Artificial Neural Network and Finite Element Modeling of Nanoindentation Tests on Silica

Two major forms of Silica include the crystalline form named Quartz which consist of the sand grains in nature, and amorphous form named Silica Glass or Fused Silica which is commonly known as glass. Fused Silica is an amorphous crystal that can show plastic behavior at micro-scale despite its brittle behavior in large scales. Due to the amorphous and ductile nature of Fused Silica, this behavior may not be explained well using the traditional dislocation-based mechanism of plasticity for crystalline solids. The crystal plasticity happens due to shear stress and stored energy in the material as dislocations which does not change the volume. In amorphous Fused Silica however, the permanent deformation is mainly caused by densification of the material under localized loading in addition to plastic flow caused by shear stress. This behavior is particularly true in the case of nanoindentation testing. Due to this densifying behavior, modeling the material using constitutive models such as Drucker-Prager/Cap can be quite helpful to further expand the model parameters to be used for geomaterials. Nanoindentation tests were performed on Fused Silica and Quartz samples and Finite Element Method (FEM) was used to further investigate the effect of different constitutive model parameters on material behavior. It was observed that, by implementing volumetric hardening in constitutive models, the FEM results were in better agreement with experimental results in case of both Fused Silica and sand grains. In the second part of the study Artificial Neural Network (ANN) models were used to predict nanoindentation test results for different material parameters as well as indenter shape and geometry. ANN models were trained using FEM results and experimental test results and verified using the reminder of the data. Trained models were then used to study of different scenarios that were not analyzed using FEM or experiments. Advisor: Chung R. Son

Mechanical behavior of fibrous root-inspired anchorage systems

Plant root-inspired geotechnics seeks to harness the principles of one of Earth’s most ubiquitous foundation elements to redesign or enhance conventional geotechnical infrastructure. In particular, the anchorage and material efficiency attributes of fibrous root systems are encapsulated in a novel root-inspired anchor that has the capability of surpassing conventional anchorage systems (e.g. tiebacks, tiedowns, plate and pile anchors) particularly in areas with weak soil or spatial constraints. The scope of this research fully exposes the application of the bio-inspired design process to the realization of root-inspired anchorage systems from 1) the reasoning behind the selection of fibrous root systems as a prime source of inspiration for sustainable, resilient anchor elements (e.g. plastic and thigmotropic adaptability properties, multifunctionality), to 2) the identification of the critical attributes of fibrous root systems to pullout behavior through testing of leek (Allium porrum) and spider (Chlorophytum comosum) plants, to 3) the design and fabrication of root-inspired anchor models, to 4) an extensive performance evaluation. More specifically, the root-inspired anchors are assessed in terms of their pullout behavior through a combination of analytical, experimental, and numerical analyses. The slip line method from plasticity theory is used as the basis to derive a solution for the prediction of plate anchor pullout capacity that was further modified to account for the more complex geometry of root-inspired anchors through mechanics-informed insights. Experimentally, a series of 1g pullout tests are performed to parametrically study the role of root-inspired anchor features (i.e. morphology, topology, material properties, and interface roughness) as well as soil properties (i.e. relative density, particle angularity, and particle size) on pullout behavior. Additionally, through a combination of x-ray CT imaging and digital image correlation (DIC), the formation and evolution of the soil failure surface during the uplift of a root-inspired anchor model is visualized and analyzed to connect the local soil kinematics to the global pullout response. With the finite volume method, the uplift process is simulated to validate experimental results and to extend the parametric study to a wider range of anchor and soil conditions. Finally a few considerations are highlighted concerning the upscale design, installation, and testing of these next generation anchor elements.Ph.D

Behaviour of sandy soil subjected to dynamic loading

This thesis presents the kinematics occurring during lab-based dynamic compaction tests using high speed photography and image correlation techniques. High speed photography and X-ray microtomography have been used to analyse the behaviour of sandy soil subjected to dynamic impact. In particular, the densification mechanism of granular soils due to dynamic compaction is the main theme of the thesis. High speed photography and digital image correlation (DIC) techniques have enabled the deformation patterns, soil strains and strain localisations to be observed. Image correlation and X-ray scans revealed the formation, rate and growth of narrow tabular bands of intense deformation and significant volumetric change and provided answers towards a better understanding of the densification mechanism in dry granular soils due to dynamic compaction. As a quantitative tool, high speed photography has allowed the propagation of localised deformation and strain fields to be identified and has suggested that compaction shock bands control the kinematics of dynamic compaction. The displacement and strain results from high speed photography showed that soil deformation in the dynamic tests was dominated by a general bearing capacity mechanism similar to that widely stated in classic soil mechanics texts. Comparative static loading tests have been conducted to enable the dynamic effects to be clearly distinguished. This has enabled the densification process taking place below the soil surface to be investigated and identified. Simulations of the physical models were carried out using LS-DYNA finite element formulations for comparison and verification purposes. The FE simulations verified the general characteristics from the photography findings. However, simulation results were unable to predict the exact details of the strain localisation due to surface impacts during physical model tests