3. High Cycle fatigue is due to flow induced vibration called “Flutter”. 3、高周疲劳的发生是由流量变化时产生的自激振动也即颤振。
4. Flutter is due to higher frequency Strouhal number which is a design Criteria. 4、颤振是由设计原因造成的高频率斯特罗哈数。
5. Higher frequency Strouhal number is due to small scale instabilities from the separation of shear layer.
5、高频率斯特罗哈数由剪切层气流脱离导致小规模的气流不稳定造成。
6. Small scale instabilities from the separation of shear layer are due to modulation of IGV with frequency coupled with fouling &fluctuation in GT load. This can be confirmed by putting sensitive probe on casing just above the blades and if any side band frequency is observed and if this side band frequency is not a multiplication of Natural frequency then it can be concluded that this side band is not due to Resonance but because of Flutter. 6、剪切层气流脱离导致小规模的气流不稳定归因于IGV的频繁调整以及压气机的结垢和燃机负荷的波动。这可通过安装高敏感的探头于叶片上部进行检测，如果任何一侧探测到了B频段的频率，且此B段频率不会因为固有频率而增加，那么，可以这么认为这一侧B段频率不是因为雷诺数产生，而是因为颤振导致的。
But it is to be kept in mind that fatigue is not the root cause. Root cause is the factor caused the material to go under fatigue. By Fractographic analysis it is also possible to identify which factor/factors caused the material to be fatigue e.g. machining defect (Notch/Tool Mark), Design deficiency, forging defect, Metallurgical defect, Corrosion, Oxidation etc.
但是请记住疲劳不是发生问题的根源。问题的根源在于材质问题导致疲劳。通过断口分析同样能辨别是由那些因素导致金属疲劳，例如制造因素（刻痕、刀痕）、设计因素、锻造因素、冶金因素、腐蚀因素、氧化因素等等。
The study of failure surface with the help of high resolution microscope is known as fractigraphic analysis and this is a pure NDT Test and test specimen is not / should not be disturbed during the study. If the failed surface is even minutely disturbed, then the fractographic analysis will not give fruitful result.
通过高分辨率显微镜研究叶片断裂表面称之为断口分析，这是纯粹的无损检测试验，在研究期间进行试验取样不会也不应该受到干扰。如果叶片断裂表面每分钟都受到干扰，那么，断口分析将不会有任何富有成效的结果。
Corrective Measures in absence of Fractographic Result.
1. Increase in Frequency of Inspection:-On fractographic analysis, if it is found that the primary cause of failure is HCF (most likely, obviously with visual inspection but actual failure can only be confirmed after fractographic analysis) If on analysis, it is established that the failure is due to fatigue, then this is due to design deficiency and can not be rectified in situ until and unless these blades are replaced with modified blades. Under this circumstances, to avoid premature failure of the components, the machines are to be opened frequently for proper inspection so that the premature damage can be detected before reaching critical value/ failure and GE, most likely recommended more inspection compare to inspection interval mentioned in GER 3620K for other utilities.
断口分析结果缺乏矫正措施 1、增加检查的频率
断口分析中如果发现了最初高周疲劳断裂的原因（最大可能、最明显是用目视检查，但是实际应用时只有进行断口分析后才会被证实）。如果据分析，已经确定叶片断裂的原因为
疲劳，那么，这就是设计因素了，而且已安装机组无法在现场进行矫正，除非这些叶片更换为改进型叶片。基于这种情况下，为避免叶片过早地失效，那么，压气机需更频繁地打开进行合适的检查，这样才可避免在到达临界值时或断裂时发生类似的事件，同时GE最好建议其他同类型机组相比GER 3620K中建议的检修间隔进行更多的检查。
2. Vibration Measurement: -It is pertinent to mention here that the vibrations of Gas turbine bearings are monitored judiciously but the existing vibration monitoring system mounted on bearings of turbines can not detect this type of problem, however, if any increasing trend is observed then the vibration spectrum is to be analysed with a potable FFT analyzer. If on FFT analysis, the peak value is found to be corresponding to a blade pass frequency then the machine is to be stopped for inspection. Existing On line vibration monitoring system can not detect this problem. If special vibration pick up is directly mounted on the casing on stage from row#R0 to Row#4 (failure prone zone) and data is collected on continuous at various load and frequency. After carrying out FFT analysis, load/ Grid frequency at which the peak value is appearing, that particular load/ Frequency is to be avoided. The amplitude of peak value is to be monitored strictly. If any increase in amplitude is observed then the machine is to be stopped and after stopping, blade/vanes of stage corresponding to a blade pass frequency is to be thoroughly checked for development of Crack. 2、振动测量
在这提到燃机轴承的振动是因为目前的监控轴承振动监测系统是无法反应此类问题的，因此，发现任何轴承振动光谱增加趋势，必须立即利用便携的频谱分析仪进行分析。若在频谱分析仪中，发现振动峰值与叶片传递的频率一致，那么，机组应停机进行相关检查。目前轴承振动在线监测系统无法反应此类故障。若安装在R0列到第四列（S1）叶片（断裂高发区域）外壳探测器捕捉了特殊的振动，且收集的数据分布于不同负荷和频率。实施频谱分析后，负荷/电网频率波动时峰值均可捕捉到，那么，AGC必须避免投入，峰值的振幅必须严格地监视。观察到任何振幅的增加，机组必须停运，停运期间，各级动叶和静叶的频率与叶片传递的频率一致，必须彻底地检查裂纹的发展。
3. Monitoring of Compressor Efficiency: -Failure of Gas Turbine Compressor takes place mainly because of Rotating Stall / Surging and this takes place due to fouling. With the fouling, compressor efficiency decreases resulting reduction of air flow and the compressor operation regime is shifted towards the Surging. All OEM supplies the Compressor efficiency curve for different ambient temperature. If the measured efficiency value is close to the recommended value mentioned in the curve then compressor should be washed immediately either by On Line, if facility is there, or by OFF Line other wise surging will take place which will cause huge damage to the Compressor, Gas turbine may also damage. 3、监控压气机效率
压气机叶片的断裂主要发生在旋转失速喘振，这主要是因为压气机结垢所造成。压气机结垢后，压气机流量的减少从而导致压气机效率的降低，压气机的运行工况朝喘振边界线靠近。所有的压气机制造商提供了不同环境温度的效率曲线，若压气机效率值下降到建议的效率值，压气机应立即进行水洗，如果有条件那就进行在线水洗，或者离线水洗会发生喘振破坏压气机，燃机透平同样会被损坏。
4. Stopping of IGV Modulating at base load :-Normally GT is started with IGV in minimum position and after synchronizing of GT, as the GT load is increased IGV started opening at 60 to 65% of the base load. On load Control mode, IGV full opens when the load is ~ 80 to 85% of the base load. After this with further increase in load on load control mode, GT exhaust temperature
increases and finally GT control changed over to Temperature control mode from load control mode. And on temperature control mode since IGV remains full open so on temperature control, IGV has no control on quantity of air therefore, it regulates fuel flow to maintain the designed TIT hence IGV normally does not modulate. But in case of Dabhol, it has been noticed that though the GT is running at base load on temperature control mode, still IGV is modulating with frequency and the frequent modulation of IGV is causing flow induced vibration owing to fluctuation in air flow, matter is be taken up with OEM for such behavior of IGV. 4、满负荷时停止IGV的调整
正常启动时IGV在最小的位置直至机组并网，随着燃机负荷的增加，IGV开至满负荷时的60%至65%。在加负荷控制模式，当机组负荷升至满负荷的80-85%时IGV全开，若要在加负荷控制模式下再继续升负荷，燃机排气温度增加，最终控制切换至温度控制模式。在温度控制模式下，IGV依然保持全开状态，IGV无法改变空气流量，因此，为维持设计排气温度必须要有足够的燃料流量，所以，IGV正常情况下是不会调整的。但是在Dabhol电厂，虽然燃机在满负荷的温度控制模式，IGV仍然随着电网频率进行调整，频繁的调整IGV导致空气流量波动引起自激振动，IGV的这种行为对压气机制造商是感兴趣的事件。
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