Experimental crack propagation and failure analysis of the first stage compressor blade subjected to vibration

This paper presents results of experimental vibration tests of the helicopter turbo-engine compressor blades. The blades used in investigation were retired from maintenance under technical inspection of engine. Investigations were conducted for selected undamaged blades, without existence of preliminary cracks or corrosion pits. The blades during experiment were entered into transverse vibration. The crack propagation process was conducted in resonance condition. During the fatigue test, the growth of crack was monitored. In the second part of work, a nonlinear finite element method was utilized to determine the stress state of the blade during vibration. In this analysis a first mode of transverse vibration were considered. High maximum principal stress zone was found at the region of blade where the crack occurred.     

某型航空发动机压气机四级转子叶片失效分析

某型航空发动机压气机四级转子叶片是故障多发叶片.对叶片典型失效件进行了断口分析,发现叶片故障是由于叶片表面发生腐蚀导致疲劳强度降低,使得叶片在振动应力下发生疲劳失效.进一步对显微组织进行分析,发现叶片材质合乎要求,叶片腐蚀主要是由于使用环境因素造成的.研究结果对于叶片的故障分析及预防具有重要的意义.     

Dynamic and fatigue compressor blade characteristics during fluid-structure interaction: Part I—Blade modelling and vibration analysis

Fatigue failure in blades resulting from alternating loads is a primary cause of aircraft engine failure. In this study, the dynamic and fatigue performance of a compressor blade, which is prone to fatigue failure in practical use, is verified through numerical analysis and tests to identify specific failure causes and ultimately prevent blade failure. Two companion papers describe this work. Part I—blade modelling and vibration analysis establishes a compressor blade model based on reverse engineering and investigates the blade's dynamic characteristics during aerodynamic and structural behaviour interactions under various flight conditions. To accurately describe the dynamic performance of a real blade, three blade models with different fitting precisions are constructed to support dynamic response analysis. In addition, the flow field of each model is constructed to simulate the aerodynamic loads that cause blade vibration. A numerically based analysis of the fluid-structure interaction is subsequently performed. A reasonable calculation model is selected by comparing the dynamic characteristics among these models. The Campbell diagram is analysed to determine resonance probability, and blade failure cause is ultimately verified based on the vibration analysis. The results show that different accuracies in the blade calculation model produce varying degrees of error; the calculation model's ability to reflect the dynamic and fatigue behaviours of a real blade is recommended to be used as a critical evaluation index in the numerical study. The first-order harmonic resonance occurring at critical speeds may contribute to blade failure.     

考虑风速分布的叶片MPC气弹控制

作为叶片失效的重要原因,颤振一直是风电领域研究重点。以抑制叶片工作过程中的颤振为目的,建立分析模型时应充分考虑实际风速分布特别是风切变和塔影效应对叶片振动的影响,并在离散化后求取平均输入风速。针对小型风力机叶片气动弹性稳定性问题进行综合分析,根据实体结构建立叶片气弹模型,考虑非线性气动力作用时叶片大攻角、大风速工况下产生的高频、高幅失速颤振,模拟典型截面仿真振动位移。通过模型预测控制的滚动优化和误差矫正控制叶片挥舞、摆振两个运动方向的振动频率和幅值,结果表明振动形式实现控制后可在短时间内达到稳定且静差可接受。     

Investigation on the effects of blade corrosion on compressor performance

Corrosion is a common reason for marine gas turbine blade failure, and salt spray is largely responsible for corrosion of metallic objects near the coastline. It will directly lead to geometrical deviations of the compressor blade, forcing an increase in roughness height of the surface of the blade. By using a three-dimensional numerical method, this paper deals with the corrosion of a 1.5 stage axial compressor. Two different schemes, respectively based on blade geometrical model modification and turbulent near wall functions, were employed in numerical simulations for corrosion rate of the blades. The compressor map, derived from the numerical simulations by adjusting the back pressure, shows that there is a decline of compressor efficiency and pressure ratio when the blade is corroded, which will result in performance degradation of the compressor. The corrosion properties of stainless steel were studied, using the static salt spray corrosion test under laboratory conditions. Additionally, combined with numerical methods, the compressor performance parameters along the extension of operating time were simulated and a binomial relationship between performance degradation and corrosion time was established. This research provides a technical guide for compressor performance prediction under less than ideal conditions.     

Evaluation of corrosion fatigue crack propagation life at low-pressure steam turbine rotor groove

The corrosion fatigue crack propagation life of Christmas-tree type rotor groove with three hooks is studied. Each corner of the hook can be a candidate for crack initiation site therefore the condition where cracks initiate and propagate simultaneously at several hook corners must be considered. When a blade is inserted in the rotor groove, narrow gap is introduced unavoidably between the rotor groove and the blade root. The effect of this narrow gap on the crack behavior must also be considered. A procedure was presented to assess the crack initiation and propagation behavior under such a condition. Using the procedure, crack initiation and propagation behavior was evaluated for several gap conditions. It was revealed that the gap condition had little effect on the relation between crack depth at the third hook corner and life consumption ratio (ratio of loading cycle to final failure life). A corrosion fatigue test was performed using a rotor groove model specimen, and the results were compared with the evaluation results.     

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.     

Vibration based damage detection of rotor blades in a gas turbine engine

This paper describes the problems concerning turbine rotor blade vibration that seriously impact the structural integrity of a developmental aero gas turbine. Experimental determination of vibration characteristics of rotor blades in an engine is very important from fatigue failure considerations. The blades under investigation are fabricated from nickel base super alloy through directionally solidified investment casting process. The blade surfaces are coated with platinum aluminide for oxidation protection. A three dimensional finite element modal analysis on a bladed disk was performed to know the likely blade resonances for a particular design in the speed range of operation. Experiments were conducted to assess vibration characteristics of bladed disk rotor during engine tests. Rotor blade vibrations were measured using non-intrusive stress measurement system, an indirect method of blade vibration measurement utilizing blade tip timing technique. Abnormalities observed in the vibration characteristics of the blade tip timing data measured during engine tests were used to detect the blade damage. Upon disassembly of the engine and subsequent fluorescent penetrant inspection, it was observed that three blades of the rotor assembly were identified to have damaged. These are the blades that exhibited vibration abnormalities as a result of large resonant vibration response while engine tests. Further, fractographic analysis performed on the blades revealed the mechanism of blade failures as fatigue related. The root cause of blade failure is established to be high cycle fatigue from the engine run data history although the blades were put into service for just 6 h of engine operation.     

Numerical stress and crack initiation analysis of the compressor blades after foreign object damage subjected to high-cycle fatigue

This paper presents results of the complex stress and crack initiation analysis of the PZL-10 W turbo-engine compressor blade subjected to high cycle fatigue (HCF). A nonlinear finite element method was utilized to determine the stress state of the blade during the first mode of transverse vibration. In this analysis, the numerical models without defects and also with V-notches were defined. The quality of the numerical solution was checked by the convergence analysis. Obtained results were next used as an input data into crack initiation (ε–N) analyzes performed for the load time history equivalent to one cycle of the transverse vibration. In the fatigue analysis the different methods such as: Neuber elastic–plastic strain correction, linear damage summation and Palmgreen–Miner rule were utilized. As a result of ε–N analysis, the number of load cycles to the first fatigue crack appearing in the compressor blades was obtained. Moreover, the influence of the blade vibration amplitude on the number of cycles to the crack initiation was analyzed. Values of the fatigue properties of the blade material according to Baumel–Seeger and Muralidharan methods were calculated. The influence of both the notch radius and values of the UTS of the blade material on the fatigue behavior of the structure was also considered. In the last part of work, the finite element results were compared with the results of an experimental vibration HCF tests performed for the compressor blades.     

Hot corrosion failure in the first stage nozzle of a gas turbine engine

First-stage nozzles of gas turbines, which are the first elements after the combustion chamber, encounter hot gases from the combustion process and have the task of directing the fluid path and increasing the velocity of combustion products. This paper reports on the incidence and failure of the first-stage nozzles of a gas turbine in September 2013 at a seaside pump-house located in the South-West of Iran. The nozzle was made of nickel-based superalloy Nimonic105. Due to nozzle failure, the turbine was damaged severely. The cause of nozzle failure was investigated. The results of visual inspection, XRD analysis of deposits on the blade airfoil, SEM images and EDAX analysis showed the characteristics of hot corrosion. Finite-element analysis (FEM) revealed that the cause of blade trailing edge failure was thermal stress leading to thermal fatigue, which accelerated nozzle blade failure in addition to the hot corrosion.     

Failure analysis of components in compressor vane

The paper presents a failure analysis of components damaged in compressor vane. In order to investigate the damage mechanism and failure causes of components, macroscopic and microscopic observations, microstructural investigations, chemical analysis and hardness measurement have been performed. The results show that the damaged components are caused by fractured blades of 1st stage rotor. In all 1st stage rotor blades, 1^# blade is fractured firstly, and is troublemaker in this incident. The fracture mechanism of 1^# blade fractured from blade root is due to first order bending vibration fatigue damage. The microstructure, hardness and chemical compositions of 1^# blade fracture, all which coincide with technical requirements. The bad shot peening qualities in the surface near blade fracture have important influence on fatigue failure.     

Fatigue crack growth simulation in a first stage of compressor blade

A first-stage rotary compressor blade of a Model GE-F6 gas turbine failed due to vibration in early March 2008. Initial investigations showed that pitting on the pressure side of the blade caused micro cracks, leading to larger cracks due to high cycle fatigue. To assess this failure, a series of experimental, numerical, and analytical analyses were conducted. Fractography of the fractured surface of the blade indicated that two semi-elliptical cracks incorporated and formed a single crack. In this study, static and dynamic stress analyses were performed in Abaqus software. Moreover, fracture mechanics criterion was accomplished to simulate fatigue crack growth. This was carried out using a fracture analysis code for 3-dimensional problems (Franc3D) in two states. Firstly, stress intensity factors (SIFs) for one semi-elliptical surface crack and then SIFs for two semi-elliptical surface cracks were taken into account. Finally, the Paris and Forman–Newman–De Koning models were used to predict fatigue life. Since stress level and crack shape in both conditions are the same and the SIF at the crack tip reaches the fracture toughness of the blade, SIFs results indicate that insertion of a second crack has no effect on the final SIF, however, the second crack facilitates the process of reaching the critical length. So, fatigue life in two-crack condition is less than in the one-crack state.     

Stress corrosion cracking of a 60 MW steam turbine rotor

The paper presents a root cause analysis of a steam turbine rotor blade groove cracking. The scope of analyses included material testing and mechanical integrity calculations. In scope of material testing, fracture microstructure was assessed and basic mechanical property characteristics of the rotor discs were determined. In scope of integrity analyses, the stress fields in the blade grooves were calculated and the possibility of cracking due to different mechanisms was assessed. Both calculations and material tests confirmed the stress corrosion cracking to be the root cause of the rotor failure. This was a basis for proposing the rotor discs repair by overlay welding with a lower strength material and modifications to the groove geometry.     

镍基单晶合金涡轮叶片开裂原因

某发动机高压涡轮叶片为镍基单晶合金叶片,在室温下进行振动疲劳试验后发现叶片开裂,通过宏观观察、金相检验和扫描电镜分析等方法对叶片开裂的原因进行了分析.结果表明:进气边叶根和榫头伸根的开裂形式均为疲劳开裂;进气边叶根气膜孔内壁存在多处小缺口及榫头伸根亚表面存在疏松缺陷,这些缺陷部位容易形成裂纹源,促进了裂纹的萌生,裂纹扩...     

风力机叶片挥舞摆振气弹失稳分析

为避免风机叶片在强风作用下发生破坏,需对其采取停机保护措施。该文研究叶片处于非旋转状态时的挥舞摆振气弹失稳现象发生的条件。基于风力机叶片简化模型,采用迭代法求解叶片的自振频率及振型,建立了非旋转叶片挥舞摆振气弹效应响应的振型叠加法,该方法可以便捷地进行叶片多工况气动弹性响应分析。计算了在不同风速不同攻角条件下叶片的挥舞摆振气弹效应响应,得到了叶片挥舞摆振响应随风速和风攻角的变化规律以及不稳定风攻角的分布特征。结果表明:在某些风攻角下,风机叶片挥舞摆振失稳现象在风速较低的情况就有可能发生,其响应幅值与结构阻尼联系紧密。另外,挥舞摆振失稳会大大增加作用于叶片上的风荷载,并进一步造成叶片结构的损伤破坏。     

Failure analysis of an MVR (mechanical vapor recompressor) Impeller

This article describes the root cause failure analysis of a mechanical vapor recompressor (MVR) impeller used in a dairy processing plant. An impeller blade was thrown during commissioning after approximately 150 h of service. Visual examination of the blade’s fracture surface indicated that the crack growth mechanism was fatigue and this was confirmed by scanning electron microscope (SEM) examinations. A detailed investigation was commissioned to determine the source of the oscillatory stresses that must have been present to cause fatigue. A vibration analysis indicated large torsional oscillatory stresses, leading to fatigue, were induced by excitation of the 1st and 2nd torsional vibration modes of the MVR impeller and motor system. The excitation was induced by the control system for the motor variable speed drive (VSD).     

Failure analysis of a second stage blade in a gas turbine engine

The failure of a second stage blade in a gas turbine was investigated by metallurgical and mechanical examinations of the failed blade. The blade was made of a nickel-base alloy Inconel 738LC. The turbine engine has been in service for about 73,500 h before the blade failure at 5:50 PM on 14 August 2004. Due to the blade failure, the turbine engine was damaged severely. The investigation was started with a thorough visual inspection of the turbine and the blades surfaces, followed by the fractography of the fracture surfaces, microstructural investigations, chemical analysis and hardness measurement.

The observation showed that a serious pitting was occurred on the blade surfaces and there were evidences of fatigue marks in the fracture surface. The microstructural changes were not critical. It was found that the crack initiated by the hot corrosion from the leading edge and propagated by fatigue and finally, as a result of the reduction in cross-section area, fracture was completed.

An analytical calculation parallel to the finite element method was utilized to determine the static stresses due to huge centrifugal force. The dynamic characteristics of the turbine blade were evaluated by the finite element modal and harmonic analyses. Finally according to the log sheet records and by using a Campbell diagram there was a good agreement between the failure signs and FEM results which showed the broken blade has been resonated by the third vibrational mode occasionally before the failure occurred.     

Failure analysis of generator rotor fan blades

The failure analysis of a generator rotor fan blade was investigated by mechanical analysis and metallurgical examination of fracture surface. Fracture took place at the airfoil root, surface examination showed that the blade had cracked by a high cycle fatigue mechanism. However, there was no evidence of material defect. A series of analytical, finite element and experimental analysis was utilized to determine the steady-state stresses and dynamic characteristic of the blade. Possibly the failure was due to aerodynamical disturbances that resulted in a state of resonant condition of vibration. The simulation of blade with final crack showed the stress intensity factor (SIF) under these condition exceed the critical SIF and final fracture could be occurred under analyzed stresses.     

Cement kiln dust induced corrosion fatigue damage of gas turbine compressor blades – A failure analysis

Compressor of one of the gas turbines installed in a power plant was stopped under emergency conditions. Primary investigation showed that almost all of the first stage blades and some of the next stages were severely damaged. In this study, one of the first stage broken blades was failure analyzed. The results showed that the corrosion pits were formed on the compressor blade surface due to the presence of Cl and S elements in the compressor inlet air. Since the power plant located in the vicinity of a cement company and also an oil refining company, the inlet air of compressor had large amounts of Cl and S containing compounds. The corrosion pits acted as stress concentration sites, and facilitated fatigue crack initiation and propagation, leading to final fracture of the blades.     

高速离心泵叶片断裂分析

对高速离心泵断裂叶片的材料成分,断口和力学性能进行了分析,确定了其断裂模式为腐蚀疲劳断裂,叶片根部加工粗糙,造成局部应力集中是导致叶片断裂的主要原因,对叶片的结构设计进行了探讨。