Browsing by Author "Pierron, Olivier N."

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    Comparison of the low and high/very high cycle fatigue behaviors in Ni microbeams under bending
    (Springer, 2021-02-16) Barrios, Alejandro; Kakandar, Ebiakpo; Castelluccio, Gustavo M.; Pierron, Olivier N.
    The present work demonstrates a micromechanical technique to investigate the low cycle fatigue (LCF) behavior of Ni microbeams under fully reversed bending loadings. The technique extends the range of measured fatigue lives from the previously reported technique for high and very high cycle fatigue (HCF/VHCF) characterization in the same microbeams. The results highlight significant differences in the slope of stress and strain-life behavior and crack propagation rates that differ from an average of 10–12 m/cycle in HCF/VHCF to an average of 10–8 m/cycle in LCF. These results, in addition to postmortem fractography work, suggest that the mechanisms follow the conventional mechanisms of crack tip stress intensification in the LCF regime. This is in stark contrast to the void-controlled mechanisms that were previously identified in the HCF/VHCF regime. These results demonstrate that the transition in governing mechanisms from void-controlled to conventional mechanisms is highly influenced by the size effects present in the microbeams.
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    A computational and experimental comparison on the nucleation of fatigue cracks in statistical volume elements
    (Elsevier, 2020-04-05) Kakandar, Ebiakpo; Barrios, Alejandro; Michler, Johann; Maeder, Xavier; Pierron, Olivier N.; Castelluccio, Gustavo M.
    The failure of micron-scale metallic components presents significant variability as a result of their size being comparable to microstructural length scales. Indeed, these components do not represent the bulk of the material but correspond to statistical volume elements (SVEs). This work investigates the role of SVEs on fatigue crack nucleation with a novel comparison between microbeam experiments and microstructure-sensitive simulations. We recreate multiple microstructural computational realizations to estimate fatigue crack nucleation lives and orientations by means of physics-based crystal plasticity models. We demonstrate a unique approach to validate microstructure sensitive models and quantify the fatigue crack stochasticity associated with small volumes.
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    Quantitative in situ SEM high cycle fatigue: The critical role of oxygen on nanoscale-void-controlled nucleation and propagation of small cracks in Ni microbeams
    (2018-02-28) Barrios, Alejandro; Gupta, Saurabh; Castelluccio, Gustavo M.; Pierron, Olivier N.
    This Letter presents a quantitative in situ scanning electron microscope (SEM) nanoscale high and very high cycle fatigue (HCF/VHCF) investigation of Ni microbeams under bending, using a MEMS microresonator as an integrated testing machine. The novel technique highlights ultraslow fatigue crack growth (average values down to ∼10–14 m/cycle) that has heretofore not been reported and that indicates a discontinuous process; it also reveals strong environmental effects on fatigue lives that are 3 orders of magnitude longer in a vacuum than in air. This ultraslow fatigue regime does not follow the well documented fatigue mechanisms that rely on the common crack tip stress intensification, mediated by dislocation emission and associated with much larger crack growth rates. Instead, our study reveals fatigue nucleation and propagation mechanisms that mainly result from room temperature void formation based on vacancy condensation processes that are strongly affected by oxygen. This study therefore shows significant size effects governing the bending high/very high cycle fatigue behavior of metals at the micro- and nanoscales, whereby the stress concentration effect at the tip of a growing small fatigue crack is assumed to be greatly reduced by the effect of the bending-induced extreme stress gradients, which prevents any significant cyclic crack tip opening displacement. In this scenario, ultraslow processes relying on vacancy formation at the subsurface or in the vicinity of a crack tip and subsequent condensation into voids become the dominant fatigue mechanisms.