Low Cycle Fatigue @ Test Devices, Inc.
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Low Cycle Fatigue TestingIntroductionHigh-speed rotating equipment is susceptible to many kinds of problems. Problems range from bearing wear and vibration to component failure from internal flaws. One of the problems that has plagued the jet engine manufactures for decades is failure due to Low Cycle Fatigue. Low Cycle Fatigue, commonly referred to as LCF, is the fatigue of rotating components brought on by the continuous imposing and relaxing of centrifugal force caused by fluctuation in speed. Typically, rotating components, much like automotive engines, have an idle or low speed and an operational or high speed. Cycling from the low speed (low centrifugal stress) to the operational speed (high centrifugal stress), continuously stresses the rotor material. Much like a paper clip is fatigued when it is bent back an forth in your hand (see Figure 1), a high speed rotor fatigues as it cycles from low to high speeds until a point at which it fails.
Figure 1 Low Cycle Fatigue failures typically have a root cause which stems from flaws in the material (impurities or voids), abusive machining which creates high stress concentrations, or wear between components. However, even "perfect" components have a finite life, and will fail after a certain number of cycles. A cycle is the completion of one repetition from low to high speed, and back to low speed. Failures of any high speed rotating components (jet engine rotors, centrifuges, high speed fans, etc.) can be very dangerous to surrounding equipment and personnel (see Figure 2), and must always be avoided. Though high speed equipment manufacturers design their equipment to be safe during operation, until tested, they cannot be certain of safety.
Figure 2 ProcedureTest Devices offers Low Cycle Fatigue testing services covering a complete speed range up to 160,000 rpm, in its robust spin test chambers (see Figure 3). Due to customer schedules and cost requirements, LCF drives have to accelerate and brake (decelerate) the test rotor in the shortest possible time to maximize the number of cycles per day. Cycle rate is productivity. Longer cycle times delay development programs, and raising the cost for high speed turbo machinery manufacturers.
Figure 3 TDI uses its proprietary air turbines (see Figure 4), specifically designed for LCF testing, to efficiently carry out low cycle fatigue tests. Test Devices' 700 series spin system air turbines can typically produce up to 5 times the number of cycles as conventional turbines in a given period of time. This time efficiency results in the customer receiving test data much earlier, providing the opportunity to make critical program decisions while keeping the project on schedule.
Figure 4 Elevated TemperatureTest Devices offers customers the option of running cycle tests at elevated temperatures. LCF tests can be run at isothermal temperatures up to 1600°F (870°C). Many customers find performing cycle testing at operational temperatures better simulates the conditions the rotor will actually experience during use. The increased temperature testing not only stresses the component with centrifugal forces but also thermal forces, producing a worst case scenario for the component during testing. Elevated temperatures in cycle testing can typically be held to within ±10°F (5.5°C). Temperature gradient simulation during testing adds an additional stress on rotating components. Differences in temperature (non-isothermal), both radially and axial, create stresses in an assembly. Variations in temperature throughout an assembly result in additional stresses, and potentially reducing a component's usable life. TDI offers high temperature LCF testing with temperature gradient simulation. Controlling the temperature of individual sections of a rotating assembly allows for better correlation with the CAD model and real world conditions. Controlled AtmosphereA problem that can arise in Low Cycle Fatigue testing of rotors with inserted (i.e. replacable) blades is fretting between the blades and rotor. Fretting is the micro movement of blades at the insertion point into the rotor (example: fir tree attachment), resulting in the transfer or release of material, and the premature failure of blades. Fretting can be accelerated in long duration LCF tests at high temperature and conventional vacuum levels of 100 - 300 millitorr. Test Devices has developed a technique to eliminate fretting during heated LCF tests by running the test at absolute pressures above typical spin pit vacuum levels. This technique requires: careful control of pressure to ensure too much oxygen is not introduced, high power drives to overcome the additional aerodynamic drag from the increased pressure, and special temperature controls to accommodate for rotor heating resulting from aerodynamic friction. Crack DetectionTest Devices offers "crack detection" for all Low Cycle Fatigue testing done at our facility. This system has proven to be invaluable to customers by allowing an LCF test to be halted once a crack initiates, but before rotor failure. Figure 5 below is an example of a rotor which was under test at TDI and was stopped prior to failure. For more information: www.testdevices.com/crackdetect.htm
Figure 5 Data AcquisitionThe graph below (Figure 5) is an example of data taken on TDI's on-line high speed data acquisition system. This digital system is used on all LCF tests done at TDI and has replaced paper data recorders for space conservation and more sophisticated data analysis.
Figure 6 Test Devices can simulate and measure a multitude of test parameters during Low Cycle Fatigue tests. This data is provided to the customer as well as archived at TDI for future reference. Benefits and ApplicationsSpin pit low cycle fatigue testing is an extremely important instrument in testing high speed component life. Customers are able to verify the number of predicted life cycles a component design can endure. This valuable test data can be obtained in a safe environment and in substantially less time and cost than from testing in the actual machine. High speed machinery benefiting from LCF testing include:
Test Devices' customers enjoy advantages in LCF testing at TDI not offered elsewhere in the spin testing service industry.
For more information, please contact one of our sales engineers at: |
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