Multiscale Modeling of the Mechanical Behavior of Clay
The goal of this thesis is to develop a better understanding of the macroscopic engineering properties of clay through a bottom-up modeling approach, involving simulations at three length scales. At the nanoscale, molecular dynamics simulations were carried out to quantify the Potential of Mean Forc...
| প্রধান লেখক: | |
|---|---|
| অন্যান্য লেখক: | |
| বিন্যাস: | গবেষণাপত্র |
| প্রকাশিত: |
Massachusetts Institute of Technology
2023
|
| অনলাইন ব্যবহার করুন: | https://hdl.handle.net/1721.1/151969 https://orcid.org/0000-0003-3652-1388 |
| সংক্ষিপ্ত: | The goal of this thesis is to develop a better understanding of the macroscopic engineering properties of clay through a bottom-up modeling approach, involving simulations at three length scales. At the nanoscale, molecular dynamics simulations were carried out to quantify the Potential of Mean Force (PMF) for water-mediated interactions between pairs of primary clay particles using a free energy perturbation method. We compare results for two commonly occurring clay minerals, illite and Na-smectite, both with 2:1 sheet structures, and nanotubular imogolite (1:1). Illite particles exhibit a well-defined potential well at 11 Å separation, enabling particle aggregation in face-face configurations, while Nasmectite (with lower surface charge density) exhibits net repulsion for separations less than 16 Å. In both cases, the free energy is affected by exclusion of counterions in the interlayer space, and are characterized by an oscillatory component of free energy at different solvation states. Imogolite tubes can also aggregate when counterions are distributed within the hollow tube structures. We then simulate mesoscale aggregation by coarse-graining primary particles, equilibrating monodisperse particle assemblies using NPT simulations. The illite particles are represented as single-site ellipsoidal particles using the Gay-Berne potential to approximate the PMF results. The equilibrated assemblies are characterized by a lognormal distribution of particles that aggregate in well-aligned face-face configurations w ith mean stack size 𝑚 = 3 ∼ 7. We study the impact of confining p ressure a nd p ressure h istory on the particle arrangement. Higher confining pressure r esults in larger stack s izes, and in the increase of overall level of preferred particle orientation. We demonstrate that mesoscale assemblies show compression properties (in terms of the compression indices in the 𝑒 − ln 𝑝 and ln 𝑒 − ln 𝑝 spaces) similar to those observed macroscopically in experimental tests with small elastic recovery of strains during unloading and reloading. We also study the quasistatic stress-strain response of the NPT-equilibrated mesoscale systems through a sequence of strain controlled NVT relaxations. The results exhibit non-linear, inelastic and hysteretic behaviors that are also observed in macroscale experimental results. Elastic properties estimated from the stress-strain data also exhibit trends similar to published experimental studies on pure clay minerals. The stiffness properties a re c onsistent with p ower l aw functions (exponent 𝑛 = 0.2 − 0.6) of confining p ressure. We s tudy t he e volution o f mesostructures by analyzing the geometric parameters (including the stack size distribution, fabric tensors, interstack pair correlation, etc.) in order to establish the relation between mesoscale and macroscopic behaviors. The potential energy is sub-divided into intra- and four inter-stack components based on the components of interstack pair correlation functions. We develop an analytical model to predict the elastic properties of mesoscale particle assemblies. The model contains a strain energy formulation for specific particle configurations based on the 5 geometric parameters describing the mesostructures of the assemblies, and a perturbation formulation by specifying changes of mesostructure to small affine transformations. We obtain good agreement between the analytical solutions and numerical estimates. Both indicate that orthotropic symmetry provides a reasonable representation of the mesoscale assemblies. The analytical model provides a basis to characterize the constitutive behavior of clay particle assemblies from a multiscale perspective, and a general framework that has potential for broader application to other engineering materials. |
|---|