Dr. Leser with Thermomechanical Fatigue Test Setup
Dr. Leser with thermomechanical fatigue test setup

High-Temperature Materials Test
Monitoring thermal gradients in preparation for thermomechanical fatigue testing

Up To Code

Dr. Christoph Leser is the product manager for materials testing software at MTS. He has nearly 20 years of experience as a researcher, test engineer, application engineer and test consultant, and is a subcommittee chairperson for ASTM. Here, Dr. Leser describes how materials researchers can achieve consistent results in thermomechanical fatigue (TMF) testing with greater confidence.

Q: What factors contribute to the rising importance of TMF testing in materials research and development?

Leser: Two of the most exciting areas in materials R&D today are related to the turbines used in aerospace and power generation. In both cases, designers are pushing operational efficiency and reliability beyond all previous expectations. To do that, they need turbine components and structures that can withstand higher temperatures for longer periods of time under a variety of cyclic loading conditions. Materials under investigation include innovative superalloys, ceramic matrix composites, ceramic coatings and others. Knowing how these materials react to simultaneous changes in temperature and load make TMF an essential part of the R&D process.

Q: Can you describe the mechanics and objectives of TMF testing?

Leser: TMF testing is a kind of fatigue testing that simulates the real-world service conditions of engineered components as closely as possible. Specifically, TMF tests characterize the response of materials to simultaneous cyclic mechanical loading and fluctuating temperature, which produce a synergistic response that is not easy to predict using isothermal fatigue testing. The data generated through TMF testing helps researchers model component behavior as well as validate existing models in a controlled test environment.

Q: What are the unique challenges of TMF testing?

Leser: TMF can be difficult because it uses in-phase and out-of-phase mechanical and thermal cycling. The phase shift between temperature-induced and mechanically induced strain invites numerous interpretations of material behavior. You need to apply heat using radiant or induction methods, which may require cooling. Test equipment and fixturing must be able to withstand high temperatures. Capturing accurate data is hard because high-precision systems behave differently at very high temperatures. The type of material being tested influences each setup, and there are many options for instrumentation, heating, cooling and fixturing. For all of these reasons, TMF testing tends to be customized and complex.

Q: Given this complexity, how can researchers hope to achieve consistency from test to test and lab to lab?

Leser: There are standards from ISO and ASTM researchers can follow, which provide best practices for consistently characterizing materials subjected to simultaneous thermal and mechanical loads. The TMF Code of Practice, which was created by a consortium of international partners, is another recent example. As I mentioned, one of the big challenges with TMF is that total strain includes thermal and mechanical components, and you have to know the precise distribution. For this reason, the TMF Code of Practice reduces the phase shift command to two possibilities: in-phase (maximum strain at maximum temperature) and out-of-phase (maximum strain at minimum temperature) cycling. The code also includes all the steps required to perform the test, from calibration to data reporting.

Q: How can researchers be confident they are meeting the requirements in the TMF Code of Practice?

Leser: Researchers can adhere to the code — and address the complexity of TMF testing — using advanced software solutions. MTS, for example, created a TMF template for MTS TestSuite™ Multipurpose software. The template is compliant with both ASTM standards and the TMF Code of Practice. It is completely transparent, providing full insight into its algorithms and calculations. You can use it to interpret standards in different ways, and you can modify the template for your own needs to gain new insight. All of this allows researchers to perform many different TMF tests for a diverse array of materials and specimen geometries.

Q: What features of the template and the software are particularly suited to TMF testing?

Leser: The template gives researchers explicit access to the activities required to perform the test through an intuitive graphical interface. Each task from the TMF Code of Practice is represented in the template by a button from the command panel, and each button is defined by a program block the user can modify. Basically, if you can see it, you can change it. This applies to test calculations and workflow, as well as the visual representation of data during the test, which offers unlimited views and is user-configurable. It also applies to results, so you can export the data and present results in the format that works best for your needs.



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