Rotor Spin Testing:
Low Cycle Fatigue Testing
The Issue
High speed rotating components typically operate at both a low (idle) speed where there is low centrifugal stress and at a high (operational) speed where high centrifugal stress exists. Repeated cycling (the completion of one repetition from low speed to high speed to low speed) stresses the rotor material much like a paper clip is fatigued when it is bent back and forth in your hand (see Figure 1). This Low Cycle Fatigue (LCF) affect results in the initiation of cracks in the higher stress regions of the component, typically near bolt holes, blade slots, or other complex geometric features, which eventually lead to failure.
Why It Matters
Failures of any high speed rotating components (jet engine rotors, centrifuges, high speed fans, impellers, etc.) can endanger personnel in the vicinity of the failure, destroy expensive equipment, and result in huge monetary losses for the operator.
LCF failures typically result from flaws in the material (impurities or voids), poor or inconsistent manufacturing processes, complex geometries (bolt holes, scallops, blade slots, etc.) that create high stress regions (hot spots) on the component, and wear between components. However, even “perfect" components have a finite life. They fatigue in operational heat and stress environments, and after a certain number of cycles they fail.
Although high speed equipment manufacturers design components to have sufficient fatigue life during field operation, they cannot be certain of the durability of a high speed rotating component until the predicted behavior is validated by testing under realistic conditions.
What You Can Do
Given the relatively high cost of spin testing, it is important to gain as much experience from the component under test as possible. The sophisticated spin test facilities of Test Devices expose components to operating loads under realistic conditions like isothermal and gradient temperatures (radial/axial), and various boundary conditions. Test Devices also utilizes state-of-the-art equipment to detect cracks during testing, measure elastic/plastic growth while running (at speed), and measure strain at designated points on the component, or over a region of interest.
Test Devices offers LCF spin testing using high performance (fast cycle/low cost) drive systems. Component tests, as opposed to engine test rigs, afford our customers the opportunity to economically push their critical parts up to the limit of failure. This aggressive testing gives our customers the ability to reach the fatigue limit of critical rotating components, well before even "lead the fleet" engines. This early exposure to potential failures gives our customers time to resolve these problems before they are experienced in the field.
Advantage of Low Cycle Fatigue Testing
- Special Purpose: Test Devices utilizes the highest productivity drive systems in the industry. Our higher power drive systems are specifically designed for LCF testing, enabling Test Devices to "fast cycle" test components. This shortens program durations, cuts costs, and reduces the risk of damaging test assets.
- Faster Test Data: In the event of in-field failures, Test Devices can provide critical fatigue test data earlier, which helps speed redesign of components that failed before analytical model prediction.
- Data for Planning: Access to actual and model component fatigue test data enables customers to plan for design improvements or modifications proactively, rather than just reacting to unexpected results from test cells or performance data from the field.
- Better Productivity: Test Devices performs LCF tests throughout the complete component speed range in robust spin test chambers designed to contain high energy component failures. Electric motors and air turbines rapidly accelerate and decelerate test components in the shortest possible time, maximizing cycles completed per day.
- Greater Accuracy: Tight speed control is an essential part of LCF testing. A component may fatigue prematurely if exposed to higher stress levels throughout the LCF test due to extreme variations in maximum cycle speed. Conversely, a test component may display an unrealistically long fatigue life if routinely cycled to a speed lower than the set maximum cycle speed. Test Devices can maintain cycle speed tolerance to ±10 rpm. Enhancements nearing completion will allow us to maintain cycle speed tolerance to ±1 rpm.
- Data Acquisition: Test Devices uses an on-line, high speed, digital data acquisition system to record all test parameters throughout the duration of the LCF test. This system replaces paper data recorders traditionally used in the industry, allowing large amounts of data to be stored, transferred and analyzed easily.
- Reporting: LCF testing customers receive daily e-mail updates on the status of their project. The final report includes all cycle test data together with certificates of conformance, balance, heat calibrations, and tooling material certificates. Test data is also archived at Test Devices for the customer’s future reference.
Applications of Low Cycle Fatigue Testing
LCF testing is beneficial for any component that cycles between low and high speeds during normal operation. Components typically LCF tested include:
| Jet engine rotors | High speed fans | High speed electric motors |
| Centrifuge rotors | Composite flywheels | Industrial gas turbine rotors |
| Turbochargers | Compressor rotors | Material characterization |
| Rocket pumps | ||