Today’s emerging technologies in high temperature applications are placing difficult demands on structural materials. These applications, including hypersonic flight and nuclear energy generation, are often limited by the maximum operating temperature of the structural and coating materials. The current list of materials that can withstand these extreme environments is short and a new material space must be explored to meet current and future demands. High entropy carbides, which comprise complex solid solution mixtures of five or more metals and carbon, represent a new class of ultra-high temperature ceramics that allow for unique properties compared to the traditional binary carbides. In this work, five-component, rocksalt structured, transition metal carbides are synthesized in bulk via mechanical alloying and current and pressure assisted densification and homogenization. The carbides are investigated for phase composition in the pursuit of randomly mixed, single-phase, solid solutions. The effect of entropy on mixing is predicted via a newly formulated entropy descriptor, the Entropy Forming Ability (EFA), that is developed using experimental data and density functional theory. The EFA is then employed in choosing candidate compositions for experimental synthesis. In total, a list of 12 compositions are investigated with 9 forming single-phase and, therefore, high entropy carbides. Phase composition and the extent of mixing is investigated via electron microscopy with energy dispersive x-ray spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray absorption fine structure. These high entropy carbides allow for electronic structures that are not available to the binary or ternary carbides. Particularly, the use of entropy to stabilize single phase structures allows for the inclusion of elements that are not stable in the rocksalt structure at room temperature. In this work, the inclusion of molybdenum and tungsten into rocksalt structured carbides allows for more electron rich compositions that exhibit increased metallic character, which is described by the increased energy distance between the Fermi level and the pseudo-gap. This parameter is used to predict and tune mechanical properties and leads to the synthesis of compositions with enhanced plasticity and high fracture toughness. Elastic properties, hardness, and fracture toughness are investigated via acoustic wave speed, nanoindentation, and microindentation techniques.

Plasmonic nanoantennas that a support localized surface plasmon resonance (LSPR) are capable of confining visible light to subwavelength dimensions due to strong electromagnetic field enhancement at the probe tip. Nanoantenna enable optical methods such as tip-enhanced Raman spectroscopy (TERS), a technique that uses scanning probe microscopy tips to provide chemical information with nanoscale spatial resolution and single-molecule sensitivities. The LSPR supported by the probe tip is extremely sensitive to the nanoscale morphology of the nanoantenna. Control of nanoscale morphology is notoriously difficult to achieve, resulting in TERS probes with poor reproducibility. In my thesis, I demonstrate high-performance, predictable, and broadband nanospectroscopy probes that are fabricated by self-assembly. Shaped metal nanoparticles are organized into dense layers and deposited onto scanning probe tips. When coupled to a metal substrate, these probes support a strong optical resonance in the gap between the substrate and the probe, producing dramatic field enhancements. I show through experiment and electromagnetic modeling that close-packed but electrically isolated nanoparticles are electromagnetically coupled. Hybridized LSPRs supported by self-assembled nanoparticles with a broadband optical response, giving colloidal nanoantenna a high tolerance for geometric variation resulting from fabrication. I find that coupled nanoparticles act as a waveguide, transferring energy from many neighboring nanoparticles towards the active TERS apex. I also use surface-enhanced Raman spectroscopy (SERS) to characterize the effects of nanoparticle polydispersity and gap height on the Raman enhancement. These colloidal probes have consistently achieved dramatic Raman enhancements in the range of 108–109 with sub-50 nm spatial resolution. Furthermore, in contrast to other nanospectroscopy probes, these colloidal probes can be fabricated in a scalable fashion with a batch-to-batch reproducibility of ~80%. This body of work serves as an important demonstration that bottom-up engineering can be used for batch fabricatation of high-performance and high-reliability devices using inexpensive equipment and materials.

The results of static and dynamic nonlinear analyses of earthen structures are dependent on the implemented numerical methods including the constitutive models, the solution scheme, and the modeling software. Recent significant static failures (e.g., Feijão Dam 1) and past dynamic failures (e.g., 4th Ave. landslide in Anchorage, Alaska) have occurred due to yielding of the plastic soils. These failures were influenced by the strain-softening tendencies of the plastic soils, which highlights the need for continued development of numerical tools to model plastic soils in nonlinear analyses. This dissertation presents the development of a new constitutive model, a relationship that connects element level viscous behavior to field scale analyses, and system level analyses with new solution schemes that incorporate viscous effects and strain- softening in nonlinear analyses.

First, a viscoplastic constitutive model for representing plastic silts and clays in geotechnical static slope stability applications is presented. The PM4SiltR model builds on the stress ratio-controlled, critical state-based, bounding surface plasticity model PM4Silt and is coded as a dynamic link library for use in the finite difference program FLAC 8.1. PM4SiltR incorporates strain rate-dependent shear strength, stress relaxation, and creep using a consistency approach combined with an internal strain rate and auto-decay process. Six parameters are introduced to control the viscous response of PM4SiltR while the parameters controlling the nonviscous portion of the response are the same as for PM4Silt. Single element simulations are presented to illustrate the influence of viscoplasticity on the constitutive response in direct simple shear loading and undrained creep. Single element responses are shown to be consistent with observed experimental results. Simulations of a hypothetical tailings dam of upstream construction are performed to illustrate use of PM4SiltR at the field scale. Results of field scale simulations show PM4SiltR can model undrained creep and progressive failure leading to delayed slope instability after relatively minor changes in loading conditions at.

A numerical study is then presented that shows the effect that viscoplasticity can have on the localization process in sensitive, saturated clays and plastic silts. Numerical simulations of laboratory specimens of sensitive, viscoplastic soil subjected to monotonic, undrained, direct simple shear loading were performed using the PM4SiltR constitutive model. Parametric analyses evaluated the effects of soil sensitivity, post-peak strain-softening rate, and strain rate-dependency, as well as specimen size, mesh discretization, and loading rate. The numerical results showed that the global strain at which a localization forms primarily depends on the strain rate-dependency of the soil's shearing resistance relative to its rate of post-peak strain-softening. A regression model is subsequently presented that relates the global strain at the onset of localization to the soil's strain rate-dependency and post-peak strain-softening rate. The results indicate that the inclusion of reasonable levels of viscoplasticity significantly increases the strain that can develop before a localization develops in clays and plastic silts with modest strain-softening rates. The consistency of the numerical results with available laboratory observations is discussed. Implications for practice and future research needs are discussed.

Next, two-dimensional viscoplastic nonlinear analyses of the 2019 Feijão Dam 1 failure in Brazil are performed using the finite difference program FLAC 8.1 with the user-defined constitutive models PM4SiltR and PM4Sand. A brief history of Feijão Dam 1, its failure, and the conflicting findings from two previous independent failure investigations are summarized. The present study uses the site characterization from those prior studies to develop the dam cross section, define material index properties, and establish groundwater conditions, but uses alternative techniques for characterizing undrained shear strengths. Simulations show that the dam was marginally stable against long-term consolidated, undrained conditions and that modest loading changes were sufficient to trigger failure with deformation patterns consistent with the observed failure. Simulations further show that collapse could have been triggered by a modest wetting event causing an increase in water content and loss of suction above the phreatic surface, by ongoing drilling activities causing a localized loss of shear strength in the tailings, or a combination of both mechanisms. Sensitivity of the results to choices in the calibration process and the numerical solution scheme are discussed. The implications of these results on the interpretation of the Feijão Dam 1 failure and long-term slope stability assessment procedures in practice are discussed.

Finally, two investigations look at the influence of sensitivity and strain-softening on the seismic deformations of a set-back levee. First, nonlinear dynamic analyses are used to investigate how seismic deformations of a 6-m tall levee founded on a deposit of normally consolidated clay may vary with the sensitivity of the clay. The analysis results showed that strain-softening and associated strength loss in the clay layer increased with increasing clay sensitivity, but it did not result in increased levee deformations for the conditions examined. Instead, the results showed that increasing clay sensitivity slightly reduced the levee deformations, which was attributed to the strength loss in the clay also causing a reduction in the accelerations that developed in the levee. Second, the results of nonlinear dynamic analyses (NDAs) of an idealized levee founded on a sensitive clay are compared with simplified procedures that combine limit equilibrium (LE) and Newmark sliding block methods. The tendency for simplified procedures to underestimate or overestimate seismic deformations compared with NDA results is shown to depend on the specific conditions (e.g., soil sensitivity and brittleness, ground motion intensity, margin of safety against instability) and how potential strength loss is accounted for in the simplified methods. Limitations of these findings for practice are discussed.

Overall, the outcomes from this dissertation contribute to an increased understanding on how to account for viscous effects and strain-softening in static and dynamic nonlinear analyses.