Monitoring low cycle fatigue damage in turbine blade using vibration characteristics (original) (raw)
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Study of fatigue damage in wind turbine blades
Engineering Failure Analysis, 2009
The inspection of damages detected in some blades of 300 kW wind turbines revealed that the nature of these damages was probably due to a fatigue mechanism. The causes that had originated the failure (superficial cracks, geometric concentrator, abrupt change of thickness) have been studied, verifying, by means of the simplified evaluation procedure of fatigue life of the ''Germanischer Lloyd" (GL) standard, that these causes can explain the failure detected in the period of time in which it happened.
Dynamic stress analysis and a fracture mechanics approach to life prediction of turbine blades
Mechanism and Machine Theory, 1997
Emerging blade technologies are finding it increasingly essential to correlate blade vibrations to blade fatigue in order to assess the residual life of existing blading and for development of newer designs. In this paper an analytical code for dynamic stress analysis and fatigue life prediction of blades is presented. The life prediction algorithm is based on a combination method, which combines the local strain approach to predict the initiation life and fracture mechanics approach to predict the propagation life, to estimate the total fatigue life. The conventional stress based approach involving von Mises theory along with S-N-Mean stress diagram suffers from the drawback that it does not make allowance for the possibility of development of plastic strain zones, especially in cases of low cycle fatigue. In the present paper, strain life concepts are employed to analyse the crack initiation phenomenon. Dynamic and static stresses incurred by the blade form inputs to the life estimation algorithm. The modeling is done for a general tapered, twisted and asymmetric cross section blade mounted on a rotating disc at a stagger angle. Blade damping is non-linear in nature and a numerical technique is employed for estimation of blade stresses under typical nozzle excitation. Critical cases of resonant conditions of blade operation are considered. Neuber's rule is applied to the dynamic stresses to obtain the elasto-plastic strains and then the material hysteresis curve is used to iteratively solve for the plastic stress. Static stress effects are accounted for and crack initiation life is estimated by solving the strain life equation. Crack growth formulations are then applied to the initiated crack to analyse the propagation of crack leading to failure. The engineering approximations involved are stated and the algorithm is numerically demonstrated for typical conditions of blade operations. NOMENCLATURE A-area of cross-section a, b-coefficients in trigonmetric series of forcing functions a~, b~-initial and final crack lengths b-fatigue strength exponent c-fatigue ductility exponent C-torsional stiffness [C]~ Samping matrix D,-depth of defect E-modulus of elasticity e-engineering nominal strain F-correction factor for stress intensity factor F~, F,-forcing functions /-shape function for bending deflections /-shape function for angular deflections H,,~-rnth harmonic response in the kth mode h-shape function for bending and twisting moments L-Ao(I-z) + 1 A I (F-z 2) + " • • , A.
A multi-frequency fatigue testing method for wind turbine rotor blades
Journal of Sound and Vibration, 2017
Rotor blades are among the most delicate components of modern wind turbines. Reliability is a crucial aspect, since blades shall ideally remain free of failure under ultra-high cycle loading conditions throughout their designated lifetime of 20-25 years. Full-scale blade tests are the most accurate means to experimentally simulate damage evolution under operating conditions, and are therefore used to demonstrate that a blade type fulfils the reliability requirements to an acceptable degree of confidence. The state-of-the-art testing method for rotor blades in industry is based on resonance excitation where typically a rotating mass excites the blade close to its first natural frequency. During operation the blade response due to external forcing is governed by a weighted combination of its eigenmodes. Current test methodologies which only utilise the lowest eigenfrequency induce a fictitious damage where additional tuning masses are required to recover the desired damage distribution. Even with the commonly adopted amplitude upscaling technique fatigue tests remain a time-consuming and costly endeavour. The application of tuning masses increases the complexity of the problem by lowering the natural frequency of the blade and therefore increasing the testing time. The novel method presented in this paper aims at shortening the duration of the state-of-the-art fatigue testing method by simultaneously exciting the blade with a combination of two or more eigenfrequencies. Taking advantage of the different shapes of the excited eigenmodes, the actual spatial damage distribution can be more realistically simulated in the tests by tuning the excitation force amplitudes rather than adding tuning masses. This implies that in portions of the blade the lowest mode is governing the damage whereas in others higher modes contribute more significantly due to their higher cycle count. A numerical feasibility study based on a publicly available large utility rotor blade is used to demonstrate the ability of the proposed approach to outperform the state-of-the-art testing method without compromising fatigue test requirements. It will be shown that the novel method shortens the testing time and renders the damage evolution with a higher degree of fidelity.
Volume 10B: Structures and Dynamics, 2020
A method of fluid-structure interaction coupling is implemented for a forced-response, vibration-induced fatigue life estimation of a high-pressure turbine blade. Two simulations approaches; a two-way (fully-coupled) and one-way (uncoupled) methods are implemented to investigate the influence of fluidsolid coupling on a turbine blade structural response. The fatigue analysis is performed using the frequency domain spectral moments estimated from the response power spectral density of the two simulation cases. The method is demonstrated in light of the time-domain method of the rainflow cycle counting method with mean stress correction. Correspondingly, the mean stress and multiaxiality effects are also accounted for in the frequency domain spectral approach. In the mean stress case, a multiplication coefficient is derived based on the Morrow equation, while the case of multiaxiality is based on a criterion which reduces the triaxial stress state to an equivalent uniaxial stress usin...
Wind Engineering, 2019
The wind energy has been recognised as one of the rising sustainable energies in the world. The wind turbines are subjected to high aerodynamic loads and they cause vibrations due to the wake formation. The magnitude of the applied loads has significant effects on the crack propagation. The fatigue loads have been identified as one of the key sources of damage, with delamination as the main cause for the failure of the turbine blades. The article presents a review of fatigue damages that have been experienced in the wind turbine blades, and factors that are influenced due to the fatigue loads are discussed. The causes and effects of the fatigue loads have been highlighted, and the ways for preventing the fatigue damage by improving the design lifetime are mainly concentrated in review. The overall review gives an idea for determining and reducing the crack growth in wind turbine blades.
Fatigue Life Estimation Procedure for a Turbine Blade Under Transient Loads
Fatigue analysis and consequent life prediction of turbomachine blading requires the stress load history of the blade. A blade designed for safe operation at particular constant rotor speeds may, however, incur damaging stresses during start-up and shut-down operations. During such operations the blade experiences momentary resonant stresses while passing through the criticals, which may lie in the speed range through which the rotor is accelerated. Fatigue due to these transient influences may accumulate to lead to failure. In this paper a technique for fatigue damage assessment during variable-speed operations is presented. Transient resonant stresses for a blade with nonlinear damping have been determined using a numerical procedure. A fatigue damage assessment procedure is described. The fatigue failure surface is generated on the S-N-mean stress axes and Miner's Rule is employed to estimate the accumulation of fatigue.
A Fracture Mechanics Approach to Life Prediction of Turbine Blades
1993
Emerging blade technologies are finding it increasingly essential to correlate blade vibrations to blade fatigue in order to asses the residual life of existing blading and for development of newer designs. In this paper an analytical code for Dynamic Stress Analysis and Fatigue Life Prediction of blades is presented. The life prediction algorithm is based on a combination method, which combines the local strain approach to predict the initiation life and fracture mechanics approach to predict the propagation life, to estimate the total fatigue life. The conventional stress based approach involving von Mises theory along with S-N-Mean stress diagram suffer from the drawback that they do not make allowance for the possibility of development of plastic strain zones, especially in cases of low cycle fatigue. In the present paper, strain life concepts are employed to analyse the crack initiation phenomenon. Dynamic and static stresses incurred by the blade form inputs to the life estima...
Nonlinear vibrations and long-term strength of turbine blades
2010
The method of a durability estimation of rotating turbomachinery blades at forced flexural-flexural-torsional vibrations is offered. The method is based on the methods of Continuous Damage Mechanics and the accurate strain analysis of the pre-twisted blades at the nonlinear vibrations with moderate displacements. The method to solve the strain analysis problem and turbomachinery blades high-cycle fatigue damage estimation as a result of nonlinear vibrations is presented.
Procedia Structural Integrity, 2022
Vibration diagnostics of damage belongs to the class of non-destructive methods, which usually do not take long time. However, the main problem of vibration diagnostics is relatively low sensitivity to the critical damage of fatigue crack type, which arises because of long time accumulation of plastic deformation. To improve the sensitivity and reliability of vibration diagnostics of damage two methods were considered. The first method was based on the fact, that a characteristic feature of vibrations of structural elements with fatigue crack is the occurrence of non-linear resonances (sub-and super-harmonic) and significant nonlinearity of vibration response at these resonances. The second oneon the fact, that quite noticeable in certain cases increase of damping characteristic caused by a crack can be observed. Analytical and experimental studies of these methods were carried out as applied to the blades of aircraft gas turbine engines. As a result of the studies, the intensity of change in parameter of superharmonic resonance and in damping characteristics at different parameters of crack was determined. Besides, the experimental techniques for vibration testing of turbine blades were developed. There was demonstrated, that the sensitivity of both considered methods of vibration diagnostics is several orders of magnitude higher than the sensitivity of conventional methods based on the change in natural frequencies and mode shapes, and they can be effectively used for the diagnostics of blades on the stage of engine repair.
Structural Integrity Analysis and Life Estimation of a Gas Turbine Bladed-Disc
Procedia Structural Integrity, 2019
Turbine blades in an aero-engine are subjected to severe conditions of high temperature and pressure, which cause high levels of stress leading to crack formation and subsequent failure in service. We have investigated the influence of crack on vibration parameters of a typical aero-engine gas turbine blade and have described a life assessment approach for blades and bladed discs. A typical transport aircraft AMT (Accelerated Mission Test) cycle has been utilized for getting operating parameters. Material data is taken from tests conducted on specimens extracted from turbine disc of a transport aircraft. Initial studies are carried out on idealizations involving cantilever beams with uniform cross-section; the procedures are then extended to free-standing turbine blades with asymmetric airfoil cross section mounted at a stagger angle on a rotating disc. Dynamic characteristics of the blade are estimated and free vibrations analysis has been carried out for healthy blades and those with cracks of different sizes. Influence of crack size on natural frequencies and mode shapes is studied. Results show a difference of less than 1% in frequency for cracks less than 1mm in length; for larger crack lengths the frequency shifts are higher. Analytical results are compared with experimental tests on a Laser Doppler Vibrometer setup. Subsequently, forced vibration analysis is performed and a methodology, using Lazan's law,is developed to extract modal damping ratios from the strain energy of the blade under nozzle excitation pressure fluctuations. Modal damping ratios, thus obtained, are indicative the energy dissipation in the component under such stress conditions. The ratios show differences of the order of 5% between healthy and cracked blades for the second mode. These observations lead illustrate that modal damping has strong correlation with blade structural integrity. The possibility of employing these modal damping ratios as indicators for the presence of cracks / defects is discussed.