Small-scale high-cycle fatigue testing by dynamic microcantilever bending
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Research Letter
Small-scale high-cycle fatigue testing by dynamic microcantilever bending Stefan Gabel , Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Materials Science & Engineering, Institute I, Martensstr. 5, Erlangen D-91058, Germany Benoit Merle , Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Materials Science & Engineering, Institute I, Martensstr. 5, Erlangen D-91058, Germany; Interdisciplinary Center for Nanostructured Films (IZNF), Cauerstr. 3, Erlangen D-91058, Germany Address all correspondence to Stefan Gabel at [email protected] (Received 24 February 2020; accepted 20 April 2020)
Abstract The lifetime of cyclically loaded devices is often limited by the fatigue resistance of their individual phases. An advanced method is presented for measuring the high-cycle fatigue behavior of materials at the micrometer scale using a nanoindenter. It is based on the cyclic deflection of focused ion beam-fabricated microcantilevers using the continuous stiffness method (CSM). In line with experimental data on bulk nanocrystalline copper, the specimens exhibit grain coarsening followed by the formation of extrusions and a fatigue strength exponent of −0.10. The method is suitable for characterizing single phases and individual components of further complex systems.
Introduction Many modern alloys consist of at least two different phases, such as ferrite and cementite in steel, or a layered architecture, such as in TiAl. Their combination in an ordered microstructure results in outstanding mechanical properties. While the mechanical characterization of the system can be easily done at the macroscale, the contribution of the individual phases is hard to determine. For this purpose, microscale setups were developed to measure their tensile,[1] compressive,[2] or fracture behavior.[3,4] However, there are only limited possibilities to characterize the fatigue behavior at the small scale.[5–7] Reliable cyclic methods are available for thin film testing[8–10] or for custom MEMS (micro-electro-mechanical systems),[11] but the knowledge obtained from these specific objects cannot be extrapolated to any 3D structure. Small-scale mechanical testing on other objects typically relies on nanoindentation approaches. Classical pyramidal geometries are poorly suited for accessing the local cyclic behavior[12] due to the complex stress state introduced in the sample. Methods with defined stress states should be preferred, such as micropillar compression tests[13,14] and microcantilever bending.[15,16] However, there are only few implementations reported and most of the time they are limited to low-cycle fatigue (LCF) testing.[17–19] Merle et al.,[20] as well as Lavenstein et al.,[21] pioneered a new approach to extend small-scale testing to the high-cycle fatigue (HCF) range based on the use of the continuous stiffness measurement (CSM) method.[22] The CSM is widely used in nanoindentation systems for monitoring the variation of the hardness and Young’s modulus during indentation.[23] Independently from each ot
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