Blade Damage Research Articles

Application of self-healing technique to fibre reinforced polymer wind turbine blade

This paper presents a novel concept of healing some of the damages in wind turbine blades (WTBs) such as cracks and delamination. This is achieved through an inherent functioning autonomous repairing system. Such wind turbine blades have the benefit of reduced maintenance cost and increased operational period. Previous techniques of developing autonomous healing systems uses hollow glass fibres (HGFs) to deliver repairing fluids to damaged sites. HGFs have been reported with some limitations like, failure to fracture, which undermines their further usage. The self-healing technique described in this paper represents an advancement in the engineering of the delivery mechanism of a self-healing system. It is analogous to the HGF system but without the HGFs, which are replaced by multiple hollow channels created within the composite, inherently in the FRP matrix at fabrication. An in-house fabricated NACA 4412 WTB incorporating this array of network hollow channels was damaged in flexure and then autonomously repaired using the vascular channels. The blade was re-tested under flexure to ascertain the efficiency of the recovered mechanical properties.

Rotor anisotropy as a blade damage indicator for wind turbine structural health monitoring systems

Structural damage of a rotor blade causes structural anisotropy of the rotor. In rotor dynamic, the anisotropy affects the symmetry of the rotor mode shapes, and the latter can be utilized to detect the blade damage. The mode shape symmetry can be characterized by relative blades’ magnitude and phase. The study examines the potential use of these parameters as rotor damage indicators.Firstly the indicators are studied analytically using a simple 6 degrees-of-freedom model of a rotating rotor. Floquet analysis is used due to the time periodic nature of the considered system. Floquet analysis allows one to perform analytical modal decomposition of the system and study the sensitivity of the damage indicators to the amount of damage. Secondly, operational modal analysis (OMA) is involved to extract the same damage indicators from simulated experimental data, which was synthesized via numerical simulations.Finally, the same procedure was applied to operating Vestas V27 wind turbine, first using the simulated experimental data obtained by using aeroelastic simulation code HAWC2 and then using the data acquired during the measurement campaign on a real wind turbine.The study demonstrates that the proposed damage indicators are significantly more sensitive than the commonly used changes in natural frequency, and in contrast to the latter, can also pinpoint the faulty blade. It is also demonstrated that these indicators can be derived from blades vibration data obtained from real life experiment.

Lumped parameter modelling of turbine blade packets for analysis of modal characteristics and identification of damage induced mistuning

Turbine blade vibration of packeted blade-disk system is generally studied on the assumption that all the blades are identical and tuned together geometrically as well as structurally. However in practice, blade to blade variations exist and sometimes the variation may be introduced by the onset of a local damage or a crack. It has been shown that even a small mistuning can lead to stress build up through mode localisation under forced vibration. There have been many analytical and FEM based studies for response prediction and stress localisation under mistuned conditions but few have attempted identification of damage from the blade-disk vibration response. Present work has two parts; in the first part tuned blade packet is analysed using lumped parameter model and eigen value problem is formulated involving special Jacobi matrices. Modal spectrum is characterised as a function of number of blades in the packet and lacing wire to blade stiffness ratio and it is demonstrated that the ratio can be estimated from the modal spectrum. In the second part mistuning in the blade packet is considered due to both blade damage and lacing wire damage and representative modal characteristics are investigated. It is mathematically shown that in case of lacing wire damage, fundamental submodal natural frequency does not change, whereas it changes for the blade damage. This fact is exploited for the damage identification and further investigations are made for determination of possible location of the damage from the measured modal spectrum.

A study of the wear damage on gas turbine blades

Abstract In the present work, a study and description of the wear damage of the first-stage compressor blades of a gas turbine were conducted. The type of the equipment was a gas turbine Centaur 40 GT and the deterioration of the blades was after around 8000 h of service. This equipment is located in the City of Campeche in Mexico where the weather conditions are extremely warm. The gas turbine combines high performance operation with rugged industrial construction. This design provides high efficiency, low maintenance and a long service life. For power generation, the components that are on direct contact with air at different stages of operation are the compressor blades which are exposed to severe wear damage for the impact of particles, environmental contaminants such as salts, sands and sulphur. Due to this fact, the wear damage on the turbine blades was analysed. Firstly, a tribological characterization was conducted for obtaining the chemical composition of the turbine blades, which were in operation, by using energy dispersive X-ray analysis (EDS). Atomic Force Microscopy (AFM) was used to measure the roughness and has a perspective of the degradation of the surfaces of the blades after real service. Hardness tests were also conducted to determine the material that was employed to manufacture the blades. Additionally, optical microscopy and scanning electron microscopy (SEM) were used to identify the wear mechanisms on the surfaces. This allowed obtaining a more complete failure analysis. The wear modes were severe pitting action, large craters similar to those observed in solid particle erosion when the samples are impacted at normal incidence, corrosion and a few irregular scratches similar to ploughing action in abrasive wear.

On‐site inspection of potential defects in wind turbine rotor blades with thermography

AbstractRecurrent non‐destructive testing inspections are necessary to prevent damages in wind turbine rotor blades, but so far, there is no established method that detects defects in blades from greater distances – although this becomes increasingly important in the context of hardly accessible offshore wind parks.Thermography is a promising method for detecting subsurface defects, but various challenges arise when this method is applied on‐site to turbine blades in operation. Disturbing influences from the environment easily lead to a misinterpretation of thermograms (i.e. thermographic images), such as thermal signatures caused by reflections, dirt and other superficial inhomogeneities. This study explores several problems and effects that arise, when (rotating) blades are monitored with thermography.It will then be demonstrated that a meaningful defect inspection in this scenario is essentially restricted to a procedure following three steps: Firstly, calculating the so‐called difference thermograms of all blade pairs for eliminating disturbing reflections. Secondly, identifying potentially relevant signals, which are associated neither with structural features nor with dynamical effects, and the identification of these signals' allocations (through comparison of all difference thermograms with each other). And thirdly, comparing these signals with (processed) photos for excluding incorrect indications by surface effects. Unlike common thermographic analysis methods, which typically only include an aspect of this procedure, the composition presented in this contribution constitutes an advanced technique for minimizing disturbing influences in thermograms. The proposed thermographic technique enables the detection of potential subsurface defects within rotating rotor blades from greater distances – such as from the ground, air crafts or vessels. Copyright © 2015 John Wiley & Sons, Ltd.

Damage and nonlinearities detection in wind turbine blades based on strain field pattern recognition. FBGs, OBR and strain gauges comparison

A 13.5m wind turbine blade prototype was designed and made of glass fiber and vinylester resin doped with carbon nanofibers. The blade was manufactured using Light RTM (Resin Transfer Molding) as a monocoque structure with a PVC foam core.Within the presented work, a methodology for instrumenting the blade with Fiber Optic Sensors (FOS) embedded into the structure during the manufacturing process was developed. Two different FOS technologies for strain sensing were embedded into the blade: Fiber Bragg Gratings (FBGs) and a plain fiber optic for distributed sensing using an Optical Backscatter Reflectometer (OBR). Besides the FOS, traditional electrical extensometers were bonded to the surface of the blade.By using Hierarchical nonlinear principal component analysis (h-NLPCA) it was possible to perform a pattern recognition technique based on the strain field inferred from the measurements acquired by different sensors. Defects and nonlinearities could be detected during the certification testing of the blades, avoiding the premature failure of the structure.Several static tests were conducted, including a test campaign with known artificial damages induced into the structure and the sensitivity of the technique was evaluated. The results showed that every damages could be detected by using different sensing techniques.

New features of solid particle erosion damage of control stage blades in supercritical steam turbine

Reducing solid particle erosion of blades is one of the most urgent problems for high-parameter steam turbine power generation technology. Based on the erosion rate model and particle rebound model of blade materials obtained through accelerated erosion test under high temperature, erosion characteristics of flaky particles in control stage of a typical supercritical steam turbine were systemically studied using three-dimensional numerical simulation method. The erosion mechanism of hard coatings on nozzle suction surface is first revealed, and erosion resistance of boride coating with large oxide particles is validated through high-temperature erosion test. Results show that serious erosion damage of boride coating on the latter half of nozzle suction surface is caused by the combined rebounded impingement of large particles between 1000 µm and 2000 µm after their initial impingement on the nozzle pressure surface and the leading edge of rotating blade. The average particle impingement velocity can reach up to 150–180 m/s, and the impingement angle is in the range of 18°–28°. High-temperature erosion test results show that boride coating will soon broken and fall off under the continuing impact of millimeter-sized particles, which confirms that hard coatings are difficult to resist the high-intensity impingement from these large particles. Therefore, separating particles before entering the nozzle chamber with the employment of an inertial separator should be the optimal choice for improving the erosion resistance of turbine. The results of this study enrich the types of blade erosion damage and provide a new idea for reducing erosion damage of control stage blades.

Wind Turbine Blade Life-Time Assessment Model for Preventive Planning of Operation and Maintenance

Out of the total wind turbine failure events, blade damage accounts for a substantial part, with some studies estimating it at around 23%. Current operation and maintenance (O&M) practices typically make use of corrective type maintenance as the basic approach, implying high costs for repair and replacement activities as well as large revenue losses, mainly in the case of offshore wind farms. The recent development and evolution of condition monitoring techniques, as well as the fact that an increasing number of installed turbines are equipped with online monitoring systems, offers a large amount of information on the blades structural health to the decision maker. Further, inspections of the blades are often performed in connection with service. In light of the obtained information, a preventive type of maintenance becomes feasible, with the potential of predicting the blades remaining life to support O&M decisions for avoiding major failure events. The present paper presents a fracture mechanics based model for estimating the remaining life of a wind turbine blade, focusing on the crack propagation in the blades adhesive joints. A generic crack propagation model is built in Matlab based on a Paris law approach. The model is used within a risk-based maintenance decision framework to optimize maintenance planning for the blades lifetime.

음향방출(AE)을 이용한 풍력 블레이드의 피로손상 평가

In this study, the flap fatigue test of a 48 m long wind turbine blade was performed for 1 million cycles to evaluate the characteristics of acoustic emission signals generated from fatigue damage of the wind blades. As the number of hits and total energy continued to increase during the first 0.6 million cycles, blade damage was constant. The rise-time result showed that the major aspects of damage were initiation and propagation of matrix cracks. In addition, the signal analysis of each channel showed that the most seriously damaged sections were the joint between the skin and spar, 20 m from the connection, and the spot of actual damage was observable by visual inspection. It turned out that the event source location was related to the change in each channel`s total energy. It is expected that these findings will be useful for the optimal design of wind turbine blades.

Description and Computer Modeling of a Ball-and-Socket Hub That Enables Teetering for Three-Bladed Wind Turbines

A horizontal axis wind turbine with a ball-and-socket hub is disclosed. The hub enables horizontal axis turbines with two or more blades to teeter in response to wind shear gradients. Computer modeling was done using existing and modified fast code in order to compare the new hub design with existing designs. Results show that a three-bladed turbine with the ball-and-socket hub provides very significant reductions in out-of-plane bending loads applied to the main shaft in comparison to a three-bladed turbine with a rigid hub. Results also show that the new hub design provides significant reductions in the out-of-plane loads applied to the blades. A blade fatigue study using a rainflow counting of multi-axial torque contributions at the blade root was performed in order to assess the impact of these reductions, and results show that the three-bladed turbine equipped with a ball-and-socket, teetering hub provides for very significant reductions in lifetime blade damage in comparison to existing wind turbine designs due to a combination of factors. The first factor is that teetering largely eliminates the cyclic variations in out-of-plane torque on the blades that are observed with rigid hubs. Here, the fatigue study shows that the three-bladed wind turbine with a teetering hub provides for an approximate sixfold reduction in lifetime blade damage in comparison to a three-bladed turbine with a rigid hub. The second factor is that the addition of a third blade reduces the load on each blade by one-third. Here, the fatigue study shows that a three-bladed turbine with a teetering hub provides for an approximate fourfold reduction in lifetime blade damage in comparison to a two-bladed turbine with a teetering hub.

Structural changes and damage of single-crystal turbine blades during life tests of an aviation gas turbine engine

The irreversible structural changes of the single-crystal ZhS32-VI nickel superalloy blades of a high-pressure turbine that occur during life tests of a gas turbine engine are studied. The main operation damages in the hottest section of the blade airfoil are found to be the fracture of the heat-resistant coating in the leading edge and the formation of thermomechanical fatigue cracks. The possibility of reconditioning repair of the blades is considered.

MEMS Multisensor Intelligent Damage Detection for Wind Turbines

Maintenance and repair of wind turbine structures have become more challenging and at the same time essential as they evolve into larger dimensions or located in places with limited access. Even small structural damages may invoke catastrophic detriment to the integrity of the system. So, cost-effective, predictive, and reliable structural health monitoring (SHM) system has been always desirable for wind turbines. A real-time nondestructive SHM technique based on multisensor data fusion is proposed in this paper. The objective is to critically analyze and evaluate the feasibility of the proposed technique to identify and localize damages in wind turbine blades. The structural properties of the turbine blade before and after damage are investigated through different sets of finite-element method simulations. Based on the obtained results, it is shown that information from smart sensors, measuring strains, and vibrations data, distributed over the turbine blades can be used to assist in more accurate damage detection and overall understanding of the health condition of blades. Data fusion technique is proposed to combine these two diagnostic tools to improve the detection system that provides a more robust reading with reduced false alarms.

Aspects of structural health and condition monitoring of offshore wind turbines.

Wind power has expanded significantly over the past years, although reliability of wind turbine systems, especially of offshore wind turbines, has been many times unsatisfactory in the past. Wind turbine failures are equivalent to crucial financial losses. Therefore, creating and applying strategies that improve the reliability of their components is important for a successful implementation of such systems. Structural health monitoring (SHM) addresses these problems through the monitoring of parameters indicative of the state of the structure examined. Condition monitoring (CM), on the other hand, can be seen as a specialized area of the SHM community that aims at damage detection of, particularly, rotating machinery. The paper is divided into two parts: in the first part, advanced signal processing and machine learning methods are discussed for SHM and CM on wind turbine gearbox and blade damage detection examples. In the second part, an initial exploration of supervisor control and data acquisition systems data of an offshore wind farm is presented, and data-driven approaches are proposed for detecting abnormal behaviour of wind turbines. It is shown that the advanced signal processing methods discussed are effective and that it is important to adopt these SHM strategies in the wind energy sector.

Modal characteristics of turbine blade packets under lacing wire damage induced mistuning

Effect of mistuning on turbo machine blade vibration in a packeted blade–disk system has become an important area of research in the recent past, mainly due to the critical applications in aero engines and power plant turbines. It has been shown that even a small mistuning can lead to stress build up through mode localization under forced vibration. Such mistuning can come from initial geometric blade to blade variation due to manufacturing tolerances or from a crack growing in the bladed disk system during operational life stages. The literature review indicates that researchers have mainly considered blade damage as a cause of mistuning. However, lacing wire damage, although not as catastrophic as blade damage, are more frequent in occurrences and often act as a precursor to subsequent blade damage. Detection of lacing wire damage is therefore equally important. Present work has investigated nature of mistuning induced by lacing wire damage and its effect on the characteristic modal properties. A damage severity index has been introduced and effect of damage on the blade group natural frequencies is investigated. Scope of developing a damage identification methodology in packeted blade–disk system is also discussed.

A Novel Method and Its Field Tests for Monitoring and Diagnosing Blade Health for Wind Turbines

A new diagnostic method is proposed to efficiently monitor the structural health and detect damages in wind turbine blades. A high-resolution real-time blade condition monitoring system that considers the harsh turbine operating environment and uses optical sensors and a wireless network is presented. A hybrid algorithm, which merges probabilistic analysis, design loads, and real-time load estimates, is introduced to enhance operational safety and reliability. Moreover, the alarm limits are updated every 10 min through a learning algorithm to further improve reliability. The proposed algorithm was implemented in a blade monitoring system. The effectiveness of the proposed algorithm was demonstrated for a 3-MW wind turbine in the Yeongheung wind farm.

Simulation and Evaluation of Different Process Strategies in a 5-axis Re-contouring Process

In the regeneration of turbine or compressor blades each blade damage is varying in location, size and shape, depending on the cause of damage. An important regeneration step is the re-contouring which is applied after the deposit welding to remove excess material. With complex blade shapes this step requires 5-axis machining methods. Due to different cases of damage, the re-contouring has to be adapted to each individual repair case to satisfy the high quality requirements regarding the final workpiece. This paper combines the two topics machining strategy and emerging workpiece quality. In this context the work demonstrates a simulation-based approach for the 5-axis re-contouring process. Hence, different 5-axis tool-paths strategies are applied on an analogy repair case including a modelled weld shape. The re-contouring is performed virtually via a dexel based material removal simulation as well as experimentally by using a 5-axis milling machine. Afterwards an evaluation of the different tool-path strategies is done considering achieved workpiece quality. The results imply that the simulation is applicable to predict certain aspects of the workpiece quality such as surface topography. With the simulation system, a tool-path evaluation is possible before re-contouring real workpieces.

Shaking-Swing Coupled Vibration Analysis of a Laminar Composite Rotating Blade by the Finite Element Method

For studying the vibration characteristics of a laminar composite rotating blade, the spin softening effect calculated by the finite element method (FEM) was incorporated with the prestress effect to analyze shaking, swing, torsion, and coupled vibrations. The mathematical formula was derived. The study shows that the centrifugal force exerts influence on the difference in vibration frequencies. The laminar composite can reduce an influence of the spin softening on the first swing vibration frequency because of its structure and smaller density. With the variation of rotating velocity, the blade spin softening can lead to the coupled first shaking and first swing vibrations. In the coupled vibrations, swing is contained in the shaking mode and shaking is also contained in the swing one, which is the reason of major blade damages. The results confirm the complexity of the dynamic characteristics of the laminar composite rotating blade. This method is also well applicable to the dynamic design of such blades.

Post-operation state and extension of service life of compressor blades of the gas-turbine power plant GTE-45-3

Gas turbine engine compressor blades are the most essential parts among all the units of gas turbine power plants. They are made of chromium stainless steels. Erosive and corrosive wear of working surfaces as well as thermal stress fatigue are the main types of damage of compressor blades after long-term use. The aim of this work was to develop methods of recovering the serviceability of the material of the power plant ГТЭ-45-3 compressor blades with a view to extending their service life. To examine the post-operation state of compressor blades and the material serviceability the compressor blades of the gas turbine engine power plant ГТЭ-45-3 made of corrosion-resistant steels ЭИ961 (13Х11Н2В2МФ) and 12Х13 were investigated after their service life expired with the total engine hours equal to 50000. To evaluate the serviceability of the blade material an unconventional approach and know-how were used. The results of analysis of the microstructure of corrosion-resistant steels as well as their physical and mechanical properties are presented in the paper. A process of material serviceability recovery is developed with a view to extending the service life of compressor blades. The process includes the following sequence of operations: machining aimed at removing corrosion and mechanical defects, argon-arc welding that restores the geometry of the rotor blades, thermal tempering for removing residual stress.

Analytical and Numerical Investigation of Lacing Wire Damage Induced Mistuning in Turbine Blade Packet

Investigations of modal parameters for a mistuned packet of turbine blades due to lacing wire damage are reported using analytical and numerical studies with a simplified model. The turbine blade is assumed to be an Euler-Bernoulli beam connected with a lacing wire which is modeled as a mass less linear elastic spring. Thus, the blade is considered as a continuous system and lacing wire as a discrete system. The analytical results using Eigen value analysis are compared with numerical results obtained using commercial finite element package. In real life situation, though not reported in the literature, it is the failure of lacing wire that occurs quite often compared to the turbine blade and acts as precursor to the subsequent blade damage if it goes undetected. Therefore, studying the modal parameters of the grouped turbine blades in the context of lacing wire failure becomes important. The effect of variation of lacing wire location and stiffness indicative of damage resulting in the loss of stiffness on modal parameters is investigated. The study reveals a lot of fundamental understandings pertaining to dynamic behavior of grouped blades compared to the stand-alone blade under the influence of damaged lacing wire.

Computational fluid dynamics simulations of blade damage effect on the performance of a transonic axial compressor near stall

Gas turbine axial compressor blades may encounter damage during service for various reasons such as damage by debris from casing or foreign objects impacting the blades, typically near the rotor’s tip. This may lead to deterioration of performance and reduction in the surge margin. The damage breaks the cyclic symmetry of the rotor assembly; thus, computational fluid dynamics simulations have to be performed using full annulus compressor assembly. Moreover, downstream boundary conditions are unknown during rotating stall or surge, and simulations become difficult. This paper presents unsteady computational fluid dynamics analyses of compressor performance with tip curl damage. Computations were performed near the stall boundary. The primary objectives are to understand the effect of the damage on the flow behaviour and compressor stability. Computations for the undamaged rotor assembly were also performed as a reference case. A transonic axial compressor rotor was used for the time-accurate numerical unsteady flow simulations, with a variable area nozzle downstream simulating an experimental throttle. Computations were performed at 60% of the rotor design speed. Two different degrees of damage for one blade and multiple damaged blades were investigated. Rotating stall characteristics differ including the number of stall cells, propagation speed and rotating stall cell characteristics. Contrary to expectations, damaged blades with typical degrees of damage do not show noticeable effects on the global compressor performance near stall.