A High-Temp Testing PrimerGenerating quality data at elevated temperatures
Dr. Yunming Hu, MTS Systems Integration Engineer, shares his expertise and insight into high-temperature materials testing, discussing why it is important, how to generate high-quality data and fundamental considerations for pursuing a variety of test applications.
Q: Why is high-temperature fatigue testing important?
Hu: High-temperature materials testing is particularly relevant in two industries: aerospace and power generation. In both cases, fuel efficiency depends on turbine performance. Components and structures in these turbines are subjected to repeated loading at extremely high temperatures. Fatigue damage in these turbines would be critical, and there is obviously zero tolerance for failure. Understanding how materials perform at very high temperatures plays a very significant role in the development of turbines that are more reliable and also more fuel-efficient.
Q: What do engineers do with high-temperature fatigue data?
Hu: High-quality fatigue data at service temperature conditions are used in three ways. Design engineers use the data to design new materials, such as superalloys, ceramic matrix composites and ceramic coatings. Stress engineers use the data to develop accurate fatigue life predictions of critical components. And quality assurance (QA) engineers use the data to qualify both materials and finished components for specific high-temperature applications.
Q: What types of fatigue tests are conducted at high temperatures?
Hu: There are three standard specimen fatigue tests. The majority are strain-controlled axial fatigue tests (conforming to ASTM E606), which are isothermal tests where strain is applied to a smooth specimen until damage, or deformation, occurs. The other two standard tests are thermomechanical fatigue (conforming to ASTM E2368), during which temperature is cycled along with load, and load-controlled axial fatigue (conforming to ASTM E466), which helps evaluate high-cycle performance as well as the effect of notched features at the edges of components.
In addition, you can also perform non-standard spectrum loading and variable amplitude loading. With these tests, researchers are usually attempting to simulate an extreme service condition, such as when overload conditions are experienced by aircraft for a very short time during takeoff.
Q: How are high-quality fatigue data generated? What type of test system configuration is required?
Hu: The setup is similar to other tests in many respects. There is a load frame, a grip interface for the specimen, strain measurement, a controller and software. Of course, high-temperature tests also require a furnace or induction heating system to generate the required temperature gradient throughout the specimen. Load train alignment is also very important.
Q: Why is load train alignment so important?
Hu: When you are performing high-temperature testing, any misalignment, whether concentric or angular, has a significant effect on fatigue life and compromises the quality of your data. Good alignment practices are really important because they reduce the variability in your test data and eliminate sources of uncertainty. MTS offers alignment tools that make it efficient to perform periodic load train alignment of a fatigue test frame, which is often required by testing specifications and/or testing standards. With an adjustable fixture on the load train that works in tandem with special alignment software, the MTS solution provides an easy way to achieve the desired alignment level and ensure that all bending strains are within an acceptable envelope.
Q: What MTS offerings should be used in a high-temperature test solution?
Hu: An ideal solution would include an MTS Landmark servohydraulic test system, which incorporates the MTS 370 load frame with an extremely stiff and lightweight crosshead, precision-machined columns, fatigue-rated actuators and best-in-class load cells. FlexTest® digital controllers would also be integrated, as well as MTS TestSuite™ software, which has a module specifically designed for isothermal and thermomechanical fatigue (TMF) testing. For heating, I recommend a multiple-zone furnace for most applications, but a combination of induction heating and active air cooling is required for thermomechanical fatigue. MTS also offers extensometers that are specifically designed to withstand very high temperatures.
Q: What can you tell us about specimen geometry?
Hu: Round gage sections are almost always preferred for low-cycle (strain-controlled) and thermomechanical fatigue tests, because they resist buckling and have no edge effect. The grip interface can be threaded, which requires preloading with a push bar to prevent backlash. Another interface is the buttonhead, which is well-suited to hot grips. Finally, there is the smooth shank end, which does not have backlash but requires collet friction grips and is best used in axial-torsional applications. You can also use tubular specimens to minimize radial thermal gradients for thermomechanical fatigue. Of course, in addition to the smooth round specimens, load-controlled high-cycle or low-cycle fatigue tests often use specimens that replicate a specific component’s grooved or notched geometry.
Q: How are hot and cold grips used in high-temperature testing?
Hu: Hot grips are used inside the furnace with threaded or buttonhead specimens. MTS 680 grips, for example, are made from nickel-based superalloy and are water-cooled. Cold grips are not heated, and are used with induction heating systems. The hydraulic seals in the collet grip (for smooth end specimens) can’t tolerate high temperatures, so you often have to use an extension bar. It is all about trade-offs. Threaded and buttonhead specimens introduce backlash, but they can go in the furnace. Smooth specimens do not introduce backlash, but they require grips that can’t go in the furnace. You have to make a judgment call based on your particular test.
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