On the Mechanics of Ripplocations: A New Defect in Layered Solids

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Recent discoveries in layered materials have opened new dimensions for tailoring better materials. Layered materials are ubiquitous in our surroundings; they encompass graphite, mica, and two-dimensional solids (with an atomic-scale thickness), among many others. It has long been understood that for solids to deform, microscopic defects must be present. Up to quite recently these defects were presumed to be line defects called dislocations, a well-known and studied material defect. In 2016, however, research suggested that conventional wisdom was incorrect, or incomplete, regarding the deformation of layered materials. A potentially new micro-mechanism called a ripplocation, best described as an atomic scale ripple, was suggested as the governing mechanism. At the macroscale, a ripplocation would be a carpet ripple. Currently, little is known about these defects, their mechanics or behavior, and how to leverage them to create improved materials. This research investigates the mechanics of ripplocation through an integrated suite of modeling and experimentation at two length scales. The overall goal integrates fields of mechanics, materials science, and scientific computing to build a platform to engineer more advanced materials as well as provide fundamental knowledgebase to the technology sectors where layers materials are relevant, such as geo-technology and microelectronics. Also, as part of the research, outreach to high school students will be made through summer programs, undergraduate and graduate students from underrepresented groups will be hired to perform research, and interactive learning modules will be added to the curriculum to educate the next generation of diverse scientists and engineers.Fundamental differences between ripplocation and dislocation behavior exist and motivate the current project. According to dislocation theory, when the planes of layered solid materials are loaded and unloaded edge-on they will either reversibly deform, and return to their original form without dissipating any energy if it's an elastic material or they will remain permanently indented. Ripplocation behavior explains the third observed option, which is the material returning to its original form, while dissipating considerable amounts of energy. The goal of this work is to understand ripplocation mechanics at multiple levels, from the atomic, through molecular modeling and theory, to the macroscopic where layered solids will be indented. Results from molecular modeling to capture the fundamental physics of ripplocations will be leveraged to create a new ripplocation dynamics model. Indentation predictions, including both strength and microstructural deformation, from the model will be directly compared to and verified with results from atomistic modeling at the nanoscale and experimental indentation studies at the microscale. The goal is to show that ripplocation mechanics are scale invariant and provide a mechanics-based predictive framework that can be extended to a myriad of other layered materials to enable improved design strategies.