of turbine blades considering the initial damage state and can be widely applied to guide drilling process optimization and blade fatigue analysis.

A probabilistic life prediction framework for vibration fatigue of turbine blade based on refined polynomial chaos expansion

AbstractVibration fatigue is a typical form of multiaxial fatigue. In this study, initial vibration fatigue tests were conducted on rocket turbine blades, and significant dispersion in fatigue life was found. Consistent with this finding, a novel probabilistic life prediction framework was proposed. This framework integrates sensitivity analysis and sequential sampling technology and introduces polynomial chaos expansion as a computationally efficient alternative to finite element analysis. And a continuous mechanics‐based damage evolution model was employed to examine the vibration fatigue life of turbine blades. The findings validate the effectiveness of the framework, as no significant difference was found between the experimental results and simulated predictions at the 95% confidence level. Furthermore, comparison with the Monte Carlo simulation indicated that this framework achieves comparable prediction accuracy, while significantly reducing the required samples by 2 orders of magnitude, which effectively addresses the fatigue problem of small sample data. This framework enables rapid and accurate multiaxial fatigue probabilistic life prediction, which holds important implications for the reliability design of reusable spacecraft.

Enhancement of high-temperature fatigue properties of 310S stainless steel welded joints by strengthened grinding process inducing gradient structure

This study employed the strengthened grinding process (SGP) for surface enhancement treatment of 310S stainless steel weld joints. The surface characteristics and high-temperature fatigue performance were investigated. Initially, the effects of different SGP durations on the surface morphology, microhardness, and residual stress of the weld joints were studied. Subsequently, the high-temperature fatigue life of samples processed through SGP was examined, and the potential mechanisms underlying the impact of SGP on the high-temperature fatigue behaviour of 310S stainless steel weld joints were discussed. The maximum hardness of SGP-6M samples was 337 HV, while the average hardness of UP samples was 163 HV, and the maximum hardness of SGP-6M samples increased by 90.18 % compared to UP samples. The surface residual stress changed from a residual tensile stress of 205.24 MPa to a residual compressive stress of −864.76 MPa. At 25 °C, 500 °C, 600 °C, and 700 °C, the fatigue life increased by 36.9 %, 31.7 %, 25.9 %, and 18.6 %, respectively. This research demonstrates that SGP is an effective method for increasing the high-temperature fatigue life of turbine blades.

Thermal–Fluid–Solid Coupling Analysis on the Effect of Cooling Gas Temperature on the Fatigue Life of Turbine Blades with TBCs

The application of thermal barrier coatings (TBCs) can increase the blade’s operating temperature and fatigue life. Previous studies have neglected the effects of cooling gas temperature variations on the temperature field, stress field, and fatigue life of blades and TBCs. For this reason, this paper considers the inhomogeneity of the high-temperature gas inlet temperature, the internal cooling gas temperature, the periodicity of the external flow field, etc., and establishes a finite element model of the gas turbine blade with TBCs. Then, the life of the blade and the TBCs is predicted based on the Ncode 2020R2. Finally, the effect of the cooling gas temperature on the temperature, stresses, and life of the blade and the TBCs is analyzed. The results show that the fatigue life of the TBCs is lower than that of the blade, and the low-life region of the TBCs is located at the leading edge of the blade, which is consistent with the TBCs shedding region of the real blade and verifies the accuracy of the life prediction method in this paper. The fatigue life of the blade and TBCs firstly increases and then decreases with the increase in the cooling gas temperature, and the trend of the stress changes in the opposite direction. When the cooling gas temperature is increased from 573 K to 873 K, the minimum life of the blade is increased by 358.5%, and that of the TBCs is increased by 138.7%. The conclusions can provide guidance for the design of long-life turbine blades with TBCs.

A dual-threshold modelling approach for fatigue life prediction under combined high and low cycle fatigue

This paper develops a dual-threshold modelling approach to characterize fatigue performance and predict the fatigue life of turbine blades and their alloys under combined high and low cycle fatigue (CCF). This approach investigates the influence of high cycle fatigue (HCF) superimposed to low cycle fatigue (LCF) on fatigue life through the HCF and LCF stress amplitude ratio threshold. Also, the damage threshold reveals the accelerated failure due to load interactions. Several available test data are applied to validate the presented approach. Results indicate that the approach can obtain higher prediction accuracy than others. This approach has the potential to be generalized to other mechanical structures.

Neural Network Optimization Algorithms to Predict Wind Turbine Blade Fatigue Life under Variable Hygrothermal Conditions

Moisture and temperature are the most important environmental factors that affect the degradation of wind turbine blades, and their influence must be considered in the design process. They will first affect the resin matrix and then, possibly, the interface with the fibers. This work is the first to use a series of metaheuristic approaches to analyze the most recent experimental results database and to identify which resins are the most robust to moisture/temperature in terms of fatigue life. Four types of resin are compared, representing the most common types used for wind turbine blades manufacturing. Thermoset polymer resins, including polyesters and vinyl esters, were machined as coupons and tested for the fatigue in air temperatures of 20 °C and 50 °C under “dry” and “wet” conditions. The experimental fatigue data available from Sandia National Laboratories (SNL) for wind turbine-related materials have been used to build, train, and validate an artificial neural network (ANN) to predict fatigue life under different environmental conditions. The performances of three algorithms (Backpropagation BP, Particle Swarm Optimization PSO, and Cuckoo Search CS) are compared for adjusting the synaptic weights of the ANN and evaluating the efficiency in predicting the fatigue life of the materials studied, under the conditions mentioned above. For accuracy evaluation, the mean square error (MSE) is used as an objective function to be optimized by the three algorithms.

An integrated microporosity model of 3D X-ray micro-tomography and directional solidification simulations for Ni-based single crystal superalloys

The shrinkage microporosity formed during the directional solidification of Ni-based single crystal superalloys can significantly reduce the fatigue life of turbine blades, which may lead to a catastrophic flight accident due to the failure of aeroengines. We developed an integrated mesoscale model, which combined the shrinkage pressure drop from dendritic growth using cellular automaton method and X-ray microtomography characterization for Ni-based single crystal superalloys. The critical shrinkage pressure drop was derived from the simulation and compared to the empirical functions and experimental measurements. It was found that the pressure drop was so critical for predicting the microporosity accurately, and its relationship with the pore size and fraction was established via direct comparison to X-ray tomography. Using this integrated model, the pore size, percentage, and distribution as a function of directional solidification conditions can be successfully simulated. The model can predict small pores at the roots of the secondary dendrite arms in the early stage of solidification, and the large ones in the interdendritic liquid at the late stage of solidification. Therefore, this proposed approach has demonstrated to predict not only the influence of the processing conditions such as withdrawal rate on pore volume fraction, but also the root cause of pore size distributions.

Investigation on geometric precision and surface quality of microholes machined by ultrafast laser

The geometric precision and surface quality of film cooling holes has significant influence on the cooling efficiency and fatigue life of turbine blades. In this paper, the investigation about fabrication process of film cooling holes on the single crystal superalloy DD6 plate specimen by ultrafast laser is carried out. By comparing three different processing paths, minimum roughness of hole wall could be obtained by concentric circular scanning, which was used in the followed-up experiment. The influence of process parameters on the geometric precision and surface quality of microholes was analyzed, and the physical mechanism was examined. The results show that among several process parameters, focus position has the most significant influence on the microholes geometric precision and surface quality, and that the divergent beam (where the focal plane is above the machined surface) can lead to smaller taper, roundness, and surface roughness. In addition, besides a layer of solidified debris adheres to the inner wall at the hole entrance, the microholes machined by ultrafast lasers have no other defects such as a recast layer, microcrack or heat affected zone.

Multidisciplinary Design Optimization Based on Parameterized Free-Form Deformation for Single Turbine

In a traditional multidisciplinary design optimization loop for complex-shaped structures, the regeneration of structure geometry and its associated analysis grids may be unsuccessful or of poor quality, which will reduce the result validity and efficiency of multidisciplinary design optimization. In this Paper, an improved multidisciplinary design optimization method based on parameterized free-form mesh deformation is proposed. Using this method, the analysis grid can be directly updated with high quality instead of regenerating the geometry model when design variables change during optimization. To achieve the mesh deformation of a blade in a single turbine stage, three-dimensional control volume needs to be constructed using two-dimensional control planes at different blade heights, where the control points in the two-dimensional control plane are arranged and parameterized according to the definition of profile design variables. The grids for both computational fluid dynamics and structural analysis are controlled by the same control volume to ensure grid consistency on the fluid–structure interface. Results for an example single turbine stage show that the proposed method can guarantee good controllability and high grid quality when updating design variables in optimization loop. Multidisciplinary design optimization for the single turbine stage is performed, and the optimization results indicate a significant improvement of isentropic efficiency and turbine blade fatigue life. Therefore, the proposed multidisciplinary design optimization method based on parameterized mesh deformation provides an efficient tool for the design of complex and high-performance engineering structures.

Effect of Aluminized Coating on Combined Low and High Cycle Fatigue Life of Turbine Blade at Elevated Temperature

Some cracks were detected on the fir-tree root of turbine blade in an in-service aero-engine, and the aluminized coating was considered to be the main cause of these cracks. To study the effect of aluminized coating on fatigue life of turbine blade, the combined low and high cycle fatigue (CCF) tests are carried out at elevated temperature on both aluminized and untreated turbine blades. Probability analysis of test data is conducted and the result indicates that the median life is decreased by 62.2% due to the effect of the aluminized coating. Further study on the mechanism of crack initiation and propagation has been conducted based on fractography and cross section morphology analysis by using scanning electron microscope (SEM), and the results indicate: (1) The aluminized coating consists of two layers, of which the inner layer is considered to contain the σ phase and it reduces the resistance to fatigue of blade. (2) Many cavities are found in the inner layer of aluminized coating, which lead to the initiation of cracks and result in the reduction of crack initiation life. (3) The marker band widths of aluminized and untreated blade are very close, which indicated the aluminized coating may have no effect on the crack propagation life of the blade.

Fluid Structure Interaction Modelling of Tidal Turbine Performance and Structural Loads in a Velocity Shear Environment

Tidal Current Turbine (TCT) blades are highly flexible and undergo considerable deflection due to fluid interactions. Unlike Computational Fluid Dynamic (CFD) models Fluid Structure Interaction (FSI) models are able to model this hydroelastic behavior. In this work a coupled modular FSI approach was adopted to develop an FSI model for the performance evaluation and structural load characterization of a TCT under uniform and profiled flow. Results indicate that for a uniform flow case the FSI model predicted the turbine power coefficient CP with an error of 4.8% when compared with experimental data. For the rigid blade Reynolds Averaged Navier Stokes (RANS) CFD model this error was 9.8%. The turbine blades were subjected to uniform stress and deformation during the rotation of the turbine in a uniform flow. However, for a profiled flow the stress and deformation at the turbine blades varied with the angular position of turbine blade, resulting in a 22.1% variation in stress during a rotation cycle. This variation in stress is quite significant and can have serious implications for the fatigue life of turbine blades.

Crystal orientation effect and multi-fidelity optimization of a solid single crystal superalloy turbine blade

Single crystal (SC) turbine blade adopts directional solidification casting, traditional design approach requires the [0 0 1] crystal orientation coincide with the blade stacking axis, and the principal deviation angle is less than 10 deg, the other crystal orientation is random. This design philosophy result in random fatigue life of the formed blade even in the same service environment. Crystal orientation effect on mechanical response of a SC turbine blade is investigated using finite element calculation, then presents a multi-fidelity optimization procedure for crystal orientation optimizations of the blade. A nonlinear turbine blade FEM model with orientation dependent crystal plastic theory and coupled temperature conversion provides the high-fidelity model, the low-fidelity model involves a surrogate model technique. Through the multi-fidelity optimization of crystal orientation design, taking into account the efficiency and accuracy to reduce the maximum resolved shear stress (Rss) at the dangerous position of solid turbine blade and effectively improve the fatigue life of the blades. The results demonstrate that crystal orientation have an appreciable variation on Rss in the dangerous position of solid turbine blade. The influence of temperature and structure must be taken into consideration, especially the maximum Rss has the significant effect on evaluating the fatigue life of turbine blade. The optimization model based on the approximation theory can accurately describe the response between the design variables and optimization targets. The optimization reduces the Rss and deformation by 28% and 17%, respectively. The multi-fidelity optimization strategy can effectively optimizes the crystal orientation of the blade and improves the blade life.

Reliability-based low-cycle fatigue damage analysis for turbine blade with thermo-structural interaction

To improve the computational accuracy and efficiency of complex mechanical component like engine turbine structure, distributed collaborative response surface method is applied to the reliability analysis of aeroengine turbine blade low-cycle fatigue damage. The improved Manson–Coffin formulas with different confidence levels are established based on the linear variance regression analysis in the application of the fatigue data of nickel-based superalloy GH4133. The distributed response surfaces of strain range Δεt and mean stress σm are established by considering the randomness of the design sizes, working loads and material parameters. And then Δεt and σm are regarded as the basic input variables of fatigue life Nf to complete turbine blade low-cycle fatigue damage reliability analysis by the Miner cumulative damage theory. The probabilistic sensitivity analyses demonstrate that the fatigue performance parameters hold important influence on the low-cycle fatigue life of turbine blade. Through the comparison of methods, it is revealed that distributed collaborative response surface method is superior to response surface method in computational precision and efficiency, especially for low confidence level. The efforts give the conclusion that distributed collaborative response surface method is a promising approach in ameliorating the computational precision and efficiency of reliability analysis, which enriches the reliability theory and method of complex mechanical structure with multi-component and multi-failure mode.

Reliability-based fatigue life investigation for a medium-scale composite hydrokinetic turbine blade

As the most important, expensive component of a hydrokinetic turbine system, the composite turbine blade must achieve a long operating life (10–20 years). The investigation of fatigue life for the composite turbine blade is essential when designing a cost-effective hydrokinetic composite turbine system. A reliability-based fatigue life analysis methodology was developed for a medium-scale, horizontal axis, hydrokinetic turbine blade. Finite element method, coupled with the blade element momentum theory, was used to find the stress response on the turbine blade. The fatigue behavior of the blade was studied in stress-critical zones. A metamodel was constructed for the stress response according to simulations at specified design points. Accounting for uncertainties in material properties and the material S–N curve, the reliability analysis method was employed to estimate the fatigue life distribution of the hydrokinetic turbine blade. The effect of river velocity models on the fatigue life of turbine blades was also studied. The fatigue life of the composite blade was sensitive to composite material properties. Transverse strain E22 is particularly dominant which is related to the matrix cracking as the fatigue failure mode. The statistical distribution of S–N data implies a significant dependence of fatigue life on composite S–N data.

PS06 小標本数試験データおよび実機検査記録を用いた信頼性設計手法の開発

Probabilistic safety assessment is an effective method to improve the conservative safety factors on low cycle fatigue life of turbine blades used in jet engines. This paper focuses on uncertainties of low cycle fatigue design life result from small samples and surface finishes. Improvement of uncertainty of design fatigue life, reflected from small sample tests, by use of inspection data is verified through Bayesian update and virtual inspection data. Furthermore, tension-compression low cycle fatigue tests on Inconel 718 were conducted with the specimens, of which surfaces were finished Ra 0.8, Ra 6.3, and Ra 12.5. The results indicates strain-amplitude dependence of the influence of surface finishes on low cycle fatigue life. In order to investigate the effect of surface finishing on low cycle fatigue life, local misorientations of surface layer of specimens are measured by EBSD.

Use of an orthogonal array based on the Kriging model to maximize the fatigue life of a turbine blade

PurposeApproximation techniques were used instead of expensive computing analysis in a traditional parametric design optimization of a complex system. A Kriging meta‐model was utilized, which enabled the fit of approximated design characteristics for a complex system such as turbine blades that incorporate a large number of design variables and non‐linear behaviors. This paper aims to discuss these issues.Design/methodology/approachThe authors constructed a Kriging meta‐model with a multi‐level orthogonal array for the design of experiments, which were used to optimize the fatigue life of turbine blades under cyclic rotational loads such as centrifugal force. By combining a seven‐level orthogonal array with the Kriging model, the non‐linear design space of fatigue life was explored and optimized.FindingsA computer‐generated multi‐level orthogonal array provided a good representation of the non‐linear design space information. The results show that not only was the fatigue life of the leading edge of the blade root significantly improved, but also that the computing analysis was effective.Originality/valueTo maximize the fatigue life of the turbine blade, the three‐design variables with seven factor levels were optimized via a Kriging meta‐model. As with the optimization technique, a desirability function approach was adopted, which converted multiple responses into a single response problem by maximizing the total desirability.

Vortex-induced vibration effect on fatigue life estimate of turbine blades

An analysis of a turbine blade fatigue life that includes the physics of fluid–structure interaction on the high cycle fatigue (HCF) life estimate of turbine blades is carried out. The rotor wake excitation is modeled by rows of Karman vortices superimposed on an inviscid uniform flow. The vortex-induced vibration problem is modeled by a linear cascade composed of five turbine blades and the coupled Euler and structural dynamics equations are numerically solved using a time-marching boundary element technique. The analysis can be applied to any blade geometries; it is not limited to the blade geometry considered here. Two major design parameters have been identified; the ratio of blade spacing to blade chord length s/ c of the stator, and the normalized frequency parameter c/ d which is related to the wake passing frequency of the rotor. For a rigid cascade, it is found that aerodynamic resonance prevails at the resonant c/ d values corresponding to an isolated blade while s/ c is responsible for the level of the aerodynamic response. If the central blades were elastic, the parameter s/ c plays a different role in the fluid–structure interaction problem. With a c/ d that could lead to structural resonance for an isolated blade, changing s/ c would stabilize the aerodynamic and structural response of the elastic blade in a cascade. On the contrary, an improper choice of s/ c might turn the elastic blade response into structural resonance even though the oncoming c/ d is non-resonant. The results of the nonlinear effects of c/ d and s/ c could be used together with the Campbell diagram to obtain an improved HCF design of rotor–stator pair.

FLUID–STRUCTURE RESONANCE PRODUCED BY ONCOMING ALTERNATING VORTICES

The present investigation examines a simple fluid–structure interaction problem, which is represented by the unsteady response of an airfoil/blade to a Karman vortex street in an inviscid uniform flow. Two different cases were examined; one with a rigid airfoil/blade, where the structural stiffness is infinite, another with an elastic blade. In both cases, the flow remains attached to the airfoil/blade surface. A time-marching technique solving the Euler equations and a two-degree-of-freedom structural dynamic model is used to examine the interactions between the fluid and the structure. The interactions between the convected vortices and the structure modify the shed wake whose energy, in turn, feeds into the forces and moments acting on the structure. For a rigid airfoil/blade, it is found that the amplitude of the aerodynamic response is not proportional to the density of the oncoming vortex street, but depends on c/d , the ratio of the chord length (c) to the axial spacing (d) of the convected vortices. When the number of vortices per unit length is increased, the amplitudes of the aerodynamic response increase and then decrease even though the density of the vorticity keeps increasing and so is the energy of the excitation wake. Maxima are observed at c/d=0·5, 1·5 and 2·5. This behaviour is analogous to the structural resonance phenomenon and is labeled “aerodynamic resonance”. The existence of such an “aerodynamic resonance” is important to turbomachinery applications where the blade is elastic, the flow is unsteady and the shed vortices from the previous row are convected downstream by the mean flow. Thus, “aerodynamic resonance” alone or in conjunction with structural resonance could impact negatively on the fatigue life of turbine blades and their combined effects should be accounted for in blade design. A preliminary attempt to assess this impact has been carried out. It is found that the relative fatigue life of a blade could be reduced by four orders of magnitude as a result.

Estimation of Fatigue Life of Wind Turbine Blade. In the Case of TAPPI Wind Park.

Wind turbine blade fatigue life was estimated with Goodman's Rule and Miner's Rule, using the Rain-Flow Procedure for stress values and numbers at TAPPI Wind Park where the terrain is mountainous and very rough. The fatigue life which calculated using faithfully measured data and following the distributions is evaluated to be about 12 years. Approximation using a well-known probability density function gives an estimate of about 23 years. This difference is very significant and it is thought that it is caused by conventional methods which exclude irregular and the large pulsating stresses on a blade imposed by wind turbulence.