Mechanical Property Experiments with Ultra-High Strength Micrometer Scale Fibers

Recent advances in materials synthesis have resulted in the development of continuous carbon fibers with one micron diameter and tensile strengths exceeding 5 GPa. Mechanical experiments on such strong fibers with very small diameters require tensile forces of the order of 5 mN or larger, which presents a major challenge to surface micromachined MEMS–based methods, while making rather impractical the use of existing macroscale methods for fiber testing. The application of large forces results in nonlinear response and out-of-plane deflections of surface micromachined MEMS. Furthermore, existing means for attaching high strength microscale fibers to MEMS can bias the measurement of the mechanical strength or the elastic modulus. This study addresses these issues via the design of microscale mechanical testing devices with a full-range linear response for several microNewtons of applied force, while also employing a new compliant gripping method, which eliminates specimen sliding or failure initiation of high strength fibers at the grips. With this new method, high strength carbon fibers of 1 μm diameter, reinforced with carbon nanotubes, were 100 % successfully tested. Notably, the fiber strength measured with the new compliant gripping method which averted fiber failure at the grips, was 20 % higher than that measured using the prior state-of-the-art approach for fiber gripping via Pt deposition by a focused ion beam.