Fan Disk Research Articles

A re-assessment of Ti-685 as a dwell sensitive titanium alloy and a definition for engineering relevant dwell behaviour

It is believed that the near-α titanium alloy Ti-685 was the first to demonstrate dwell sensitive fatigue behaviour whilst under engineering service as a gas turbine fan disc material. New fatigue data, micro-textural characterisation and fractography from a recent study which utilised remnant Ti-685 specimens originally manufactured for previous laboratory campaigns based at Swansea dating back to the late 1980s are now reported. In isolation, the present results failed to demonstrate a significant dwell effect in either aligned or basketweave microstructural variants. On this occasion, the limited sensitivity to dwell loading was explained by the combination of heat treatment artifacts and selection of a relatively small test specimen geometry. However, the present data were also reviewed in the context of the wider fatigue database generated by multiple studies over the past fifty years. In combination, stress-life (SN) curves defined the magnitude in applied peak stress necessary to demonstrate a dwell effect in this alloy. Ultimately, this facilitated a fundamental definition of dwell sensitivity that is relevant to the engineering community.

An appraisal of dwell sensitive fatigue in Ti-6Al-4V and the governing role of inhomogeneous micro-texture

The fact that the α/β titanium alloy Ti-6Al-4V is sensitive to cold dwell loading regimes has recently been endorsed by a combination of evidence gathered from fatigue studies performed on laboratory scale specimens and the findings of a failure investigation into an in-service fan disc failure. This work confirms that Ti-6Al-4V can demonstrate dwell characteristics under fatigue loading, subject to the microstructure and micro-texture developed during thermo-mechanical processing. Additional fatigue data are presented to support this conclusion, together with detailed metallographic and crystallographic characterisation to identify the factors controlling inhomogeneous strength, stress distribution and the crack initiation process on the micro to macro scale.

Vibration prediction of rotating composite fan blades comprising viscoelastic damping treatments

To reduce the risk of fatigue problems and flutter phenomena, the structural damping of turbomachinery components is increased by integrating constrained viscoelastic patches into rotating composite blades. However, the temperature, the frequency (of the vibration mode to be damped) and the rotation speed are all parameters which may affect significantly the efficiency of the viscoelastic damping treatment.In the presented work, a numerical tool is developed to predict the structural damping of a dynamic rotating composite fan blade while varying the temperature and the rotation speed. This study is based on a finite element model of a fan disk sector equipped with constrained viscoelastic patches. This work will allow quantifying the influence of temperature and rotation speed on the efficiency of the passive damping treatment and contribute to the design of higher performing blades.

Toward an understanding of dwell fatigue damage mechanism of bimodal Ti-6Al-4V alloys

Dwell fatigue effect is a long-standing problem threatening the long-term service reliability for fan blades and fan disks of an aircraft engine. To understand the basic mechanism of dwell fatigue damage, pure fatigue and 60 s dwell fatigue properties of bimodal Ti-6Al-4V alloys with different volume fractions of the primary α (αp) phase were examined comparatively. The results showed that both pure fatigue and dwell fatigue life decreased with increasing the volume fraction of the αp phase and the dwell fatigue life was lower than the pure fatigue one. The quasi-in-situ test results and the quantitative characterization of damage behaviors of the local microstructure units defined by the αp-secondary α (αs) combination reveal that the αs phase close to the αp phase with extensively slip activities was gradually damaged under dwell fatigue loading, while that under pure fatigue loading was undamaged, demonstrating that the dwell loading induced the damage of the αs phase, and further reduced the fatigue life. A stress relaxation-based model is proposed to describe the physical mechanism on dwell fatigue damage of the bimodal Ti-6Al-4V alloy, i.e. the elastic deformation of the αs phase caused by the strain incompatibility would be gradually transformed into plastic deformation during the dwell stage, and thus promotes fatigue damage. The model provides new insights into the microscopic process of stress/strain transfer between the soft and hard microstructure units under dwell fatigue loading.

Crack growth regularities and stress equivalent for in-service fatigued fan disk of titanium alloy Ti–6Al–3Mo–2Cr

The in-service fatigue fracture of the fan disk made of titanium alloy is analyzed by complex metallophysical research. The crack path consists of the formation of facetted pattern relief and striations on the fracture surface. From the results of striation spacing measurements, the equivalent stress for various crack sizes is estimated. The number of damaging-loading cycles per one flight is discussed and established by the analysis of the schematized flight loading cycle of the aero-engine. The inspection interval for reliable crack detection in the disk is evaluated. It is demonstrated that, without fan disk damage, its in-service durability can exceed 15,000 flights.

Comparison of aero engine component lifing methods

Failure of critical engine components such as compressor, fan, and turbine disks during flight can cause the loss of the engine, aircraft, or even life. To reduce the risk of this failure during flight, different methodologies and tools have been developed to determine the safe operating life of these critical disk components. The two most widely used lifing methods, safe-life and damage tolerance, are inherently conservative, retiring all components when a predetermined operating limit is reached. Both methods retire components with theoretical useful life remaining. Additional lifing methods can be used to reduce this conservatism and extend the life of these components. Retirement for cause, developed within the United States Air Force is a lifing method that can extend the life of components by retiring a component only when there is cause to do so. Military and industry standards on lifing methodologies were reviewed. Both deterministic and probabilistic approaches to disk lifing methods are discussed as well as current tools. This paper provides a comparison of the methodologies and tools currently being used today by both the government and industry.

Crack path of in-service fatigued fan titanium disk of D-18T gas-turbine engine of the aircraft An-124

The in-service fatigue fracture of the fan disk from the titanium alloy was considered and analyzed. Fatigue crack started from the cut which was produced during disk manufacturing and located along the slot for the blade. The crack path was performed by the mechanism of crack meso-tunneling with the facetted pattern relief and striation formation on the fracture surface. Based on the fractographic analysis, equivalent stress for various values of crack length was estimated. The inspection interval for reliable crack registration in the disk was provided. It was demonstrated that without cuts the disk in-service period will be not less than 15,000 flights.

Crystal plasticity analysis of temperature-sensitive dwell fatigue in Ti-6Al-4V titanium alloy for an aero-engine fan disc

This study investigates the temperature-dependent dwell fatigue behavior of Ti-6Al-4V alloy using crystal plasticity finite element method. The dislocation mechanism-based crystal plasticity parameters are calibrated with the data of quasi-static tensile and constant load creep tests at ambient and intermediate temperatures. In particular, the rate-dependent parameters related to slip property are employed to establish the relationship between strain rate sensitivity of a soft α-titanium single crystal and temperature. The variation of strain rate sensitivity with temperature influences the stress redistribution occurring within the hard-soft grain combination, which in turn affects the dwell fatigue sensitivity. Further, the structural analysis for a Ti-6Al-4V fan disc provides the stress fields at takeoff and cruise phases to examine the in-service stress redistribution. The highly localized hoop stress and presence of large macrozones at the bore of the fan disc, rather than the fair rate sensitivity of soft macrozones at working temperatures, are responsible for triggering basal stress enhancement and dwell facet nucleation in hard macrozones.

3D optimization of the hollow fan blade internal structure

Design optimization to reduce fan impeller mass is an important and urgent task. One way to solve the problem is to design a hollow blade. In addition to lightening the blade this also reduces the fan disk load, that in turn makes it possible to minimize the weight of the disk and the weight of the impeller as a whole. The hollow blade has a complex shape, and this significantly increases development time because modification of the structure in order to define its optimal shape is a very time-consuming and costly task. Therefore, automating this process and transforming it into an optimization task is very important and relevant. In this paper, an approach to optimal hollow fan blade design in the form of an optimization problem in 3D formulation is developed. To do this, a parameterized 3D model of the hollow blade was created in the ANSYS APDL environment. Using the developed model, a comparative optimization of a typical hollow blade was carried out.

Containment and Arrest of Blade Shedding in Gas Turbine Engines Using Novel Dual-Ring Design

Abstract In this paper, we examine the energy absorption and containment capabilities of a newly proposed dual-ring design accounting for interactions between a released blade and fully bladed fan disk using three-dimensional finite element analysis. The components of this dual-ring design are strategically selected to ensure high energy absorption and high impact resistance, thus leading to reduced damage of the disk and increased safety. Three containment ring designs are examined: (i) conventional single-ring design composed of one of titanium, aluminum, or Kevlar; (ii) a newly proposed aluminum-Kevlar dual-ring arrangement; and (iii) dual-ring arrangement with an interfacial gap between them to arrest and contain the released blade and ensure free passage of the trailing blades. The results of our numerical simulations indicate that although the single-ring design resists penetration and contains the released blade within the confines of the disk, it does not remove the released blade from the path of the trailing blades leading to severe damage to the fan disk. On the contrary, our new dual-ring design, which contains an interfacial gap, has potential to successfully arrest the released blade within the confines of the ring and out of the path of the trailing blades. This design can significantly reduce the impact damage to the fan disk and reduce kinetic energy of the released blade to near zero in less than half a rotation of the fan disk.

Experimental and analytical investigation for fatigue crack growth characteristics of an aero-engine fan disc

Crack growth evaluation should be performed to certify that the fan disc is capable of maintaining adequate integrity in presence of cracks. The spin test was carried out using the pre-flawed fan disc to obtain a more intensive insight of crack growth characteristic in the bore area. The variation of the surface crack length was measured by the replication method, and then the crack surface fractography was observed with scanning electron microscope (SEM). Both 3D numerical simulation and rectangular plate model were utilized to predict growth behavior of this crack. The crack surface fractography revealed a typical striated transgranular fatigue fracture characteristic. The comparison of the spin test result and the predicted crack growth showed that the numerical and theoretical analysis provided a good prediction for crack growth behavior for the fan disc. Taking into account the effect of plasticity-induced crack closure, the crack growth disparity at the disk surface and interior was analyzed and as a result, the shape of crack propagation front can be described more realistically.

Effect of Cooling Rate on Microstructure and Hardness during Solution Treatment and Aging Process of Ti-6Al-4V Alloy for Aerospace Components

The microstructure and hardness of Ti-6Al-4V alloys were evaluated before and after aging to investigate the effect of the cooling rate at the solution treatment stage of the solution treatment and aging (STA) process for aerospace components. The cooling rate distribution across a Ti-6Al-4V alloy sample representing practical STA process of an aerospace fan disk component was estimated using finite element analysis. The Ti-6Al-4V alloy specimens were solution-treated with the obtained cooling rates, and aged. Experimental WQ of the Ti-6Al-4V alloy specimen in the solution treatment facilitated the retention of the primary α phase, while the β phase was transformed to α′. Controlled cooling at slower rates compared to WQ led to the formation of coarser globular α and the transformation of β to α+β lamellae. Subsequent aging of the specimens led to the observation of α+β lamellae in all specimens regardless of the cooling conditions, where the average thickness of the plates in lamellae was higher at slower cooling rates. The specimens exhibited higher hardness values when cooled more quickly at the solution treatment, which was attributed to the formation of thinner α plates and/or a higher α′ fraction. The hardness value of all specimens increased further after aging, regardless of the cooling conditions, due to precipitation of Ti3Al in the α phase and/or fine α plates from the α′ phase.

Containment of blade shedding in gas turbine engines: part II—experimental and numerical investigations

In Part I, we purposely designed, developed and commissioned an instrumented functional scaled down test rig to examine blade shedding in gas turbine engine experimentally and numerically using the finite element method. Specifically, in Part II, we examine two cases involving blade shedding incidents. In the first, we study the interaction between a released and a trailing blade and in the second, we study the interaction between a released blade and a fully bladed disk. Three aspects of the work were accordingly examined in these two cases: (1) blade shedding trajectory, its interaction with the trailing blades and the containment ring, (2) the contact stresses in the containment ring as a result of the impingement of the released blade, and (3) the dynamic forces transmitted to the rotor mounting. Remarkably, the experimental results of the scaled down tests confirm the finite element predictions of a typical fan disk. In particular, they clearly show the influence of the trailing blades on the trajectory of the released blades, the large deformation of the trailing blades as a result of the momentum transfer between the trailing blades and the released blade, the final confinement of the released blade near the tip of the trailing blades and the stresses induced in the containment ring as a result of contact impingement. The results of our work should aid in the design of a proper containment ring and enhance the safety of gas turbine engines.

Modeling and characterization of bilayer containment ring in gas turbine engine

In this paper, we provide comprehensive study of lightweight bilayer containment rings in gas turbine engines to contain a released blade. Three different materials were considered to form bilayer rings: aluminum foam, titanium, and Kevlar fabric. The effectiveness of the bilayer combinations is established through fundamental oblique impact studies on a planar target and blade shedding response of a fan disc. The effect of a gap between the two layers on its shock absorption and impact response is also evaluated. The results of our studies indicate that a bilayer aluminum foam-Kevlar ring with a gap offers greater energy absorption capability.

Research on the prediction method of unbalance responses of dual-rotor system based on surrogate models

In this work, the prediction methods of unbalance responses based on the surrogate models were studied, where the simulation data of vibration responses of a dual-rotor system with four disks and five supportings were involved. Firstly, based on the Latin hypercube sampling and random sampling with uniform distribution of unbalance distribution of the fan disc and the hyper-compressor disc, the input variables of the training samples and the testing samples were respectively obtained. According to the sampling results, the multi-body dynamics simulations were conducted to extract the vibration responses at the corresponding measuring points as the output variables of the sample space. Then, the algorithms of multivariate adaptive regression splines (MARS), radial basis function (RBF) and Kriging, were selected to respectively construct the response-predicted models of the rotor system. Finally, predicted vibration responses were figured out by surrogate models and the prediction accuracies were verified by comparison with output parameters in the testing samples. The results showed that the prediction methods of unbalance responses based on MARS, RBF and Kriging enjoyed high prediction accuracies according to the standards, which were proved to be feasible in theoretically.

Contact Stress Analysis and Fatigue Life Prediction of a Turbine Fan Disc

AbstractFan discs are critical components of an aero engine. In this paper, contact stress and life prediction of a turbine fan disc were investigated. A simplified pin/disc model was conducted to simulate the practical working condition under applied loads using finite element (FE) analysis. This study is devoted to examining the effects of interface condition of pin/disc such as gap and coefficient upon the maximum stress. The FE model indicated that the maximum stress occurs at the top right corner in the second pin hole, and larger gap or friction coefficient has a significant effect on the maximum stress. In addition, FE analysis without considering friction is also conducted. The results show that the dangerous point is similar to the result which considers friction and the stress state is relatively larger than that of considering friction. Finally, based on FE analysis result, life prediction for the fan disc is conducted to combine the material

비행시험과 전산해석을 통한 소형무인기 항력 예측

This paper presents the procedure of drag prediction for EAV-1, based on a numerical analysis correlated to an in-flight test. EAV-1, developed by Korea Aerospace Research Institute, is a small-sized UAV to test a hydrogen-fuel cell power system. The long-endurance test flight of 4.5 hours provides numerous in-flight data. The thrust and drag of EAV-1 during the flight test are estimated based on the wind-tunnel test results for EAV-1`s propeller performance. In addition, the CFD analysis using a commercial Navier-Stokes code is carried out for the full-scale EAV-1. The computational result suggests that the initial CFD analysis substantially under-predicts the in-flight drag in that the discrepancy is up to 27.6%. Therefore, additional investigation for more accurate drag prediction is performed; the effect of propeller slipstream is included in the CFD analysis through fan disk modelling. Also, the additional drag from airplane trim and load factor that actually exists during the flight test in a circular path is considered. These supplemental analyses for drag prediction turn out to be effective since the drag discrepancy reduces to 2.3%.

Titanium alloy developments for aeroengine fan systems

Recent research relating to two α+β processed titanium alloys, designed for specific employment in aeroengine fan disc and aerofoil components, is described. Timetal 575 (Ti575) and Timetal 639 (Ti639) are both novel compositions that provide relatively high strength compared to Ti–6Al–4V. Static and cyclic behaviour of the two developmental alloys were evaluated at room temperature. A dwell fatigue assessment of Ti575 is reported, while the effects of unidirectional rolling and associated evolution of microtexture were studied for Ti639.

Titanium Alloy Developments For Future Fan Disc Applications – The Fatigue Response of <i>“Alloy 104”</i>

Alloy 104 is a novel high strength, α+β titanium alloy primarily aimed at aero-engine fan disc applications. Two microstructural variants of Alloy 104 have been assessed. Room temperature tensile strength and elongation have been investigated alongside a more detailed study of low and high cycle fatigue behaviour. The alloy clearly demonstrated an improved fatigue resistance in both microstructural conditions, whilst maintaining forgeability and a comparable density to Ti-6Al-4V. Furthermore, the alloy has been subjected to a load regime with a hold period at peak loads and proven to be insensitive to dwell fatigue.

Bladed disc crack diagnostics using blade passage signals

One of the major potential faults in a turbo fan engine is the crack initiation and propagation in bladed discs under cyclic loads that could result in the breakdown of the engines if not detected at an early stage. Reliable fault detection techniques are therefore in demand to reduce maintenance cost and prevent catastrophic failures. Although a number of approaches have been reported in the literature, it remains very challenging to develop a reliable technique to accurately estimate the health condition of a rotating bladed disc. Correspondingly, this paper presents a novel technique for bladed disc crack detection through two sequential signal processing stages: (1) signal preprocessing that aims to eliminate the noises in the blade passage signals; (2) signal postprocessing that intends to identify the crack location. In the first stage, physics-based modeling and interpretation are established to help characterize the noises. The crack initiation can be determined based on the calculated health monitoring index derived from the sinusoidal effects. In the second stage, the crack is located through advanced detrended fluctuation analysis of the preprocessed data. The proposed technique is validated using a set of spin rig test data (i.e. tip clearance and time of arrival) that was acquired during a test conducted on a bladed military engine fan disc. The test results have demonstrated that the developed technique is an effective approach for identifying and locating the incipient crack that occurs at the root of a bladed disc.