Technique for In-situ Meso-Scale Uniaxial Mechanical Testing in the SEM

Abstract

Satisfying the varied demands of a versatile mechanical testing lab requires dynamic range in force measurement, positional control, and testing speeds. Furthermore, many labs are outfitted with numerous types of optical microscopes and other advanced imaging systems that need to be accommodated by insitu testing methods. Using a novel actuator architecture based on the Psylotech µTS platform, the authors present a technique using precision machinery and industrial controllers to achieve: 25 nanometer crosshead control, 5 kN axial force, and up to 35 mm/sec at 600 mm/sec 2 . The system is specifically suited for digital image correlation (DIC), an image processing technique capable of tracking displacement down to 0.02 pixels [1]. The load frame's inherently planar design and attention to limiting out of plane motion results in stable preservation of a specimen's area of interest within the imaging focal plane. With low out-gassing materials and vacuum chamber feed-through compatibility, the system is meant for SEM chambers; however, it is also superbly capable in air when operating under optical, confocal, or µCT imaging systems. The authors present an overview of the design constraints, system evolution, and performance specifications.An SEM compatible load frame must handle high vacuum conditions in the region of 10 -5 to 10 -7 Torr. Additionally, most SEM stages are specified to handle up to 500 grams of mass, though in practice can handle up to 2 kg and still be translated with enough precision to enable the type of tests performed with mechanical load frames. While a large range of chamber volumes and configurations exist, it was found that a footprint of roughly 100 mm x 180 mm would satisfy the medium and larger size chambers. Also constraining the design was forcing the center of the specimen to be no more than 110 mm from one end of the load frame (in the axial direction) and as close in z-position to the electrode tip as possible. The first application required testing of metallic superalloy dogbone specimens up to 4.5 kN; these specimens were approximately 20 mm gauge length and required up to 50% strain. Quasi-static strain rates were required for these tests. The system was also expected to be used with optical microscopy, and needed a method to keep an area of interest centered under the optical or electron microscope objective (which is provided through a secondary centering stage). Less than 2 microns of out of plane motion and 1 micron displacement resolution were required to perform the test and in order to prevent false strains in DIC.To satisfy these varied requirements, the µTS platform was used as a starting point. The µTS is a welltested actuator design encompassing a pre-loaded ballscrew and direct-drive servomotor actuator. The use of high efficiency permanent magnet motors along with the direct-drive architecture allows for high force capacity in a compact form. Additional benefits include high speed capability due to lack of a gearbox. The µTS also incorporates Psylotech's proprietary...