Rotor Health Monitoring / Crack Detection @ Test Devices, Inc.
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Crack DetectionIntroductionEvery high speed machinery development program requires extensive life testing, to establish the useable life of the rotating components. Failure to do so could lead to an unpredictable failure in the field that would pose significant safety hazards. Test Devices offers customers Low Cycle Fatigue (LCF) testing services, which includes cycling rotating components from low speed (low stress) to operational speed (high stress) repeatedly for a set number of cycles, at both ambient and elevated temperatures. Historically, rotor health information was not available real-time during LCF tests. Essentially, the operator was "flying blind". In response to the need for more real-time data, Test Devices developed a patented1 "crack detection" system for detecting and monitoring developing rotor cracks or assembly problems during LCF testing. This rotor health monitoring system allows operators to halt LCF tests when there is a small change in balance, resulting from the formation of a crack, but prior to rotor failure. Previous to its development, all spin pit LCF tests ended in one of two ways: the rotor under test successfully completed the intended number of cycles, or a rotor burst leaving little indication of cause other than a bag of shrapnel.
Figure 1 The rotor health monitoring system, also referred to as crack detection, allows for the performance of LCF tests with substantially lower risk of burst. Simple vibration monitor systems cannot do this because cracks or other anomalies rarely cause observable change in vibration amplitude. Using the rotor health monitoring system and halting a test before burst allows the metallurgist an opportunity to view the disk without collateral damage and to determine the crack initiation site with complete certainty. With no burst damage, the crack surface is perfectly preserved, allowing crack growth rates to be determined precisely. ExplanationThe crack detection system uses non-contact probes, a high speed signal conditioner, PC, and software algorithms to monitor and graphically display rotor health real-time. The system measures the unbalance vector of the rotating component and compares the amplitude and phase of this unbalance to a baseline previously established. Any distortion in the strain field, or any relative movement of the components of the assembly will cause a change in unbalance and will appear as a very small but detectable vector with a distinct amplitude change ("crack amplitude" - see graphs below). A vibration vector change usually indicates that a crack has developed or reveals some other important change in the assembly such as a loose tie bolt, arbor crack, etc. In the spin chamber, this signal is used as an early warning of a crack. Once the amplitude change indicates the presence of a crack the part can be removed for inspection to determine the location and size of the crack, or to determine if there has been some slipping, fretting or movement between components of the assembly.
Figure 2 Test Devices' crack detection system can be operated manually, requiring the operator to stop the test prior to failure, or automatically, where the system stops the test prior to failure. Test ResultsTest Devices' crack detection system has been used for over seven years on countless LCF tests for customers ranging from jet engine OEMs to energy storage flywheel developers. The system has proven time and again to be invaluable to customers, preserving test rotors from burst and allowing for metallurgical inspection. Shown below are crack graphs, displaying amplitudes diverging from the baseline values, resulting in the tests being stopped before rotor failure. "I can confirm that the Test Devices crack detection method has proved valuable to Goodrich in detecting a crack in a rotor (service retired for LCF life) that was being used for a life extension exercise. It has provided the opportunity for the damaged rotor to be examined without the usual secondary damage that occurs when a rotor bursts. The crack detection system facility also saves manufacturing time and cost of replacement tooling that becomes inevitable when a rotor bursts.
The crack detection system was one of the primary reasons for selecting Test Devices as the test house for LCF testing." The graph below shows an indication of a crack seen during an actual test of a power turbine rotor. At the end of this test there were sixteen (16) cracks in the part, all approximately 0.2" in size. The graph shows the relative size of the crack compared to an initial baseline reading. The first solid indication of a crack in this test came at approximately the 15,000th cycle where the amplitude had increased (which is a sign that an unbalance has occurred).
Figure 3 The graph below shows a long duration LCF test of an impeller assembly. The test was stopped due to the exponential change in the crack amplitude from the baseline reading. Upon inspection the impeller was found to have a substantial, visible, crack, which is shown at the top of this section.
Figure 4 The turbine disk pictured below clearly shows a bolt hole crack that was detected during LCF testing by Test Devices' rotor health monitoring system.
Figure 5 The large linear crack shown in the impeller picture below was detected by Test Devices' rotor health monitoring system during an LCF test.
Figure 6 Test Devices' crack detection system has also been monitored remotely, for tests at customer facilities. Our engineers are able to monitor the system, installed at a customer's site, by the use of a dedicated telephone connection or VPN. In this way customers are able to draw upon the experience of Test Devices' engineers in monitoring live LCF tests at their own facilities. During a remotely monitored test conducted by the US Government, Test Devices repeatedly validated its crack detection monitoring system. The quotes below are from the Naval Air Systems Command spin test system operator who observed the performance of TDI's crack detection monitoring system during recent tests. "...the rapidly increasing change in the Test Devices output (rotor health monitoring system) caused us to shut down the test to avoid a catastrophic failure. Just prior to the failure, a phone conference between Test Devices, the ... (customer names)... was assembled to quickly discuss the data and make the decision to stop the test." - Test of December 2002 "An indication from the Test Devices system just prior to the release of fragments (small pieces were liberated from the test disk) and increasing dramatically for several cycles after that event lead to the shutdown of the test." - Test of May 2003 BenefitsTest Devices offers crack detection as an option to all of its customers on all LCF tests. The crack detection method has been proven to be reliable and allows a significant reduction in the need for interrupting the test for interim inspections. Benefits from halting an LCF test prior to rotor failure include the items listed below:
As part of the crack detection monitoring service, Test Devices supplies customers with periodic crack detection data to keep the customer informed of the rotor's health. An additional benefit customers receive for use of the crack detection monitoring service at Test Devices is that it absolves customers of the "burst contingency". The burst contingency, which is the equipment repair costs following a rotor failure, is an industry standard and pays to rehab the equipment damaged during high energy rotor failures, and is the responsibility of the customer. SummaryCustomers can now have more predictable test program costs by using our Crack Detection system, lower development costs overall and preserve test specimens for analysis of failure root cause. __________________________ 1(US patents - 6,098,022 & 6,456,945) |
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