SEM and fatigue-testing capabilities in a single instrument
For material scientists, understanding the relationship between a material’s mechanical properties and microstructure under real-world conditions is critical to advancing the understanding of its behavior. Scanning electron microscopes (SEM) are widely used to study microstructure, but are limited in the volume of material that can be studied, and are typically not able to simulate loading or real world environmental conditions.
Using a large-chamber scanning electron microscope (LC-SEM) to examine large material specimens during fatigue testing would shed considerable insight into how those materials fail over time. Accomplishing this, however, presents a host of engineering challenges — and to date, no technology has been developed that integrates fatigue testing with scanning-electronic microscopy.
The primary challenge involves the required near-perfect synchronization of the test software, control system and actuators with the LC-SEM used for observation. Specifically, test engineers must know precisely when an image was captured during a test, so that those observations can be reconciled with the exact test conditions under which they were acquired.
Further complicating matters, many of the materials under LC-SEM observation necessitate a low-pressure vacuum and high-temperature operating environment, making it a daunting prospect to introduce a high-pressure servohydraulic load frame into the equation.
That’s why the University Erlangen-Nuerenberg’s Institute for Material Science brought together two leaders in their respective fields — MTS in material testing, VisiTec in material observation — to help the University better understand the effects of lifecycle fatigue on certain materials that require a relatively large specimen size.
“We were basically trying to combine fire and water with this initiative, and we knew we had a major challenge on our hands,” said Adriana Romero, director of VisiTec America. “But we also knew that the complementary expertise of VisiTec and MTS would offer the best means of finding a workable solution.”
The joint efforts of MTS and VisiTec ultimately yielded a cutting-edge instrument, offering full servohydraulic fatigue testing and LC-SEM capabilities, while perfectly synchronizing the functionality of the two technologies. This breakthrough opens significant potential for material scientists to better understand why a wide variety of materials degrade and fail in their intended applications.
In addition to its engineering expertise, MTS contributions to the solution include a 100kN servohydraulic load cell and actuator, hydraulic power supply, Multipurpose TestWare application software and a FlexTest SE control system. The solution also employs an MTS high-temperature extensometer, specially designed hydraulic grips and an alignment kit with a strain-gaged specimen to ensure data integrity. German test technology company Sigmatest provided a key support role to MTS, supplying a loading cage, piston rod clamp and miscellaneous auxiliary devices.
Pioneered by the German company VisiTec, the LC-SEM facilitates the close observation of relatively large material specimens (up to 1500 mm in height and diameter), at temperatures up to 1100˚C, in a clean vacuum environment. An integrated positioning system enables measurements to be taken at multiple locations on the specimen.
The test equipment is mounted on a steel frame inside the LC-SEM vacuum chamber, with an opening in the chamber that allows the servohydraulic load frame to operate alone. When the opening to the chamber is closed, the necessary vacuum is created to operate the LC-SEM simultaneously. For the first time, a material under LC-SEM observation can be subjected to a full cyclic deformation test, complete with standard control channels including stroke, stress and strain.
“Technology and innovation might be overused terms, but these were the two key ingredients in making this project a success,” said VisiTec’s Romero. “By combining the unique domain expertise of MTS and VisiTec, we were able to resolve a pressing customer challenge and create the ultimate testing device at the same time. It is a clear example of various technologies coming together to create value far beyond the sum of the individual parts involved."
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