Dynamic Spin Testing
Overview
Managing resonant vibration of air turbine rotor blades presents a technical challenge for jet engine and industrial gas turbine programs. Under certain conditions of operation, blades vibrate in response to pulsing flow from upstream stages. When the vibration matches the natural frequency of the component, the blades can crack and fracture from High Cycle Fatigue. Repair of integrally bladed rotors (IBR’s and Blisks) often results in changes to the tuning of these components. HCF testing offers a means of validating the repair as well as retuning the component for further service.
Why It Matters
Blade failure and retiring components early because of fatigue are serious safety and economic problems for aircraft operators. The U.S. Department of Defense has reported that retiring components for fatigue or HCF failure costs $200 million per year in engine repair and reduced availability of aircraft. Failure in service can cause major engine damage with potentially disastrous consequences to people and property.
Conventional Approaches to Blade and Damper Testing
To address this issue, engine developers can modify IBR, blade, disk, and stator geometry, and introduce damping strategies to attenuate dynamic response. In order to evaluate resonant behavior and the effectiveness of damping mechanisms, engine builders can use a variety of methods.
Finite Element Analysis (FEA)
This computer-based approach can be used to model blade vibratory behavior. While potentially important, FEA cannot fully duplicate blade vibratory response in service because of the inherent difficulty in modeling boundary conditions or the effects of variations in manufacturing processes, material composition, component repair, and Foreign Object Damage (FOD) that occurs in service.
Life Engine Testing
The most accurate method of evaluation, not surprisingly, is also very expensive, with costs up to $100,000 per hour in operating and staffing charges. Other drawbacks include the danger of damage to the entire engine if an individual component fails.This loss of an engine during a test significantly reduces the scope of investigation and makes it impossible to explore conditions near failure.
Bench Testing
Blade behavior can be tested on the bench using shakers and various kinds of vibrometry. While some helpful data can be collected, these bench tests do not include the effects of centrifugal loads, a critical driver of blade behavior.For IBR's, bench testing does account for influences from the rest of the component. It is the equivalent of finding the tone of a bell by just pinging a small piece of it. As a result, frequencies and mode shapes determined with bench methods are not adequate to understand resonance in service.
Superior Approach to Blade and Damper Testing
Test Devices, Inc., has developed a new approach to blade resonance evaluation, called Dynamic Spin™ Testing, to help engine manufacturers evaluate blade/IBR resonance behavior and damping effectiveness in order to avoid or resolve blade failure issues.
Using Dynamic Spin Testing, bladed rotor components are tested in fully assembled, engine ready states. The rotor assemblies are accelerated to actual engine speeds while the blades are subjected to vibratory and thermally induced stresses matching the stresses experienced in engine operation. The excitation force used to produce resonant blade vibrations simulates engine blade-to-stator interaction, causing blade distortion.
Special Capabilities for Blade and Damper Spin Testing
| Precise speed control to allow for “on resonance” dwelling in order to accumulate HCF cycles and evaluate the life of blades under realistic operating conditions. |
| Elevated temperature testing with axial and radial thermal gradients accounts for both mechanical and thermal stresses. This leads to more accurate prediction of component hot spots. |
| Detection of cracks in rotating assemblies with a patented system that automatically shuts down a test when a crack has been detected, saving the flawed component for analysis. |
| Excitation of resonant vibrations in bladed rotors to evaluate blade/damper design performance and HCF life by simulating excitation force imparted by engine stators. |
Advantages of Dynamic Spin Testing
- More Accurate, Less Risky, More Cost-Effective: Dynamic Spin Testing more accurately evaluates the blade/damper performance of turbine engine components and stages than traditional test methods, and it is both less risky and an order of magnitude less costly than live engine testing.
- Realistic Resonant Vibration: Dynamic Spin Testing produces realistic resonant vibration of the blades so that modal frequencies can be determined accurately. Precise speed control allows very slow resonance crossings and dwelling at resonance for characterization of fully developed modes and assessment of blade/damper performance. Slow resonance crossing also enables calculation of blade amplification factors for measuring the severity of various modes and the risk they present.
- Fatigue Life Evaluation: Dynamic Spin Testing allows speed to be held at resonance for long periods of time so blade cracking can be detected to measure time to crack. Resonance dwell testing is important for measuring the reduction in fatigue life that results from various kinds of blade damage from foreign object impacts.
- Test Options and Flexibility: Dynamic Spin Testing offers options to test characterization of airfoil response, evaluation of damper performance, and life assessment of blades until they crack.
- Flexibility: By including the static centrifugal stress and tailoring tests for specific rotor configurations by adjusting speed, temperature, and excitation force, Dynamic Spin Testing provides a powerful new method for testing bladed turbine engine assemblies.
- IBR/Blisk Repair Validation and Retuning: IBR/Blisk components that experience FOD can be repaired using various methods and returned to service. These various repair methods often change the natural resonance of the part, which can result in premature failure. Evaluation of the new resonance characteristics enables retuning of the component to avoid such situations. HCF testing is also a great means of validating the repair.
Contact
Please contact Christine Murner (+1-978-562-4930) in our sales department for more information and to learn how we can help you.