High Strain Rate Testing from Micro-to-Meso Scale

Rajaprakash Ramachandramoorthy^1,2, Szilvia Kalácska², Patrik Schürch³, Manish Jain², Jakob Schwiedrzik², Wabe Koelmans³, Laetitia Philippe², Xavier Maeder², Johann Michler²

Max-Planck-Institut für Eisenforschung¹, EMPA (Swiss Federal Laboratories for Materials Science and Technology)², Exaddon AG³

Dynamic properties of materials at high strain rates are vital to assess their suitability and reliability in applications ranging from common drops to demanding impact-protection applications. Macroscale mechanical testing at high strain rates, though challenging, is already a well-established field of research. But recently, given the push towards miniaturization, small scale mechanics has also been a topic of intense research over the last two decades. However, till date the micro and nanomechanical experiments have been largely limited to testing samples made using focused ion beam (FIB) milling at quasi-static speeds. Small scale sample preparation using FIB-based milling is a serial and time-consuming process that typically limits the number of samples tested in micromechanical studies, while allowing the fabrication of only simple geometries. On the other hand, the limitation in testing speed is primarily attributed to the lack of instrumentation capable of high speed actuation and simultaneous high speed capture of loads and displacements with micronewton and nanometer resolution.

This presentation will introduce a newly developed piezo-based micromechanical tester capable of conducting high strain rate micromechanical testing at speeds up to 10mm/s. The relevant hardware requirements and protocols specific to high speed testing at the small scale will be elaborated. Further, a localized electrodeposition based metal additive micromanufacturing (μAM) technique will be introduced as a viable method for fabricating full-metal microarchitectures. A large array of ideal copper micropillar test-beds built using this technique will be presented along with electron backscatter diffraction (EBSD) based microstructural characterization showing two distinct microstructures: microcrystalline and ultrafine grain (UFG). The mechanical properties of these copper micropillars, explored as a function of strain rate from 0.001/s to 500/s, will be shown as a function of initial pillar microstructure. Relevant, stress-strain signatures, thermal activation parameters and post-deformation microstructural analysis using EBSD and transmission kikuchi diffraction (TKD) for the copper micropillars will also be presented.

The last part of the presentation will highlight the capabilities of μAM to fabricate complex full-metal 3D microarchitectures such as microlattices and microsprings. Copper microlattices with different geometries, chosen from an energy absorption perspective, including honeycomb, octet and kelvin foam will be presented along with their dynamic compressive properties upto a strain rate of ~150/s. Further, a detailed structural and microstructural characterization conducted using FIB-based 3D slice-and-view and EBSD/transmission electron microscopy (TEM) respectively will be compared between the undeformed and deformed lattices. Finally, a finite element methods (FEM) based simulation aimed at understanding the structural evolution of the metal lattices under high speed deformation will be presented. It will be explained that the FEM simulation model takes into account the defect structures determined using the reconstructed 3D slice-and-view lattices and the appropriate constitutive laws identified from the high strain rate compression of copper micropillars.