Fatigue Fracture of a Compressor Blade of an Aeroengine: What Caused this Failure?
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The success of failure analysis depends on the use of appropriate analytical techniques in proper sequence. The approach and the methodology to be adopted for a particular failure investigation largely depend on the nature of failure. Collection of background information leading to development of a complete case history of the failure is, therefore, essential. This is followed by examination of the components, to establish the physical cause of failure, mostly through metallurgical investigation and evaluation of the material of construction. Metallurgical investigation can pinpoint the physical cause of failure but often falls short of identifying the true cause of failure. It has been seen that many a time, the true cause of failure is not established due to overlooking of minor details and not because of lack of advanced knowledge. This aspect of failure analysis has been highlighted in this article through a practical example. The failure involved fatigue fracture of a high pressure compressor blade of an aeroengine. The failure occurred within 12.2 h of engine run on the test bed.
KeywordsFailure analysis Background information Physical cause of failure True cause of failure
There are compelling reasons for investigating engineering failures. Unless the true cause of the failure is established, no remedial action can be initiated. The lessons learned from failure analysis are vital for the engineering profession and the industries that aim at design and manufacture of products with minimum probability of the service failure .
The first and foremost task in conducting a successful failure investigation is to develop a complete case history of the failure . This requires information about the failed component/system and the failure itself. In case of failure in complex engineering systems, collecting information before proceeding with the investigation becomes a formidable and challenging task. If the information is incorrect or concealed or overlooked, the investigation may take a wrong direction resulting in not only wasteful work but also loss of important evidences. Many a time, the failure analysts do not have access to all information that the manufacturer or the user may have. This poses difficulties to the investigators in deciding the approach to be followed and draw a detailed test plan for a particular failure case. It is, however, the job of the failure analyst to make attempts in obtaining relevant information about the failure as and when required throughout the investigation process. It is known that the same physical failure can occur due to several different reasons, and hence identification of cause of failure for the case at hand is important .
The goal of failure analysis is to determine what caused the failure. Metallurgical investigation can reveal the physical cause but may not always lead to determination of the true cause of failure. This is because in most cases, failure occurs as a culmination of many events or factors . Sometimes, with the information available, it is impossible to determine the primary factor that was responsible for the failure to occur.
In this article, failure investigation of a high pressure compressor blade (HPCR) of an aero-engine has been discussed. Despite exhaustive metallurgical investigation and consideration of all available information, the true cause of failure could not be established. Yet, circumstances demanded that a conclusion be formulated based on educated speculation and opinion. However, while compiling data for recommending rejection of the entire batch of the blades, certain information was fortuitously revealed. Probing this information further, the primary cause of the failure of the blade could be identified.
Chemical composition of fractured HPCR blade material
Fractured HPCR blade
Visual and Stereo-Binocular Examination
Scanning Electron Microscopy
Material of Construction
Analysis of Failure
Fractography study confirmed that the failure of the HPCR blade was by fatigue mechanism. It was established through metallurgical studies that fatigue crack initiation was not due to any stress concentration effects arising from localized mechanical or metallurgical abnormalities. Also, no deficiency in the material of construction was responsible for the premature fatigue failure of the blade.
The fracture in the HPCR blade was developed in a plane normal to the blade axis. The major part of the fracture surface had smooth appearance, and it contained several clearly delineated beach marks, having semi-elliptical shape starting from the convex surface. Only about 15% of the surface was created by fast fracture. Absence of any ridges or stretch zone marks between the beach marks implies that the fatigue failure of the blade was under relatively low stress levels. Further, the symmetry of beach marks with reference to the blade axis indicates that once initiated, the fatigue crack had propagated mainly under the symmetrical loading with respect to the blade axis.
None of the remaining HPCR blades showed any evidences which could be suggestive of high amplitude vibration in the engine.
Since the newly built engine was being run in the test bed as a mandatory requirement prior to fitting onto the aircraft, all important engine parameters were recorded, and they were closely monitored. Study of the records showed that the engine was healthy, and the vibration level was significantly lower than the allowable maximum limit till the failure of the HPCR blade occurred.
Conclusion Based on Laboratory Investigation
Despite thorough metallurgical investigation, the primary cause of premature fatigue failure of the HPCR blade could not be established. However, as a preventive measure, it was decided that all blades from this batch of production be rejected.
On enquiry, the manufacturer informed that the identification “2FT” stood for blade-2 of the six blades that were identified from a production batch for qualification through “fatigue test.” After selection of six blades by random sampling, they were sent to the laboratory for qualification test, and during this time, each blade was identified by a test sample number followed by “FT.”
Fatigue testing of the blades was performed on a vibration shaker. This testing consisted of mounting the base of the blade to a vibration plate and tuning in the frequency as well as the amplitude to prescribed levels. The entire test was then monitored based on the blade tip displacement. As per the technology requirement, the blade life specified for qualification was 2 × 106 cycles. Once the blades survived the specified number of cycles, the tests were terminated. Procedure demanded that the tested blades were properly tagged and kept in the inventory for a certain period of time before they could be destroyed.
Investigation revealed that the fractured blade of the engine was one of the fatigue tested blades that were sent to the laboratory for qualification of a particular batch of production. Hence, the blade had already accumulated fatigue damage during the laboratory test and prior to the fitment in the engine. The primary cause of the failure of the blade within 11.2 h of engine run was thus established. In the normal course of metallurgical investigation, the investigators could never have been able to find that a previously “fatigue tested” blade would find its way into the engine fitments. What caused this failure? The onus was on the manufacturer to investigate further and establish the root cause.
The physical cause of a failure, namely stress corrosion cracking or fatigue or hydrogen embrittlement, can be established through metallurgical investigation. But, it may not be sufficient enough for establishing the true cause of failure. It is widespread that too much attention is paid to the physical aspect of failure, and in the process, important information available on the failed component goes unnoticed or remains unread. Often, the true cause of failure is not established due to overlooking of minor details and not because of lack of advanced knowledge. As shown in this article, if these details are read, analyzed, and linked appropriately, the outcome of the investigation can be revealing, sometimes, astonishing.
The assistance provided by Mr. M. Madan during investigation of the failure reported is acknowledged.
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