Blade Off Research Articles

Research on active vibration control of blade using variable-step-size filtered-x least mean square algorithm

The engine blade is one of the important parts of the aero engine, vibration can lead to engine blade off or fracture. Active Vibration Control (AVC) has a higher flexibility is increasingly used to suppress the vibration of the blade. The filtered-x least mean square (FxLMS) algorithm is an adaptive algorithm commonly used in active vibration control, but its fixed step size has a contradiction between convergence speed and steady-state error. A new variable-step-size FxLMS (VSSFxLMS) algorithm based on hyperbolic secant function was proposed, and the sech function ensures the control signal is bounded. In the early stage, a larger step size enables the error to decrease rapidly. The step size decreases as the error decreases, and a smaller step size ensures a smaller steady-state error. Firstly, this paper summarizes three VSSFxLMS algorithms and proposes an improved algorithm; Secondly, it compares and analyses the mentioned algorithms; Then, a joint simulation platform using ADAMS and Simulink is built to verify the effectiveness of the proposed algorithms; Finally, an experimental platform was built to complete the blade vibration active control experiments using several VSSFxLMS algorithms, and the results of the experiment verify the better vibration suppression effect of the proposed algorithm.

Theoretical study on the propagation of high impact energy in the rotor with local plastic deformation after blade off

A theoretical model of an elastic supported rotor system under impact by blade off is established based on d'Alembert principle. The gyroscopic moment of the blade-disk with asymmetrical inertia is considered in the dynamic model. The formation criterion of the plastic hinge in the cylinder rotor are derived and applied in the dynamic model to consider the local plastic deformation of the rotor with large deformation. This model extends the impact model of non-rotating beam with rigid support to the rotating beam and can describe the main mechanical properties of the rotor with blade off. The influence of the main factors, such as sudden unbalance, gyroscopic moment, geometric dimensions and concentrated mass, on the propagation of the impact energy in the rotor are analyzed. The gyroscopic moment can suppress the wave propagation characteristics of the impact energy in a rotating beam, resulting in an overall bending deformation, compared with the wave propagation characteristics of the impact energy in the non-rotating beam. Larger local concentrated mass can reduce the effect of impact, and increasing the cross-section size can significantly improve the plastic limit bending moment of the rotor. The variation of the bending moment in the low-pressure rotor is mainly determined by gyroscopic moment and bending deformation. Due to the constraint of the 1# and 2# pivots, the strain energy generated by fan blade off is mainly distributed in the shaft segment in front of the 2# pivot. Therefore, the impact resistance of this shaft segment should be improved to fulfill the safety design requirement of fan blade off failure.

Probabilistic method for impact load estimation by Stochastic Model – a Fan Blade Off event

PurposeThe front bearing mount structure in an aero engine has been severely loaded under the fan blade off (FBO) event since imbalance forces at high amplitude but low frequency is transformed to the engine front mount structure. The bearing mount structural forces are estimated by an integrated implicit-explicit analysis process of whole engine model of an aero engine. Since there are many dependent factors which are governing those predicted loads, experimental evidence on FBO is becoming necessary to validate the model used for the load prediction which is more expensive and also time consuming. This paper aims to discuss the above mentioned issues.Design/methodology/approachThe current paper deals with the high impact but low probability nature of FBO load prediction on the bearing mount structure by stochastic approach which could be replaced for FBO experiments which is highly essential for current economic conditions. Several influential factors on the predicted loads have been chosen in the stochastic model and sensitive analysis has also been performed to bring down the variation involved in the predicted load.FindingsThe predicted load by proposed stochastic model is then compared with the experimental results. The conclusion on the predicted load with various dependent influential factors is matching well with certain value of damage factor from planned FBO test event.Research limitations/implicationsLimitation of this paper could be that it does not cover with range of load amplitude and is only applicable for civil small and medium engines.Practical implicationsThe high amplitude but low frequency load pattern is assessed with impact condition by stochastic model.Originality/valueCombining experimental and probabilistic load prediction was never done before and read across from previous engine test program could be effectively performed with stochastic model approach.

A Reduced-Order Modeling Framework for Simulating Signatures of Faults in a Bladed Disk

This paper reports a reduced-order modeling framework of bladed disks on a rotating shaft to simulate the vibration signature of faults like cracks in different components aiming towards simulated data-driven machine learning. We have employed lumped and one-dimensional analytical models of the subcomponents for better insight into the complex dynamic response. The framework seeks to address some of the challenges encountered in analyzing and optimizing fault detection and identification schemes for health monitoring of rotating turbomachinery, including aero-engines. We model the bladed disks and shafts by combining lumped elements and one-dimensional finite elements, leading to a coupled system. The simulation results are in good agreement with previously published data. We model the cracks in a blade analytically with their effective reduced stiffness approximation. Multiple types of faults are modeled, including cracks in the blades of single and two-stage bladed disks, Fan Blade Off (FBO), and Foreign Object Damage (FOD). We have applied aero-engine operational loading conditions to simulate realistic scenarios of online health monitoring. The proposed reduced-order simulation framework will have applications in probabilistic signal modeling, machine learning toward fault signature identification, and parameter estimation with measured vibration signals.

Dynamic Modeling and Vibration Characteristics for a High-Speed Aero-Engine Rotor with Blade Off

Blade off that occurs during operation will generate a sudden imbalance excitation and make the rotor become inertially asymmetric, which leads to large instantaneous impact load and induces more complex rotor dynamic phenomena. In order to study the transient dynamic characteristics for complex rotors suffering from blade off, a mathematical model for solving the response of the gas generator rotor in the aero-turboshaft engine is established based on the FE method and DOF condensation, in which the complex structural characteristics, transient impact load, and inertia asymmetry of the rotor are considered. The complex impeller structure is modeled by piecewise linear fitting with cylindrical beam elements and tapered beam elements. Without loss of generality, the modeling method suitable for complex rotors is verified through a general complex test rotor with modal experiments. Based on this, the responses are solved for carrying out parametric studies and an understanding of the transient dynamic characteristics of the rotor under the extreme working conditions of blade off. The results show that the blade off has a great impact effect on the time-domain waveform, frequency components, and rotor orbits. At the instantaneous stage after blade off, the complex motion is composed of synchronous motion and some lower-order natural modes excited by blade off. Although the transient responses with blade off at different rotational speeds have similar time-varying characteristics, the impact factor is sensitive to the rotating speed. Most important is that the parameter of the blade off location will not only have a significant effect on the impact factor, but also on the frequency spectrum. These dynamic characteristics as well as impact effect provide certain guidance for the fault recognition and dynamic analysis to these complex rotors suffering blade off.

Investigation on nonlinear lateral-torsional coupled vibration of a rotor system with substantial unbalance

Substantial unbalance may be caused by fan blade off during the operation period of gas turbine engines, and related dynamic problems are very critical to the safety design of rotor system in aero-engine. This article aims to understand lateral-torsional coupled vibration of the rotor system with substantial unbalance. The governing equation of a modified unbalanced rotor system is established based on Lagrangian approach. Then, a mathematical analytical method is proposed in which a linear approximation is derived and the Floquet theory and Hill’s method are incorporated, from which the modal characteristics of the unbalanced rotor are obtained. The modal characteristics of the unbalanced rotor system are revealed comprehensively for the first time. Furthermore, the relation between the modes and responses of the unbalanced rotor is discussed in detail. The results show that the lateral vibration and torsional vibration of the unbalanced rotor are coupled through the inertial terms in the governing equations. Due to the coupling, veering and lock-in phenomena occur between the frequencies of the forward whirl mode and the torsional mode. Furthermore, lock-in can lead to a kind of principal instability. With regard to the response of the unbalanced rotor, both natural vibration components and enforced vibration components appear in the lateral response, while only natural vibration components appear during torsional vibration. Moreover, natural vibration components play a crucial role in the response within the principal instability region and cause divergence of the vibration amplitudes in the lateral and torsional directions.

Prediction of transient vibration response of dual-rotor-blade-casing system with blade off

Fan blade off occurring in a running rotor of the turbofan engine dual-rotor system will cause a sudden unbalance and inertia asymmetry, which results in large impact load and consequently induces the rubbing between blade and casing. In order to reveal the transient dynamic response characteristics of actual aero-engine when fan blade off event occurs, the dynamic model of dual-rotor-blade-casing system is developed, in which the distribution characteristics of the stiffness and mass, the load transfer, and the coupling effects of dual-rotor and casing are included. Considering several excitations caused by blade off, the physical process and mechanical characteristics of the fan blade off event are described qualitatively. Considering that only the casing acceleration signal can be used for condition monitoring in actual aero-engine, the transient response including rotor vibration displacement and casing vibration acceleration during the instantaneous status are obtained. Due to the time-varying and highly nonlinear characteristics of vibration responses, frequency slice wavelet transform is employed to isolate the vibration signal features. The results show that the impact load induced by the sudden imbalance causes significant increase of vibration amplitude. The rubbing action between blade and rotor will impose constraint effects on the rotor, which decreases the transient vibration amplitude. The inertia asymmetry has a big impact on the transient response. The vibration characteristics of casing acceleration under blade off are similar to those of rotor displacement, while casing acceleration response attenuates to stable value faster and is more sensitive to high-frequency components of vibration.

Study on Vibration Characteristics of Fan Shaft of Geared Turbofan Engine With Sudden Imbalance Caused by Blade Off

Based on the requirements of the dynamic design of geared turbofan (GTF) engines, the vibration characteristic of the fan shaft is investigated. The effect of sudden imbalance caused by blade off, the time-varying meshing stiffness, and meshing errors on the vibration characteristics is fully considered in the dynamic model. The improved Euler–Bernoulli beam element considering the effects of shear deformation is employed and a coupled relationship between the gear–shaft–bearing–casing is established. Under windmilling condition with a single blade completely lost, the vibration characteristics of the fan shaft of the turbofan engine with and without a gearbox system are compared. The effect of the gear system on the vibration of the fan shaft under different rotating speeds is examined. The results show that the orbit of the fan shaft center in the turbofan engine with the gearbox system exhibits a multifrequency whirling motion and has a stable limit cycle. Under windmilling condition, the meshing frequency and the modal frequency have multiple intersection points. The critical speed is dense and the peak value of the transient vibration of the GTF engine gearbox shows a wave rise with an increase in speed. The results of this study could provide a reference for the parameter design and optimization of GTF engines.

Numerical simulation methodology of multi-layer Kevlar 49 woven fabrics in aircraft engine containment application

ABSTRACTA numerical simulation method was developed to provide effective tools to estimate the ballistic resistance of Kevlar 49 fabric for gas turbine engine containment system in LS-DYNA. Material model based on representative unit cell and three-element viscoelastic constitutive equation was employed to describe the dynamic response of single-layer and multi-layer fabrics. The influences of mesh size, hourglass control method in the material model were discussed. The numerical simulation method is used to predict the ballistic performance of Kevlar fabrics in testing condition. The failure mechanism, the stress wave propagation and energy absorption mechanism were investigated and compared with testing results and achieved good agreements. It is indicated that the developed numerical method provides promising results in modelling the high-speed ballistic impact events and engine fan blade off events.

Dynamic behavior of aero-engine rotor with fusing design suffering blade off

Fan blade off (FBO) from a running turbofan rotor will introduce sudden unbalance into the dynamical system, which will lead to the rub-impact, the asymmetry of rotor and a series of interesting dynamic behavior. The paper first presents a theoretical study on the response excited by sudden unbalance. The results reveal that the reaction force of the bearing located near the fan could always reach a very high value which may lead to the crush of ball, journal sticking, high stress on the other components and some other failures to endanger the safety of engine in FBO event. Therefore, the dynamic influence of a safety design named “fusing” is investigated by mechanism analysis. Meantime, an explicit FBO model is established to simulate the FBO event, and evaluate the effectiveness and potential dynamic influence of fusing design. The results show that the fusing design could reduce the vibration amplitude of rotor, the reaction force on most bearings and loads on mounts, but the sudden change of support stiffness induced by fusing could produce an impact effect which will couple with the influence of sudden unbalance. Therefore, the implementation of the design should be considered carefully with optimized parameters in actual aero-engine.

Comparison of simulations on the NewMexico rotor operating in pitch fault conditions

The present paper aims to simulate and reproduce the experiments on a imbalanced rotor with one blade off pitched by 20degees carried out within the IEA task 29 Mexnext Phase III. Models of increasing complexity were used (Blade Element Method simulation (BEM) lifting line with free vortex wake (FVW) and URANS CFD) and the results were compared. The rotor has a diameter of 4.5m and three different load cases corresponding to a tip speed ratio of 5.24, 7.6 and11.2 were chosen from the test matrix performed during the experiments. The rotor speed was kept constant at 324.9RPM. BEM and FVW simulations were performed using the Aero Module tool developed by ECN that offers both an advanced BEM formulation and a FVW formulation named AWSM. The commercial solver STARCCM+ was for the CFD simulations and an Unsteady RANS approach with the k-ra SST turbulence model was chosen. Loads recorded with strain gauges during the experiments and pressures on the equipped blades were compared with results from the numerical simulations. The comparison provided good agreement in the low tip speed ratio cases while differences were more noticeable for the highest tip speed ration load case.

Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures

An experimental technique has been developed which allows loading of heated sheet material under impact conditions with simultaneous measurement of the impact force history. The combined characteristics of impact loading at elevated temperature makes the experiment ideal for validation of models used to simulate the containment structure surrounding aircraft engines. In this paper experimental results for Alloy 718 are presented, a nickel based super alloy commonly used in hot parts of the containment structure. The experimental results are then compared to simulations in order to validate previously calibrated material parameters. The basic principle of the validation experiment is based on reverse ballistics, in which a thin circular specimen with free boundaries impacts the end of an instrumented rod. Using induction heating the specimen is heated to temperatures up to 650°C and a gun driven by compressed air accelerates the specimens to desired velocity. In the reported work velocities are kept low enough to avoid cracking and thus the study is limited to plastic conditions, even though the technique is applicable also for fracture studies. The free boundaries of the experiment makes numerical modelling and simulation straightforward, making it valuable as a validation tool. All numerical simulations are performed using the commercial finite element code LS-Dyna and plastic behaviour of the material was modelled with the Johnson-Cook material model.

Theoretical and experimental investigation on the sudden unbalance and rub-impact in rotor system caused by blade off

Blade loss from a running turbofan rotor will introduce sudden unbalance into the dynamical system, and as a consequence leads to the rub-impact, the asymmetry of rotor and a series of interesting dynamic characteristics. The paper focuses on the theoretical study on the sudden unbalance and rub-impact caused by blade loss, in particular investigates the response of the rotor on a rotor test rig with sudden unbalance and rub-impact device designed respectively. The results reveal that the sudden unbalance will induce impact effect on the rotor, and critical speed frequency is excited in frequency spectrum. Meantime, the impact effect is more obvious for the rotor operating above critical speed. The influence of rub-impact is considered as additional constraint to the rotor, analyzed by the theory of time-varying system for the first time, and the results are evaluated by experimental tests. The study shows that great attention should be paid to the dynamical design for the overhung rotor system, additional constraint and corresponding analysis method in rub-impact need to be intensively studied.

Fatigue life estimation of aero engine mount structure using Monte Carlo simulation

The estimation of fatigue life of mount structures in aero engine has become crucial for safe fly-home period in the event of a fan blade off situation since it has many random factors which will be influencing the durability of these structures either direct or indirect way. The imbalanced rotation during windmilling post fan blade off (FBO) creates vibratory loads of such low frequency but in high amplitude that leads to have the shorter fatigue life of these mount structures. Regression analysis during the evaluation of fatigue life under this circumstance provides the frequency of the front rotor shaft as most influencing parameter affecting the durability of the mount structures even though there are many other factors like, tribological conditions, material properties, impact fraction, environmental and thermal effects. Since it has low probability of occurrences but with high impact, the degree of conservatism in the estimated life is still hidden. Monte Carlo simulation of randomly selected lives from a predicted distribution of all dependent factors brings the variations in fatigue life of the mount structure. This can be estimated as a function of populated size. The mean and standard deviation of the simulated fatigue life converges with more number of randomly varied samples.

Ratcheting behavior of bearing support structures during windmilling in aero engines*

Abstract A major safety concern in commercial civil aviation is losing a fan blade from jet engine during its normal operation either due to bird strike or limited fatigue. The assessment of fatigue strength of the support structures like bearing and intermediate connection in aero engine during windmilling has become an essential requirement for safe-fly-home situation. Windmilling is the rotation of a non-operating engine due to the airflow induced forces on the blades caused by the forward motion of the aircraft. The imbalanced rotation during windmilling post fan blade off (FBO) creates unavoidable vibratory loads in transient nature which leads to shorter fatigue life of the support structures. Since the repeated imbalance loads beyond elastic limit of the structure can cause the accumulation of plastic deformation, which exceeds the critical strain limit of the structure. This article deals with the ratcheting behavior (low cycle fatigue) of bearing support structures during windmilling post FBO in aero engine. When the accumulated plastic strain exceeds the critical strain limit of the bearing structure, then it experiences ratcheting fatigue failure.

Bi-objective optimization of pylon-engine-nacelle assembly: weight vs. tip clearance criterion

A realistic application of advanced structural and multi-objective optimization for the design of a fully assembled aircraft powerplant installation is presented. As opposed to the classical design process of powerplant installation that does not consider the influence of pylon sizing over engine efficiency, we develop in the present a fully integrated approach where both pylon and compressor intercase are designed at once. The main objective is to consider the impact of weight over tip clearance performance criterion and see how these two objectives are antagonistic. In this work, we perform in the same design session tasks traditionally devoted to the airframe manufacturer and aero-engine manufacturer. The overall weight of the assembly is minimized with respect to Specific Fuel Consumption (SFC) criterion. One interesting aspect of the process is that SFC criterion is based on highly proprietary models and its simulation and call within an optimization process is made available through the development of a webservice. One major phenomenon to consider in both pylon and engine design is Fan Blade Off (FBO) event, i.e. the sudden release of a blade. This event causes high impact loads and must be considered carefully in the design. Such a simulation is not an easy task and several nonlinear phenomena must be addressed (e.g. rotordynamics), not to mention the integration of this nonlinear dynamic response in a static structural optimization process. This article describes how the design of the full assembly is performed taking into account both objectives. Such a problem lies in multi-objective optimization field and then we describe the method we use to solve such a problem. The simulation of an FBO post-impact rotor dynamics is also described and we end up with the final results that show the influence of pylon-engine weight sizing over SFC.

Material development for fan blade containment casing

This paper describes the physics reasoning and the engineering development process for the structured material system adopted for the containment system of the Trent 900 engine. This is the Rolls-Royce engine that powers the Airbus A380 double-decker aeroplane, which is on the point of entering service.The fan blade containment casing is the near cylindrical casing that surrounds the fan blades at the front of the engine. The fan blades provide the main part of the thrust of the engine; the power to the fan is provided through a shaft from the turbine. The fan is approximately three meters in diameter, with the tips of the blade travelling at a little over Mach speed. The purpose of the containment system is to catch and contain a blade in the extremely unlikely event of a part or whole blade becoming detached. This is known as a ‘‘Fan Blade Off (FBO)’’ event. The requirement is that no high-energy fragments should escape the containment system; this is essential to prevent damage to other engines or to the fuselage of the aircraft.Traditionally the containment system philosophy has been to provide a sufficiently thick solid metallic skin that the blade cannot penetrate. Obviously, this is heavy. A good choice of metal in this case is a highly ductile steel, which arrests the kinetic energy of the blade through plastic deformation, and possibly, a controlled amount of cracking. This is known as ‘‘hard wall’’ containment. More recently, to reduce weight, containment systems have incorporated a Kevlar fibre wrap. In this case, the thinner metallic wall provides some containment, which is backed up by the stretching of the Kevlar fibres. This is known as ‘‘soft wall’’ containment; but it suffers the disadvantage of requiring a large empty volume in the nacelle in to which to expand.For the Trent 900 engine, there was a requirement to make a substantial weight saving while still adopting a hard wall style of containment system. To achieve this, a hollow structured material system was developed, with much of the kinetic energy arrest provided by the mechanism of crushing. A number of structural elements were included within the containment system to maximise the area of material involved in the arrest and thereby minimise the overall weight.

High strain rate and high temperature behaviour of metallic materials for jet engine turbine containment

This work presents a study on the mechanical characterisation of the materials involved in air jet engine turbines. The final objective is to analyse the phenomenon of a turbine blade off failure, to verify the requirements of the case containment. The materials in the turbine are under high temperatures, ranging from 400°C to 800' C and when the fail of the blade occurs if impacts against the case, reaching strain rates up to 10 3 s -1 . To obtain the behaviour of the materials, testing at high strain rate and high temperature at one time is necessary. The experimental set-up used was a split Hopkinson pressure bar, with a high temperature furnace adapted. The bars used on the device were high strength nickel alloys with a cooling system to decrease the temperature of the measurement devices. The effect of wave dispersion due to the temperature gradient has been also studied to correct the measurements if necessary. The material tested has been the FV535 stainless steel used on the case. The full stress-strain curves at different temperatures and at strain rates up to 10 3 s -1 have been obtained. The experimental results show a marked influence of the strain rate and the temperature that cannot be neglected. The Johnson-Cook material model has been used to fit the results of the material tests.