It has long been recognized that when basal planes of crystalline layered solids, be they graphite, mica, phyllite or MAX phase, are loaded - edge-on - in compression, they fail by kink band, KBs, formation. The latter were long assumed to form by the nucleation and movements of basal dislocations. In 2015, the idea of atomic layers rippling - christened a *ripplocation* - noticed in MoS~2~, was first introduced. In 2016, we made the case that bulk ripplocations are the operative deformation mechanism in, not only in van der Waals systems, but also in most crystalline layered solids, defined herein as solids in which the deformation, at least initially, is confined to two dimensions. This insight allowed us to explain a number of important observations that were not understood among them, the large material piles-up around the indenter when single crystals were loaded normal to the basal planes, and delamination cracks produced by loading parallel to the layers. Using high-resolution transmission electron microscopy, we and others, presented crucial direct evidence for bulk ripplocations in Ti~3~SiC~2~, phyllosilicates and graphite. Through atomistic simulation of nanoindentations on graphite, we showed that the response is initially elastic, followed by nucleation of multiple opposite sign ripplocation boundaries that rapidly span, wave-like, away from the perturbation tip and through the simulation cell. Upon unloading, the buckled layers fully recover leaving no trace of the indentation. Ripplocations are thus the key mechanism, not only for the fully reversible, and recoverable hysteresis stress-strain loops, but also for c-axis strain accommodation. To shed further light on ripplocations we loaded confined layered decks of thin sheets of steel, plastic cards, aluminum and transparencies edge-on while filming the response. In all cases, the layers failed by confined buckling, a response that was almost identical to the response of the graphite layers in our molecular dynamic simulations. We further applied a nonlinear, folding mechanics model, which accounts for frictional, bending and foundation energies, to model our experimental results. We further show that this model - that predicts the buckling loads and wavelengths of the ripplocations’ standing waves - applied equally well to the layered decks of cards/steel/paper etc. as to graphite, a span of over nine orders of magnitude. We also demonstrate that Coulombic friction between the layers can account for the energy dissipated per cycle. From the totality of our results there is little doubt at this time that the atomic layers in layered solids behave quite similarly to other layered systems in that they fail by buckling rather than be shear. 


On the Deformation of Crystalline Layered Solids by Ripplocations

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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