Blade Crack Research Articles

Crack evaluation of the fourth stage blade in a low-pressure steam turbine

A crack of the fourth stage blade in a low-pressure turbine of a 500 MW steam turbine is studied. From non-destructive inspection, the crack was found at the leading edge vane of the fourth stage blade. Material composition analysis, hardness measurement and microstructure analysis were performed to study the cause and process of the cracked blade. On further examination using the replication of the cracked surface for the blade vane, the crack was induced by corrosion pits. It is assumed that the causes of the corrosion pits are the induction of seawater from the condenser tube that has small leakage, and improper water treatment. It is shown that the corrosion pits acted as stress concentration site and facilitated crack initiation under cyclic loading experienced during normal operation of the blade. From the fracture surface of the blade, it is concluded that the crack is initiated from the corrosion pits at the leading edge of the blade vane, and propagated inside by fatigue due to the vibration of the blade.

Crack FEA of the First Stage Blade of Gas Turbines

To investigate the physical cause of premature blade cracking during the acceleration mission test (AMT) in a test cell environment, an in-depth finite element analysis (FEA) of the blade was conducted using a life prediction system. The results obtained showed that the blades had suffered excessive airfoil creep damage, leading to excessive blade lengthening and airfoil untwisting particularly in the trailing edge region. It is predicted that the uneven rubbing action might have contributed to the fatigue crack nucleation and growth process just below the platform in the shank region of the blade under AMT fatigue cycling conditions, and the excessive creep deformation made a significant effect on the overall crack nucleation process.

Reliable thermosonic inspection of aero engine turbine blades

The application of Thermosonics as a screening technique for identifying cracked aero engine turbine blades is described. Thermosonics is a rapid and potentially, a cost-saving non-destructive testing (NDT) technique for rejecting cracked turbine blades as part of the regular inspection cycle. However, there remains a question over the reliability of the Thermosonic approach. The aim of this work is to assess the reliability of Thermosonics as a screening method for cracks in turbine blades. This work involved the use of 60 turbine blades with known cracks. First, a repeatability study was conducted on a subset of the 60 blades to establish a robust and repeatable experimental procedure, after which all 60 blades were inspected. Results from this work showed the experimental process to be robust and also that the known cracks in 52 out of the 60 blades were detected.

Impact of startup scheme on Francis runner life expectancy

Francis runners are subject to complex dynamic forces which might lead to eventual blade cracking and the need for corrective measure. Damage due to cracks in runner blades are usually not a safety issues but might generate unexpected down time and high repair cost. Avoiding the main damaging operating conditions is often the only option left to plant operators to maximize the life expectancy of their runner. The startup scheme is one of the available parameter which is controlled by the end user and could be used to minimize the damage induced to the runner. In this study, two startup schemes have been used to investigate life expectancy of Francis runner using in situ measurements. The results obtained show that the induced damage during the startup event could be significantly reduced with change to the startup scheme. In our opinion, an optimization of the startup scheme with regard to fatigue damage could extend significantly the life expectancy and the reliability of Francis runner.

Crack evaluation and subsequent solution of the last stage blade in a low-pressure steam turbine

A crack of the last stage blade in a low-pressure steam turbine is studied. From non-destructive inspection, the crack was found at the trailing edge fillet of the last stage blade. On further examination using the replication of the crack and scanning electron microscopy, the crack was induced by stress corrosion cracking. The blade integrity is evaluated for several crack-removal conditions to repair the cracked blade. Finite element analysis is utilized to determine the stresses and dynamic characteristic of the blade for these conditions. The assessment of high cycle fatigue is performed by modified Goodman diagram. From the results, it is found that the crack-removal blade satisfies the design criteria for the fatigue life and has the sufficient margin from resonant condition of vibration. Repair for the cracked blade would be recommended on the basis of the careful analysis. For the repaired blade, periodic inspection is needed and replacement of the blade will be necessary when an indication is found.

Compressor Blade Health Monitoring with Use of Tip Timing and Modal Analysis Method

AbstractThe intended aim of this report is to deliver a short review of AFIT experience in the field of the blade health monitoring. A special attention has been put down for diagnostic methods (tip timing, modal analysis, metal magnetic memory) and symptom verification. Knowledge, which is necessary for reliable diagnosis and prognosis, during engine operation, as well as during overhaul and numerical simulation. It has been proven that the resonance characteristic shape analysis gives an ability to determinate if we deal with fatigue or with a blade crack. Early fatigue symptoms, i.e. modal properties of material strengthening and weakening phases have been shown. A new source of diagnostic information to ‘actively’ control blade fatigue (VHCF, HCF & LCF) has been described. The discussed problems have been illustrated with some examples. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of a Cracked Blade on Mistuned Turbine Engine Rotor Vibration

An efficient methodology for predicting the nonlinear forced vibration response of a turbine engine rotor with a cracked blade is presented and used to investigate the effects of the damage on the forced response. The influence of small random blade-to-blade differences (mistuning) and rotation on the forced response are also considered. Starting with a finite element model, a hybrid-interface method of component mode synthesis (CMS) is employed to generate a reduced-order model (ROM). The crack surfaces are retained as physical degrees of freedom in the ROM so that the forces due to contact in three-dimensional space can be properly calculated. The resulting nonlinear equations of steady-state motion are solved by applying an alternating frequency/time-domain method, which is much more computationally efficient than traditional time integration. Using this reduced-order modeling and analysis framework, the effects of the cracked blade on the system response of an example rotor are investigated for various mistuning levels and rotation speeds. First, the advantages of the selected hybrid-interface CMS method are discussed and demonstrated. Then, the resonant frequency shift associated with the stiffness loss due to the crack and the vibration localization about the cracked blade are thoroughly investigated. In addition, the results of the nonlinear ROMs are compared with those obtained with linear ROMs, as well as blade-alone ROMs. It is shown that several key system vibration characteristics are not captured by the simpler models, but that some insight into the system response can be gained from the blade-alone response predictions. Furthermore, it is demonstrated that while the effects of the crack often appear similar to those of mistuning, the effects of mistuning and damage can be distinguished by observing and comparing the response across multiple families of system modes.

Feasibility analysis for monitoring fatigue crack in hydraulic turbine blades using acoustic emission technique

In order to investigate the feasibility of monitoring the fatigue cracks in turbine blades using acoustic emission (AE) technique, the AE characteristics of fatigue crack growth were studied in the laboratory. And the characteristics were compared with those of background noise received from a real hydraulic turbine unit. It is found that the AE parameters such as the energy and duration can qualitatively describe the fatigue state of the blades. The correlations of crack propagation rates and acoustic emission count rates vs stress intensity factor (SIF) range are also obtained. At the same time, for the specimens of 20SiMn under the given testing conditions, it is noted that the rise time and duration of events emitted from the fatigue process are lower than those from the background noise; amplitude range is 49-74 dB, which is lower than that of the noise (90-99 dB); frequency range of main energy of crack signals is higher than 60 kHz while that in the noise is lower than 55 kHz. Thus, it is possible to extract the useful crack signals from the noise through appropriate signal processing methods and to represent the crack status of blade materials by AE parameters. As a result, it is feasible to monitor the safety of runners using AE technique.

Simplified model of the turbine runner blade

Francis turbine blade is modelled by circular beams loaded by uniformly distributed pressure. This simplification allows for analytical calculations of maximum stresses followed by comparisons with FEM analysis for the same turbine. Two beam models have been proposed and one of them showed good correlation with FEM for different operation conditions. This model has been applied to analyse the operation data recorded during the whole time of operation of the Unit 2 of Derbendikan hydropower station. The results show evolution of the maximum tensile stresses, which makes clear that the appearance of cracks in turbine blades cannot be explained by the high stresses level alone but by combination of different factors.

Small turbine blade inspection using laser strain techniques

High-pressure turbine blades in a gas turbine engine are constantly exposed to extremely hostile operating conditions, where temperatures can regularly exceed the melting point of the parent material. These conditions make the blades susceptible to thermal fatigue cracking, corrosion and, in some cases, damage due to particle impact. The required inspection method in aerospace for finding surface-breaking defects is Fluorescent Penetrant Inspection (FPI). However, the blade must be extensively cleaned prior to FPI and this represents a considerable cost in terms of time and resource, particularly if the blade is subsequently found to be unserviceable. Therefore, the ability to inspect such components prior to the cleaning stage is an attractive proposition. This paper describes the use of laser strain techniques to identify cracks in high-pressure turbine blades as a means of conducting inspections in the engine-ran condition. A set of nine blades were cleaned and inspected in the conventional manner using FPI, with all highlighted defects recorded photographically. Next, laser shearography techniques were applied to the test set using a bespoke laboratory system. Distortions in the resultant fringe patterns caused by defects in the material were then used to identify the cracks. All cracks highlighted by FPI were also found using the shearography method. In addition, the laser shearography technique highlighted a region of impact damage on the aerofoil of one blade, not identified at the first inspection.

Efficient Nonlinear Vibration Analysis of the Forced Response of Rotating Cracked Blades

The efficient nonlinear vibration analysis of a rotating elastic structure with a crack is examined. In particular, the solution of the forced vibration response of a cracked turbine engine blade is investigated. Starting with a finite element model of the cracked system, the Craig–Bampton method of component mode synthesis is used to generate a reduced-order model that retains the nodes of the crack surfaces as physical degrees of freedom. The nonlinearity due to the intermittent contact of the crack surfaces, which is caused by the opening and closing of the crack during each vibration cycle, is modeled with a piecewise linear term in the equations of motion. Then, the efficient solution procedure for solving the resulting nonlinear equations of motion is presented. The approach employed in this study is a multiharmonic hybrid frequency∕time-domain technique, which is an extension of the traditional harmonic balance method. First, a simple beam model is used to perform a numerical validation by comparing the results of the new method to those from transient finite element analysis (FEA) with contact elements. It is found that the new method retains good accuracy relative to FEA while reducing the computational costs by several orders of magnitude. Second, a representative blade model is used to examine the effects of crack length and rotation speed on the resonant frequency response. Several issues related to the rotation are investigated, including geometry changes of the crack, shifts in resonant frequencies, and the existence of multiple solutions. For the cases considered, it is found that the nonlinear vibration response exhibits the jump phenomenon only when rotation is included in the model.

Cracking Mechanism and Retrofit Design of Governing Stage Blades for a Steam Turbine

Cracking of blade dovetails occurred in a governing stage of a 50MW steam turbines twice within 5 months. To find out the failure mechanism and measures to avoid the failure accident, one of the cracked blades has been inspected by the metallography and a scanning electronic microscope. The inspection results show that the cracking of the blade is caused by the high cycle fatigue. When governing stage blades are in operation, the vibration of blades can extend to platform or even to dovetail parts over radial supporting surfaces depending on temperature, centrifugal force and assembly tolerance. The calculated results by FEM show that not only natural frequencies of blades in operating are smaller than that in stationary but also the modal order shifts when the blade runs from stationary to operation states. The effect of mode shift on the failure was investigated. According to the conclusion, the blades have been retrofitted by increasing neck of dovetail to avoid dangerous resonance, and the blades have been operating in healthy condition for two years after the modification. Contrarily, if the phenomenon of mode shift were ignored, it would be difficult to find out the right reason of resonance and ways to retrofit the damaged blades.

Failure analysis of air fan blades

Abstract The failure of all 12 blades of an air fan was investigated by metallurgical and mechanical experiments and an examination of the fracture surface. The experimental results show that the cast aluminium-silicon alloy without any modification had a number of material defects, such as coarse grains, a loose structure, a large number of shrinkage holes, a long and thin bold-pin shaped silicon-phase, poor material strength and serious brittleness. In addition, installed on the spindle without elastic conjunction, blade No.10 vibrated and inevitably. spun off due to the large centrifugal force. Therefore, blade No.10 first cracked at the locking handle, then broke at the root, which caused all the other 11 blades to be broken by the crack of blade No.10.

Last stage blades failure analysis of a 28 MW geothermal turbine

A last stage (L-0) turbine blades failure was experienced at a 28 MW geothermal unit after seven years of operation period. This unit has one flow intermediate/low-pressure turbine composed of nine stages with 25-in./3600 rpm last stage blades. The last stage row contains 62 free standing blades. Visual examination indicated that the 37 L-0 blades had cracks in their airfoils initiating at the trailing edge, near the blade platform. Laboratory evaluation of the cracking indicates the failure mechanism to be high cycle fatigue (HCF), and the cracks initiation was accelerated by erosion picks on the blade surface due to steam recirculation flow and corrosion. A last stage blade failure evaluation was carried out. The investigation included a metallographic analysis of the cracked blades, natural frequency analysis, blade stress analysis, unit operation parameters, history of events analysis and crack initiation and propagation analysis. This paper provides an overview of the failure investigation, which led to the identification of some operation periods with low load as the primary contribution to the observed failure.

PIVによる水力発電用ポンプ水車内部非定常流れ計測

It is thought that the pressure fluctuation can occur due to the interaction between flow through guide vanes and flow into runner blades, resulting in a vibration of turbine and a blade cracking, in a hydraulic turbine operated in a wide range for flexible power demand. High accurate velocity measurement with high time/spatial resolution can help to clarify the mechanism of the interaction and to provide good experimental data for the validation of numerical procedure. So the aim of present study is to estimate the unstable velocity field quantitatively in the area between guide vanes and runner blades, using high time-resolved particle image velocimetry (PIV). Two types of velocity measurements were carried out, i.e., phase-locked measurement and high time sequential velocity measurement, in a pump-turbine model with 20 guide vanes and 6 runner blades. The characteristic of the flow field varied corresponding to the operating conditions such as flow rate and rotational speed. Opening angles of guide vanes were kept uniform. A clockwise vortex was generated at inside of the runner blade under smaller rotational speed. A counterclockwise vortex was separated at the backside of the runner blade under higher rotational speed. At any operating conditions, the velocity between guide vanes and runner blades oscillated periodically at the blade passing frequency.Copyright © 2009 by ASME

On the Modal Analysis of a Cracking Compressor Blade

On the Modal Analysis of a Cracking Compressor Blade The paper deals with issues related to reliable recognition and prediction of health of steel compressor blades. It outlines symptoms that indicate the blade cracking, and identification methods that can be used during operation, maintenance and repairs/overhauls of turbine jet engines of the SO-3 type. An optical method involving laser measurements of blade vibration while on the excitation source is presented in details as it is implemented in the WZL-3 (Military Aviation Works No. 3). The method has been verified in practice and some findings from the verification process are discussed. Next, new testing capabilities offered by the method are presented, including identification of early symptoms of material fatigue (the strengthening and the weakening of the material) and the crack initiation. The topics under discussion have been illustrated with some examples taken while measuring modal properties typical of both defect-free and faulty blades as well as from fatigue tests conducted according to the LCF and HCF profiles. It has been proved that the analysis of resonance responses for specific modes of operation allows of reliable differentiation between material's fatigue and cracking.

Steam turbine blade failure analysis

A last stage turbine blades failure was experienced in two units of 660 MW. These units have one high-pressure turbine and two tandem-compound low-pressure turbines with 44-in. last-stage blades. The blades that failed were in a low pressure (LP) turbine connected to the high pressure turbine (LP1) and in LP turbine connected to the generator (LP2). The failed blades had cracks in their roots initiating at the trailing edge, concave side of the steeple outermost fillet radius. Laboratory evaluation of the cracking indicates the failure mechanism to be high cycle fatigue (HCF). The last-stage blades failure evaluation was carried out. The investigation included a metallographic analysis of the cracked blades, natural frequency test and analysis, blade stress analysis, unit’s operation parameters and history of events analysis, fracture mechanics and crack propagation analysis. This paper provides an overview of this failure investigation, which led to the identification of the blades torsional vibrations near 120 Hz and some operation periods with low load low vacuum as the primary contribution to the observed failure.

Cracking-Induced Mistuning in Bladed Disks

Abstract : A study of the mechanisms of cracking-induced mistuning in bladed disks is presented in this paper. An analytical model for a bladed disk was formulated using lumped-mass-beams, and a cracked blade was represented by a beam with a through-crack at the root. Local stiffness reduction due to cracking was incorporated using a flexibility matrix method. The dynamic characteristics of the bladed disk were analysed for the tuned system and then for the mistuned system with cracks of various depths introduced at the root of a blade. The dynamic characteristics of the mistuned system with a cracked blade were evaluated and the mistuning pattern due to blade cracking was investigated. The mechanisms of cracking-induced mistuning were elucidated by analysing the mode localisations and the reductions in natural frequency for different coupling ratios between the disk and blades. The results show that the occurrence of a cracked blade significantly increases the response amplitude of the other uncracked blades and therefore gives rise to enhanced HCF damage for all coupling ratios studied. In the present study, the relationship between the cracked blade and its vibration response signature is established, hence providing the essential guidance required for blade crack detection procedures and for blade failure investigations.

Stability in a Twisted Periodic Blade System with Cracks

This paper investigates the variations in dynamic stability of a cracked rotating periodic bladed disk assembly. A bladed disk composed of periodically shrouded blades is used to simulate the coupled periodic structure. To study mode localization, a number of apparently identical blades are distributed periodically around a rigid hub. For simplicity, the rigid hub is considered to be attached to a rigid shaft. The modal localization equations of the rotating cracked assembly are formulated using Hamilton's principle and Galerkin's method. The regions of instability in this cracked assembly are identified by means of the multiple scales perturbation method. The numerical results indicate that the modal localization effect introduced by local blade cracks destabilizes the shrouded blade group. One additional instability band at the localization frequency is introduced from the local crack. Furthermore, it is shown that the stability regions of a cracked rotating bladed disk assembly are significantly dependent on the rotational speed, shroud stiffness, and crack depth.

Failure analysis and retrofit design of low pressure 1st stage blades for a steam turbine

Cracking of blade fingers occurred in a few numbers of low pressure 1st stage blades for a certain type of steam turbines. In order to find out the failure mechanism, one of the cracked blades has been inspected. The inspection results showed that the cracking of the blade finger was caused by high cycle fatigue and surface defect coming from rough machining induced the initiation of crack. Vibratory modes of the blade group have been calculated and measured using a 3D finite element S/W and impact test, respectively. The results showed that resonance of the second type group axial vibration mode with nozzle passing frequency was the source of high cycle fatigue load. To avoid the dangerous resonance, the blade groups have been modified into 10 blades per group without any change of vane and dovetail. The new blade groups have been operated safely more than 2 years since the modification.