Understanding high-cycle fatigue in aerospace turbine parts

Along with foreign object damage (FOD) and erosion/corrosion, high-cycle fatigue (HCF) of airfoils from aero-elastic excitation has a significant impact on the safety and reliability of turbo-machinery. Failure of rotating turbine parts from combined cycle fatigue – low-cycle fatigue (LCF) and HCF – accounts for up to 40% of problems in engine development and an equivalent amount of in-service issues. HCF is categorized by a cyclic loading failure mode that typically occurs above 10,000 cycles, in comparison to LCF, where failures occur below 10,000 cycles.

Evaluating HCF is a critical aspect of aircraft engine blade design. Blade behavior in turbomachines is difficult to replicate accurately outside of the engine because of the complexity of the operating parameters and the harsh internal environment in the engine. Rotation of the bladed rotor results in aerodynamic loads from unsteady, periodic flow field, causing vibrations. The centrifugal effect of the rotation itself produces inertial loads within the blade structure that affect the damping and boundary conditions of the blade attachment mechanisms.

While shaker tables provide fundamental information about blade response, many OEMs would also like to evaluate airfoil HCF performance in a realistic, rotating environment. Conducting HCF testing with Test Device’s trademarked Dynamic Spin Rig (DSR) is substantially more affordable than engine testing and provides better data from a more-focused, densely instrumented evaluation of the blade response.

Test Devices has two distinct methods for exciting resonance frequencies of blades during DSR rotation testing: liquid jet excitation and aerodynamic pulse generation (APG).
 

Pulse-generation testing

The APG test method uses special hardware in a partial-atmosphere test chamber. Both methods are used to excite blades at different conditions, evaluating their resonance response in rotational conditions. One unique benefit of APG is that it can be performed in high-temperature environments.

The APG method was developed more than 20 years ago, and although proven effective, the parameters that contributed to the test successes were not well understood. In 2011, the company began the process of evaluating and analyzing the legacy APG test method in response to material advances, including the industry’s move toward non-metal airfoils, which require advanced evaluation techniques. Through analysis and additional experimentation, the company’s engineers have gained a more thorough understanding of the mechanics of dynamic spin testing with APG excitation.

The first step was to increase the drive power of the DSRs at the company’s test facility. Higher powered drive systems provide the muscle to run APG test rigs at higher atmospheric levels, producing a stronger blade response to the APG exciters. Improvements also were made to the existing air system for testing with Test Devices’ high powered air turbines, while at the same time adapting the 42" spin test machine as well as its largest (54") machine with a 260kW electric drive system.

The next step was to assess what was contributing to the experimentally validated excitation response (to the APG hardware) within the DSR. A computational fluid dynamics (CFD) analysis of the complex conditions within the DSR provided insight. The air flow, pressure plots, and resulting stress on test hardware were evaluated for basic understanding and characterization. The model was then iterated to evaluate different static hardware geometries and investigate locations that produced the maximum excitation pulse. When the analysis was completed, the results were evaluated internally by technical experts.
 

Test-system improvements

Engineers designed a modified test rig using excitation hardware with revised geometry as suggested by the analysis. Testing completed in early 2014 validated that the modified hardware had the desired effect and evaluated how the hardware’s placement contributed to the response. Results showed significantly increased blade response and confirmed the underlying principles of the APG excitation method.

“Based on test results, we gained more confidence in achieving blade excitation levels up to six times higher than before,” says Hiro Endo, chief technology officer, Test Devices. “We validated many of our hypotheses, which gave us a firm direction to take as we worked to optimize HCF excitation methods.”

The analysis of the test methods and improvements in hardware, facilities, and system design all led to a more realistic test environment for turbine parts. In addition to realistic centrifugal force on the airfoils and blade roots, the testing can be conducted at elevated temperatures. The APG test method has been demonstrated at 750°C (1,380°F).
 

Next-generation test methods

In 2001, Test Devices engineers patented liquid-jet excitation technology. By creating an advanced collection and pumping system within the evacuated chamber, the system allowed the use of atomized oil in the DSR for focused, customizable, direct excitation force that could be applied to rotating airfoils at specific locations.

However, as aerospace OEMs continue to use more new, advanced materials, they are demanding new testing methods as well. Test Devices now is developing an active airjet method that can create the same focused, high-amplitude excitation as the oil method. The goal is to create a test method that can hold resonance at required frequencies, including minor adjustments, as the rig temperature or mechanical condition is altered.

“The objective is to do more than replicate existing horizontal ‘blow down’ rigs in a vertical frame,” states Test Devices President Rob Murner. “The existing HCF test methods offer a unique ability to maintain the resonance conditions for up to 10 million cycles in an afternoon, which is a valuable offer to engine OEMs. We want to create an active airjet method that can meet this criterion, because it is unique in the industry.”

The goal is to evaluate the active airjet approach and quantify the resonance response that can be achieved without sacrificing the ability to maintain resonance conditions in a rotating test environment. If successful, the next step will be to identify possible hurdles in scaling to larger hardware. Follow-up phases will explore ways to conduct the testing in a heated environment.

Endo explains that Test Devices is focusing its efforts on new test methods to meet the upcoming needs of aerospace companies dealing with fatigue issues in turbine parts.

“Our intention is to see if customers’ materials are robust enough to withstand HCF forces as well as to capture the force and conditions that allow us to identify at what precise point the part will fail,” Endo adds. “HCF is a complex phenomenon and one that provides great insight for customers to use in their designs.”

Test Devices Inc.
www.testdevices.com