Modeling mechanical response of heterogeneous materials

Heterogeneous materials are ubiquitous in nature and as synthetic materials. These materials provide unique combination of desirable mechanical properties emerging from its heterogeneities at different length scales. Future structural and technological applications will require the development of advanced light weight materials with superior strength and toughness. Cost effective design of the advanced high performance synthetic materials by tailoring their microstructure is the challenge facing the materials design community. Prior knowledge of structure-property relationships for these materials is imperative for optimal design. Thus, understanding such relationships for heterogeneous materials is of primary interest. Furthermore, computational burden is becoming critical concern in several areas of heterogeneous materials design. Therefore, computationally efficient and accurate predictive tools are highly essential. In the present study, we mainly focus on mechanical behavior of soft cellular materials and tough biological material such as mussel byssus thread. Cellular materials exhibit microstructural heterogeneity by interconnected network of same material phase. However, mussel byssus thread comprises of two distinct material phases. A robust numerical framework is developed to investigate the micromechanisms behind the macroscopic response of both of these materials. Using this framework, effect of microstuctural parameters has been addressed on the stress state of cellular specimens during split Hopkinson pressure bar test. A voronoi tessellation based algorithm has been developed to simulate the cellular microstructure. Micromechanisms (microinertia, microbuckling and microbending) governing macroscopic behavior of cellular solids are investigated thoroughly with respect to various microstructural and loading parameters. To understand the origin of high toughness of mussel byssus thread, a Genetic Algorithm (GA) based optimization framework has been developed. It is found that two different material phases (collagens) of mussel byssus thread are optimally distributed along the thread. These applications demonstrate that the presence of heterogeneity in the system demands high computational resources for simulation and modeling. Thus, Higher Dimensional Model Representation (HDMR) based surrogate modeling concept has been proposed to reduce computational complexity. The applicability of such methodology has been demonstrated in failure envelope construction and in multiscale finite element techniques. It is observed that surrogate based model can capture the behavior of complex material systems with sufficient accuracy. The computational algorithms presented in this thesis will further pave the way for accurate prediction of macroscopic deformation behavior of various class of advanced materials from their measurable microstructural features at a reasonable computational cost

Computationally efficient black-box modeling for feasibility analysis

Computational cost is a major issue in modern large-scale simulations used across different disciplines of science and engineering. Computationally efficient surrogate models that can represent the original model with desired accuracy have been explored in the recent past. However, with the exception of few efforts, most of these techniques rely on a reduced order representation of the original complex model, resulting in a loss of information. In this paper we demonstrate the applicability of high dimensional model representation (HDMR) technique in addressing this issue while preserving the original model dimension. We will discuss the applicability of this surrogate modeling technique in the field of feasibility analysis drawing examples from process systems and materials design. It will be shown that the original physical models can be essentially considered as a black box, and same methodology can be applied across all the examples studied. It is found that the accuracy of the surrogate models depends on the order of the approximation and number of sampling points employed. While first-order approximation is largely inadequate, second-order approximation is sufficient for the model systems studied. Sampling requirement is also dramatically low for the construction of these surrogate models. © 2010 Elsevier Ltd

Effect of microscopic deformation mechanisms on the dynamic response of soft cellular materials

Cellular materials show progressive or uniform collapse during impact loading depending on their microstructural and material properties. It is generally agreed that a complex interplay among microinertia, microbuckling and microbending of the cell walls of these materials plays an important role in determining their macroscopic stress-strain response. However, an evaluation of the dependency of the overall deformation behavior on these parameters requires sophisticated modeling approach due to extremely fast and complex wave propagation events occurring during dynamic deformation. We have developed a transient finite element based computational framework that can examine the contribution of each of these effects on the deformation history of this class of materials. An in-depth parametric study for different loading, microstuctural and material parameters has been undertaken in this study. Our significant finding is that at high strain rate, shorter pulse rise times lead to higher microinertial stress enhancement due to an increase in apparent microbuckling strength. A variation of cell size shows insignificant effect of microinertia and microbuckling at initial stage but localization can be found at later stage of deformation due to increasing microbuckling and microbending activities. Deformation localization occurs in lower Young\u27s modulus specimens due to lower buckling and bending strength of the cell walls. A significant inertial stress enhancement can be noticed in the specimens with higher bulk density of the constituent material leading to increased microbuckling activities resulting in localized collapse at the impact end

Modeling the delamination of amorphous-silicon thin film anode for lithium-ion battery

Sputter-deposited amorphous silicon thin films on metallic copper current collectors are widely studied as lithium-ion anode systems. Electrochemical results indicate these electrodes exhibit near theoretical capacity for first few cycles; however delamination at the thin film-current collector interface causes rapid capacity fade leading to poor cycling performance. Primary reason for this interfacial delamination is the mechanical stress generated due to colossal volume expansion of silicon during lithiation. The focus of the current study is to present a mechanistic understanding of the role of mechanical properties of the current collector on this characteristic delamination behavior during electrochemical cycling. Toward this end, we have developed a computational framework that accounts for the coupled diffusion induced large deformation in silicon, elasto-plastic deformation of the current collector, as well as the nucleation and propagation of interfacial delamination. We have also performed a detailed parametric study to investigate the effect of mechanical properties of the current collector on the delamination of the thin film-current collector interface. We have accordingly determined that current collectors with low elastic modulus such as graphite can completely suppress interfacial delamination. Our analysis thus provides a sound mechanistic approach for designing next generation Si thin film anodes with improved capacity retention. © 2013 Elsevier B.V. All rights reserved