Blade Attachment Research Articles

Experimental study of friction damping of blade with loosely assembled dovetail attachment

In order to study the friction damping characteristics of blade with loosely assembled dovetail attachment, an experimental method and a test rig are presented in this article. In the experiment, a axial load is imposed at the bottom of blade attachment to simulate the blade centrifugal force. A harmonic force is imposed on the blade as airflow force. The forced responses of a group of plane blades and a compressor blade are measured at different centrifugal forces. The results show that the resonant frequency increases with the centrifugal force, and gradually reaches the maximum. When the centrifugal force increases, the amplitude of resonant response decreases at the beginning and then increases. There is an optimal centrifugal force under which the resonant amplitude is minimal and the friction damping effect is the best. There is softening spring effect when the friction forces worked on blade system. From the results of compressor blade, it is found that when the centrifugal force is 1.5 kN, the resonant amplitude reaches the least value of 3.42 g, which is only 22.4% of the biggest value 39.88 g. This means that 1.5 kN is the optimal centrifugal force.

Hybrid contact stress analysis of a turbine engine blade to disk attachment

The present investigation examines an analysis methodology for fretting fatigue in a turbine engine fan disk. This is an important problem for the operators of turbine engines, since it is a significant driver of fatigue damage and failure risk of disks. Fretting fatigue in turbine engines occurs when the blade and disk are pressed together in contact and experience a small oscillating relative displacement due to variations in engine speed and vibratory loading. Fretting causes a very high local stress near the edge of contact resulting in wear, nucleation of cracks, and their growth, which can result in significant reduction in the life of the material. It is dependent on geometry, loading conditions, residual stresses, and surface roughness, among other factors. These complexities are not just physically based, but also computationally challenging. For example, the determination of the local contact stresses accurately depends on the mesh resolution of the finite element method (FEM) model. This has been addressed using various approaches. Recently, a computational hybrid technique was implemented successfully to investigate fretting fatigue of turbine engine blade and disk attachments. The present work extends application to specifically investigate the effects of surface contact in an actual blade and disk assembly using a representative loading mission. The results show consistency with available experimental data. Finally, the knowledge gained from this investigation could be used as a basis for uncertainty analyses of an actual blade and disk assembly.

Fretting Damage of Ni—MoS2 Coatings: Friction Coefficient and Accumulated Dissipated Energy Evolutions

This article describes fretting wear damage of Ni—MoS2 coatings in terms of the friction coefficient (COF) and accumulated dissipated energy. Multi-layer dry lubricant coatings containing the elements Mo and S are known to maintain low friction under reciprocal relative displacement, and are considered for this purpose in fan blade attachments within aero-engines. The durability of these coatings for a dovetail connection needs to be investigated in order to help design calculations and overhaul scheduling. Experimental characterization of durability can be achieved by measuring COF as a function of the number of fretting cycles, or, alternatively, the accumulated sliding distance. Dissipated energy is also considered as an alternative indicator for characterizing fretting wear damage. In this article, fretting wear tests with Ni—MoS2 coatings and various surface treatments were performed using idealized contact geometries and loading conditions similar to those found in aero-engine components. Additionally, COF evolutions of Ni—MoS2 coatings were numerically determined as a function of accumulated dissipated energy. A direct comparison between a numerical evolution and a measured one was performed, and excellent agreement between the two evolutions was obtained.

Probabilistic Fretting Fatigue Assessment of Aircraft Engine Disks

Fretting fatigue is a random process that continues to be a major source of damage associated with the failure of aircraft gas turbine engine components. Fretting fatigue is dominated by the fatigue crack growth phase and is strongly dependent on the magnitude of the stress values in the contact region. These stress values often have the most influence on small cracks where traditional long-crack fracture mechanics may not apply. A number of random variables can be used to model the uncertainty associated with the fatigue crack growth process. However, these variables can often be reduced to a few primary random variables related to the size and location of the initial crack, variability associated with applied stress and crack growth life models, and uncertainty in the quality and frequency of nondeterministic inspections. In this paper, an approach is presented for estimating the risk reduction associated with the nondestructive inspection of aircraft engine components subjected to fretting fatigue. Contact stress values in the blade attachment region are estimated using a fine mesh finite element model coupled with a singular integral equation solver and combined with bulk stress values to obtain the total stress gradient at the edge of contact. This stress gradient is applied to the crack growth life prediction of a mode I fretting fatigue crack. A probabilistic model of the fretting process is formulated and calibrated using failure data from an existing engine fleet. The resulting calibrated model is used to quantify the influence of inspection on the probability of fracture of an actual military engine disk under real life loading conditions. The results can be applied to quantitative risk predictions of gas turbine engine components subjected to fretting fatigue.

Dynamic Analysis of Fretting-Wear in Friction Contact Interfaces

Fretting wear is a very important phenomenon occurring in bladed disks. It causes the blades to be replaced in turbomachines during their life-cycle. Methods exist to predict fretting-wear in quasistatic analysis. However, they do not predict all the phenomena observed in blade attachments on real turbomachines. That is why this study assumes that dynamics plays a role in fretting-wear. This paper is devoted to the realistic modeling and calculation of fretting-wear and dynamical response of structures in unilateral contact with friction. Vibration and wear phenomena present very different scales both in time and space. Therefore the difficulty is in finding methods that enable one to solve the nonlinear problem with a good compromise between the approximations made about the dynamical aspects and those linked with fretting-wear issues. Here, phenomenological examples are studied. They involve a small number of degrees of freedom with a view to understanding the complex coupling between vibration and fretting-wear. This way, they will show the relative importance of parameters.

Analytical solutions for a single blade in vertical axis turbine motion in two-dimensions

An analytical model for a time dependent two dimensional flow around a moving profile is developed. The model is suitable for fast aerodynamic and aeroelastic coupling calculations. It determines the inviscid pressure distribution in the vicinity of one blade and the force on the blade in arbitrary two dimensional motion. The method is more flexible than previous analysis: it can represent any profile, pitching motion and blade attachment position. The method is based on conformal mapping techniques and Laurent's series decomposition and is faster and more accurate than standard panel methods. A main idea is to directly treat the singularities of the flow in a mapped plane where any geometrical plane is simplified to a circle. The vorticity is assumed to be shed in the form of a continuous vortex sheet near the trailing edge.

Using phased array ultrasonic technique for the inspection of straddle mount-type low-pressure turbine disc

Discovery of stress corrosion cracking (SCC) in the blade attachment region of nuclear power plant low-pressure steam turbine discs leads to industry concern about this problem. Reliable inspection methods for the detection of SCC in steam turbine blade attachment area are essential to the rotor run/repair/retire decision making process. The conventional broad beam ultrasonic inspection technique usually has inadequate resolution for flaw sizing and tendency for false calls. But the phased array technique by improved detection capability and resolution could increase coverage of the examination areas with less scanning required. Using phased array ultrasonic technique to straddle mount-type low-pressure turbine disc in nuclear power plant located in Korea was studied in this paper.

Numerical and Experimental Study of Friction Damping Blade Attachments of Rotating Bladed Disks

In order to mitigate high cycle fatigue risks in bladed disks, the prediction of the vibration levels early in the design process is important. Therefore, the different sources of damping need to be modeled accurately. In this paper the impact of friction in blade attachments on forced response is investigated both numerically and experimentally. An efficient multiharmonic balance method is proposed in order to compute the forced response of bladed disks with contact and friction nonlinearities in blade roots. For experimental validation purposes, a rotating bladed disk was tested in a vacuum chamber, with excitation being provided by piezoelectric actuators. A model of the rig was built and numerical results were obtained with a normal load dependent coefficient of friction and a constant material damping ratio. Nonlinear behavior observed experimentally at resonances was well reproduced and an acceptable correlation was found with experimental resonant frequencies, amplitudes, and amount of damping throughout the spinning speed and excitation level range. The proposed numerical method can therefore serve to enhance the prediction of the alternating stresses in bladed disk assemblies.

Subsurface Stress Fields in Face-Centered-Cubic Single-Crystal Anisotropic Contacts

Single-crystal superalloy turbine blades used in high-pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent high-cycle fatigue (HCF) failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single-crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two- and three-dimensional (3D) subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single-crystal nickel-base superalloy turbine blades. An analytical procedure is presented for evaluating the subsurface stresses in the elastic half-space, based on the adaptation of a stress function method outlined by Lekhnitskii (1963, Theory of Elasticity of an Anisotropic Elastic Body, Holden-Day, Inc., San Francisco, pp. 1–40). Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis. Effects of crystal orientation on stress response and fatigue life are examined. Obtaining accurate subsurface stress results for anisotropic single-crystal contact problems require extremely refined 3D finite element grids, especially in the edge of contact region. Obtaining resolved shear stresses on the principal slip planes also involves considerable postprocessing work. For these reasons, it is very advantageous to develop analytical solution schemes for subsurface stresses, whenever possible.

Fretting fatigue crack analysis in Ti–6Al–4V

A study was conducted to verify the efficacy of a fracture mechanics methodology to model the crack growth behavior of fretting fatigue-nucleated cracks obtained under test conditions similar to those found in turbine engine blade attachments. Experiments were performed to produce cracked samples, and fretting fatigue crack propagation lives were calculated for each sample. Cracks were generated at 10 6 cycles (10%-of-life) under applied stress conditions previously identified as the fretting fatigue limit conditions for a 10 7 cycle fatigue life. Resulting cracks, ranging in size from 30 to 1200 μm, were identified and measured using scanning electron microscopy. Uniaxial fatigue limit stresses were determined experimentally for the fretting fatigue-cracked samples, using a step loading technique, for R=0.5 at 300 Hz. Fracture surfaces were inspected to characterize the fretting fatigue crack front indicated by heat tinting. The shape and size of the crack front were then used in calculating Δ Kth values for each crack. The resulting uniaxial fatigue limit and Δ Kth values compared favorably with the baseline fatigue strength (660 MPa) for this material and the Δ Kth value (2.9 MPa√m) for naturally initiated cracks tested at R=0.5 on a Kitagawa diagram. Crack propagation lives were calculated using stress results of FEM analysis of the contact conditions and a weight function method for determination of Δ K. Resulting lives were compared with the nine million-cycle propagation life that would have been expected in the experiments, if the contact conditions had not been removed. Scatter in the experimental results for fatigue limit stresses and fatigue lives had to be considered as part of an explanation why the fatigue life calculations were unable to match the experiments that were modeled. Analytical life prediction results for the case where propagation life is observed to be very short experimentally were most accurate when using a coefficient of friction, μ=1.0, rather than for the calculations using μ=0.3

Fatigue crack propagation simulation in an aircraft engine fan blade attachment

This paper discusses the computation of three-dimensional fatigue crack growth rates in a typical military aircraft engine fan blade attachment under centrifugal and aerodynamic loads. The three-dimensional crack growth simulations utilize FRANC3D, a state-of-the-art crack propagation software developed at Cornell University, which uses boundary elements and linear elastic fracture mechanics. With an existing three-dimensional finite element contact stress analysis with a prescribed coefficient of friction (COF) along the contact surface, the displacements and stress intensity factors are calculated on the crack leading edge to yield crack propagation trajectories and growth rates. Due to complex geometry of the fan blade attachment and loading conditions, all three-fracture modes are considered and the associated stress intensity factors (SIF) are calculated using the Crack Opening Displacement (COD) approach. Crack propagation trajectories under mixed-mode conditions are obtained using the planar and maximum tangential stress crack-extension criteria. The fatigue crack in the blade attachment is subjected to an over speed mission cycle that includes high cycle frequencies (i.e., spectrum load) and the crack growth rate is predicted utilizing the Forman–Newman–de Koning (FNK) model. Scanning Electron Microscope (SEM) images of a cracked component from an engine ASMET (Accelerated Simulated Mission Endurance Test) are used to evaluate and compare the simulation results. The calculated SIF's from the simulations indicate a strong Mode-I ( K I) and Mode-III ( K III) interaction at the edge of contact (EOC). However, on the free surface it is primarily a crack opening ( K I) condition only. The crack growth rates are determined using the planar extension criterion which correlates better with the test data than the maximum tangential stress extension criteria.

Gas turbine disk-blade attachment crack

Evaluation of a gas turbine disk revealed a crack in the blade attachment area. The subsequent effort to understand the origin of this crack led to a series of analyses that included computing the stresses on the attachment, characterization of fatigue crack growth, and a model for fretting fatigue crack growth. These elements were brought together to simulate the conditions that led to the cracking. It is concluded that the crack was probably caused by fretting fatigue induced by the stresses related to normal takeoff and landing cycles and exacerbated by aircraft maneuvers, and that short periods of blade resonance may have contributed to the cracking. If material had not been removed from the attachment surface of the disk by service-induced wear, it is likely more cracks would have been found.

Calculation of loads on slewing gears of turn in motion blades of axial fan impeller

A procedure is proposed for calculating the loads acting on double sheet blade of impeller of the mine axial fan. It is established that torque of aerodynamic forces of blade creates an alternating load on slewing gear of the blades, and the main load of the gear is the frictional torque of rolling at the blade attachment point under the action of centrifugal forces.

Powered instrumentation in laryngeal surgery

The endoscopic shaver system employs a motorized handpiece and specially tailored blades that allow access to glottic, subglottic, and tracheal sites. The system and setup is identical to that used in endoscopic sinus surgery except for the blade attachments. This similarity confers a degree of familiarity to the surgeon as well as the operating room staff. The system can be utilized for resection or biopsy of lesions of the upper aero-digestive tract. Endoscopic shaver resection of laryngeal tumors is a safer and more rapid alternative to conventional laser resections. Many of the risks associated with laser resections including inadvertent burn injury and airway fires are avoided. Benign tumors of the larynx, subglottis, and trachea requiring conservative management are especially amenable to this form of therapy. This technique is well suited for resection of papillomas as well as other lesions of the airway. A drawback of this technique is the inadequate assessment of surgical margins that limits its utility in the management of malignancies.

Experimental Study of Aerodynamic and Structural Damping in a Full-Scale Rotating Turbine

Damping in turbomachinery blades has been an important parameter in the study of forced response and high-cycle fatigue, but because of its complexity the sources and physical nature of damping are still not fully understood. This is partly due to the lack of published experimental data and supporting analysis of real rotating components. This paper presents the results of a unique experimental method and data analysis study of multiple damping sources seen in actual turbine components operating at engine conditions. The contributions of both aerodynamic and structural damping for several different blade vibration modes, including bending and torsion, were determined. Results of the experiments indicated that aerodynamic damping was a large component of the total damping for all modes. A study of structural damping as a function of rotational speed was also included to show the effect of friction damping at the blade and disk attachment interface. To the best of the authors’ knowledge, the present paper is the first report of independent and simultaneous structural and aerodynamic damping measurement under engine-level rotational speeds.

Fracture analysis of turbine disks and computational–experimental background of the operational decisions

In this paper, fatigue crack growth under operation conditions for rotating disks of aircraft gas turbine engines is analyzed. Initiation and growth of surface cracks for compressor disks made from two-phase titanium alloy has occurred in a disk and blade attachment. Damage accumulation and growth for turbine disks made from steel took place on the inner surface of hole in a hub of wheel. Suggested approach of simulation modeling is used for an analysis and prevention of operation failures of engine rotating components. In the approach described, finite-element models (FEMs) in two and three dimensions were applied to the study of stress–strain state and stress intensity factors for the basic configurations of compressor and turbine disks and their operational damage. Proposed design modifications and repair technologies to existing in-service aircraft gas-turbine engine rotating components are analyzed and substantiated on a static strength and fatigue life basis.

Fatigue Failure in High-Temperature Single Crystal Superalloy Turbine Blades

Fatigue failures in PWA1480/1493 single crystal nickel-base superalloy turbine blades used in the Space Shuttle Main Engine fuel turbopump are discussed. Many blades in the turbopump had small HCF cracks initiating from the leading edge radius and extending across the airfoil tip on the pressure side. Stage II non-crystallographic fatigue cracks with multiple origins were present at the core leading edge radius and extending down the airfoil span along the core surface. The longer cracks transitioned from stage II fatigue to crystallographic stage I fatigue propagation, on octahedral planes. Reasons that contributed to initiation and growth of tip cracks were the small leading edge fillet radius, the non-uniform wall thickness on pressure and suction sides, the high mean stress, and consequently low allowable alternating stress. Many blades also showed fatigue damage in the attachment regions leading to fretting induced cracking with multiple origins with stage II cracks. An investigation of crack depths on the population of blades as a function of secondary crystallographic orientation revealed that for β = 45+/- 15 degrees tip cracks arrested after some growth or did not initiate at all. Detailed 3D FE stress analysis of AHPFTP/AT SSME single crystal turbine blades subjected to rotational, aerodynamic, and thermal loads was conducted. Evaluation of stress response at the blade tip and attachment regions, as a function of primary and secondary crystal orientation, revealed that there are preferential β orientations for which crack growth is minimized at the blade tip. The FE analysis results give further evidence to the conclusion that control of secondary and primary crystallographic orientation has the potential to significantly increase a component's resistance to fatigue crack growth without adding additional weight or cost. Fretting contact stresses in the attachment region were seen to reach peak values at locations where fretting cracks have been observed. Contact stresses at critical attachment regions also varied significantly as a function of crystal orientation alone. The tangential normal stress σ X in the attachment region increased with increasing coefficient of friction at the critical contact location. This is of practical interest since cracks are thought to initiate at locations where σ x reaches a maximum tensile value, and hence high temperature metal coatings applied to the attachment region, to reduce the coefficient of friction, will be beneficial in enhancing fretting fatigue life. Additional fretting damage parameters need to be evaluated in detail for single crystal materials.

Fretting Stresses in Single Crystal Superalloy Turbine Blade Attachments

Single crystal nickel base superalloy turbine blades are being utilized in rocket engine turbopumps and turbine engines because of their superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over polycrystalline alloys. High cycle fatigue induced failures in aircraft gas turbine and rocket engine turbopump blades is a pervasive problem. Blade attachment regions are prone to fretting fatigue failures. Single crystal nickel base superalloy turbine blades are especially prone to fretting damage because the subsurface shear stresses induced by fretting action at the attachment regions can result in crystallographic initiation and crack growth along octahedral planes. This paper presents contact stress evaluation in the attachment region for single crystal turbine blades used in the NASA alternate advanced high pressure fuel turbo pump for the space shuttle main engine. Single crystal materials have highly anisotropic properties making the position of the crystal lattice relative to the part geometry a significant factor in the overall analysis. Blades and the attachment region are modeled using a large-scale three-dimensional finite element model capable of accounting for contact friction, material anisotropy, and variation in primary and secondary crystal orientation. Contact stress analysis in the blade attachment regions is presented as a function of coefficient of friction and primary and secondary crystal orientation. Fretting stresses at the attachment region are seen to vary significantly as a function of crystal orientation. The stress variation as a function of crystal orientation is a direct consequence of the elastic anisotropy of the material. Fatigue life calculations and fatigue failures are discussed for the airfoil and the blade attachment regions.

Effect of Crystal Orientation on Fatigue Failure of Single Crystal Nickel Base Turbine Blade Superalloys

High cycle fatigue (HCF) induced failures in aircraft gas turbine and rocket engine turbopump blades is a pervasive problem. Single crystal nickel turbine blades are being utilized in rocket engine turbopumps and jet engines throughout industry because of their superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over polycrystalline alloys. Currently the most widely used single crystal turbine blade superalloys are PWA 1480/1493, PWA 1484, RENE’ N-5 and CMSX-4. These alloys play an important role in commercial, military and space propulsion systems. Single crystal materials have highly orthotropic properties making the position of the crystal lattice relative to the part geometry a significant factor in the overall analysis. The failure modes of single crystal turbine blades are complicated to predict due to the material orthotropy and variations in crystal orientations. Fatigue life estimation of single crystal turbine blades represents an important aspect of durability assessment. It is therefore of practical interest to develop effective fatigue failure criteria for single crystal nickel alloys and to investigate the effects of variation of primary and secondary crystal orientation on fatigue life. A fatigue failure criterion based on the maximum shear stress amplitude [Δτmax] on the 24 octahedral and 6 cube slip systems, is presented for single crystal nickel superalloys (FCC crystal). This criterion reduces the scatter in uniaxial LCF test data considerably for PWA 1493 at 1200°F in air. Additionally, single crystal turbine blades used in the alternate advanced high-pressure fuel turbopump (AHPFTP/AT) are modeled using a large-scale three-dimensional finite element model. This finite element model is capable of accounting for material orthotrophy and variation in primary and secondary crystal orientation. Effects of variation in crystal orientation on blade stress response are studied based on 297 finite element model runs. Fatigue lives at critical points in the blade are computed using finite element stress results and the failure criterion developed. Stress analysis results in the blade attachment region are also presented. Results presented demonstrates that control of secondary and primary crystallographic orientation has the potential to significantly increase a component’s resistance to fatigue crack growth without adding additional weight or cost.

A New Name in Chinese Dioscorea (Dioscoreaceae)

During preparation of the account of Dioscoreaceae for the Flora of China, it was noticed that one species is illegitimately named, being a later homonym: Dioscorea parviflora C. T. Ting (1979), not Philippi (1864). The following new name (nomen novum) is therefore proposed here: D. sinoparviflora C. T. Ting, M. G. Gilbert & Turland. Dioscorea sinoparviflora C. T. Ting, M. G. Gilbert & Turland, nom. nov. Replaced name: Dioscorea parviflora C. T. Ting, in C. Pei et al., Acta Phytotax. Sin. 17(3): 69. 1979. Not Philippi, Linnaea 33: 257. 1864. TYPE: China. Yunnan: Yongsheng Xian, Xinghe, 1600 m, Mar. 1963, Q. Y. Liang 63009 (holotype, NAS). Dioscorea parvflora C. T. Ting (in Pei et al., 1979: 69-70) was described from Yongsheng County (Xian) in northwestern Yunnan Province, China. Unfortunately, the name is illegitimate under Article 53.1 of the Tokyo Code (Greuter et al., 1994) because an earlier homonym exists: D. parviflora Philippi (1864), described from Chile. The Chinese species will be accepted by Ding and Gilbert, as endemic to Yunnan Province, in their forthcoming account of Dioscoreaceae in the Flora of China (in press). Therefore, a new name for D. parviflora is required, and D. sinoparviflora C. T. Ting, M. G. Gilbert & Turland is proposed here. Dioscorea sinoparviflora is most similar to D. zingiberensis C. H. Wright, which is also endemic to China, occurring in Gansu, Henan, Hubei, Hunan, Shaanxi, Sichuan, and Yunnan provinces. Dioscorea sinoparviflora differs from D. zingiberensis in having perianth lobes 0.8-1.2 X 0.6-0.8 mm (vs. 1.2-1.5 X 0.8-1 mm) and capsules longer than wide (vs. ca. as long as wide). Both species can be readily distinguished from others in China by the peltate attachment of most leaf blades. Moreover, D. zingiberensis is the only Chinese species recorded as being monoecious.