Ripplocations in layered materials

Kaustubh Sudhakar

doi:10.17918/00011198

This dissertation presents a systematic and multi-scale investigation into the deformation mechanisms of layered crystalline solids (LCSs), with a primary emphasis on graphite and composite systems. The central argument advanced throughout this work is that ripplocations--atomic-scale ripples confined to basal planes--are the fundamental defects governing deformation in LCSs, rather than classical kink boundaries (KBs) or basal dislocations (BDs). By integrating theoretical analysis, molecular dynamics (MD) simulations, nanoindentation experiments, and finite element analysis (FEA), this thesis establishes ripplocations and ripplocation boundaries (RBs) as the operative deformation mechanisms, providing a unified framework that explains phenomena ranging from elastic nonlinearity and reversibility at the atomic scale to macroscopic energy dissipation in composite materials.The first component of this work redefines the distinction between KBs and RBs. While KBs in metals are well-characterized, this study demonstrates that most boundaries traditionally identified as KBs in LCSs should instead be classified as RBs. Unlike KBs, RBs are not atomically sharp; their strain fields are delocalized, they delaminate at high curvatures, and they form nanobridges across adjacent layers. Crucially, RBs are highly strained yet largely reversible unless trapped or driven to extreme curvatures, where irreversibility and fracture emerge. This distinction has significant implications for understanding deformation, failure, and the resilience of layered materials. Building on this, MD simulations were performed on 60-layer graphene systems subjected to uniaxial (UA) compression and double indentation (DI). These studies reveal that RBs nucleate spontaneously beyond linear elasticity, with nucleation energies nearly twice as high in UA loading compared to DI. Buckling thresholds were quantified at total strains of ~0.5% in UA and ~0.3% in DI. Post-buckling, interlayer van der Waals interactions dominate, and remarkable reversibility is observed up to ~30% strain. Cyclic loading induces a memory effect by reducing nucleation energies, confirming that RB formation is energetically more demanding than their subsequent motion. Beyond this regime, delamination pins RBs, yielding partial reversibility. Importantly, post-delamination structures from simulations closely replicate experimental results, thereby validating the predictive capacity of ripplocation theory. Complementary nanoindentation experiments on single-crystal graphite further corroborate these findings. Stress-strain curves consistently show three regimes: linear elasticity, nonlinear elastic response associated with ripplocation nucleation and propagation, and fully reversible hysteresis loops due to interlayer friction. From these data, critical nucleation stresses ([sigma]_c), propagation stresses ([sigma]_RB), and interlayer friction coefficients ([mu]) were extracted. The friction coefficients were remarkably consistent, [mu] ~ 0.21-0.24, confirming the validity of a Coulombic friction model even at atomic scales. Additionally, stochastic "pop-in" events, evident as sudden displacement bursts, were found to depend strongly on indenter radius and local defect distributions. These experiments decisively demonstrate that ripplocations, not basal dislocations, are the operative deformation mechanism in LCSs, and they highlight the universality of ripplocation behavior across scales and materials. The final component of the dissertation extends ripplocation theory to engineered layered composites subjected to confined edge-on indentation. Experiments and FEA simulations on laminated decks composed of transparencies, steel sheets, and plastic cards show that localized buckling coupled with delamination--i.e., ripplocation boundaries--govern the deformation response and energy dissipation in these macroscopic systems. The results demonstrate how filler type, layer thickness, friction coefficient, and stacking sequence systematically influence hysteresis and localization. Even in systems of comparable overall thickness, differences in stiffness mismatch and interfacial shear resistance significantly alter the extent of plastic energy dissipation. Stress-strain analysis further reveals consistent offsets between theory and experiment, attributable to interfacial friction [mu], reinforcing the critical role of friction in ripplocation-mediated deformation. Taken together, these findings establish ripplocations and RBs as the universal deformation mechanism in layered materials, spanning atomic to macroscopic length scales. By distinguishing RBs from KBs, quantifying their energetics, and demonstrating their universality across both crystalline and engineered composite systems, this thesis provides a paradigm shift in understanding deformation in layered solids. The implications extend broadly, offering new opportunities for designing advanced materials with tailored mechanical responses, improved energy absorption, and enhanced resilience through deliberate manipulation of interlayer friction and microstructural architecture.

Ripplocations in layered materials

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Ripplocations in layered materials

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