The main objective of this study is to develop and validate a framework for microstructural modeling of asphalt composite materials using a coupled thermo-viscoelastic, thermo-viscoplastic, and thermo-viscodamage constitutive model. In addition, the dissertation presents methods that can be used to capture and represent the two-dimensional (2D) and three-dimensional (3D) microstructure of asphalt concrete. The 2D representative volume elements (RVEs) of asphalt concrete were generated based on planar X-ray Computed Tomography (CT) images. The 2D RVE consists of three phases: aggregate, matrix, and interfacial transmission zone (ITZ). The 3D microstructures of stone matrix asphalt (SMA) and dense-graded asphalt (DGA) concrete were reconstructed from slices of 2D X-ray CT images; each image consists of the matrix and aggregate phases. The matrix and ITZ were considered thermo-viscoelastic, thermo-viscoplastic, and thermo-viscodamaged materials, while the aggregate is considered to be a linear, isotropic elastic material. The 2D RVEs were used to study the effects of variation in aggregate shape, distribution, volume fraction, ITZ strength, strain rate, and temperature on the degradation and micro-damage patterns in asphalt concrete. Moreover, the effects of loading rate, temperature, and loading type on the thermo-mechanical response of the 2D and 3D microstructures of asphalt concrete were investigated. Finally, the model parameters for Fine Aggregate Mixture (FAM) and full asphalt mixture were determined based on the analysis of repeated creep recovery tests and constant strain rate tests. These material parameters in the model were used to simulate the response of FAM and full asphalt mixture, and the results were compared with the responses of the corresponding experimental tests. The microstructural modeling presented in this dissertation provides the ability to link the microstructure properties with the macroscopic response. This modeling combines nonlinear constitutive model, finite element analysis, and the unique capabilities of X-ray CT in capturing the material microstructure. The modeling results can be used to provide guidelines for designing microstructures of asphalt concrete that can achieve the desired macroscopic behavior. Additionally, it can be helpful to perform 'virtual testing' of asphalt concrete, saving numerous resources used in conducting real experimental tests.

abstract: This study employs a finite element method based modeling of cementitious composite microstructure to study the effect of presence of inclusions on the stress distribution and the constitutive response of the composite. A randomized periodic microstructure combined with periodic boundary conditions forms the base of the finite element models. Inclusion properties of quartz and light weight aggregates of size 600μm obtained from literature were made use of to study the effect of their material (including inclusion stiffness, stiffness of interfacial transition zone and matrix stiffening) and geometric properties (volume fraction of inclusion, particle size distribution of inclusion and thickness of the interfacial transition zone) on the composite. Traction-separation relationship was used to incorporate the effect of debonding at the interface of the matrix and the inclusion to study the effect on stress distribution in the microstructure. The stress distributions observed upon conducting a finite element analysis are caused due to the stiffness mismatch in both the quartz and the light weight aggregates as expected. The constitutive response of the composite microstructure is found to be in good conformance with semi-analytical models as well as experimental values. The effect of debonding throws up certain important observations on the stress distributions in the microstructure based on the stress concentrations and relaxations caused by the stiffness of the individual components of the microstructure. The study presented discusses the different micromechanical models employed, their applicability and suitability to correctly predict the composite constitutive response. / Dissertation/Thesis / Masters Thesis Civil Engineering 2016

<div>This research aims at investigating the direct in-situ synthesis of high-performance multi-component alloys such as high entropy alloys, bulk metallic glasses, and metal matrix composites using the directed energy deposition (DED) process, and modeling the entire solidification and microstructure evolution of these alloys via a novel three-dimensional cellular automata-phase field (3D CA-PF) model. These alloys are currently the focus of significant attention in the materials and engineering communities due to their superior material properties. In the 3D CA-PF model, the growth kinetics including the growth velocity and solute partition at the local solid/liquid interface is calculated by the multi-phase and multi-component PF component, and the 3D CA component uses the growth kinetics as inputs to calculate the dendrite morphology variation and composition redistribution for the entire domain, which could save the computational cost more than five orders of magnitude compared to the PF modeling that can only be applied to small domains due to its heavy computational requirements. Coupled with the temporal and spatial temperature history predicted by the experimentally validated DED model, this computation-efficient 3D CA-PF model can predict the microstructure evolution within the entire macro-scale depositions, which is known to be nonuniform due to the particular nature of additive manufacturing (AM) processes. </div><div>To achieve the final goal of direct in-situ synthesis of five-component CoCrFeCuNi high entropy alloys (HEA), and modeling of the solidification and microstructure evolution during the DED process, the proposed research is carried out in progressive stages with the increasing complexity of alloy systems. First, a simple binary material system of Ti-TiC composite was studied. The thermodynamically-consistent binary PF model is used to simulate the formation mechanism of detrimental resolidified dendritic TiCx. To capture the polycrystalline solidification, a grain index is introduced to link different crystallographic orientations for each grain. This PF model simulates the microstructure evolution of TiCx in different zones in the molten pool by combining the temperature history predicted by the DED model. The simulated results provide the solution of limiting the free carbon content in the melt, according to which, the formation of TiCx dendrites is successfully avoided by experimentally controlling the melting degree of premixed TiC particulates.</div><div>Second, the solidification, grain structure evolution, and phase transformation in the DED-built ternary Ti6Al4V alloy under the influences of thermal history are systematically simulated using the established simulation framework and a phase prediction model. The thermal history in a three-track deposition is simulated by the DED model. With such thermal information, the 3D CA model simulates the grain structure evolution on the macro-scale. The thermodynamically-consistent PF model predicts the local grain structure and concentration distributions of solutes Al and V on the micro-scale. The meso-scale CA-PF model captures the sub-grain microstructure evolution and concentration distributions of solutes within the entire molten pool. The dendritic morphology is captured within the large β grains. When the temperature drops below the β-transus temperature, the solid-state phase transformation of β→α/ is studied by the phase prediction model. Based on the predicted volume fractions of and α, the microhardness is also successfully assessed using rules of mixtures. </div><div>Third, the material system is expanded to a four-component ZrAlNiCu bulk metallic glass composite, whose raw composition is prepared by premixing the four pure elemental metals. The DED model is employed to obtain the temperature field and heating/cooling rates in single-track ZrAlNiCu bulk metallic glass composite, which provides insights for microstructure evolution. By delicate control of the material composition and utilization of the thermal history of the DED process, an amorphous-crystalline periodic structure is produced with in-situ formed crystalline particulates embedded in the amorphous matrix. This crack-free microstructure is successfully maintained within bulk parts, where a high fraction of the amorphous phase and crystalline phases are produced in the fusion zone and heat-affected zone, respectively. The large volume percentage of the amorphous phase contributed to the hardness, strength, and elastic modulus of the composite while the various soft crystalline phases improve the ductility by more than three times compared to monolithic metallic glasses. Nanoindentation tests are also performed to study the deformation behavior on the micron/sub-micron length scale. </div><div>Fourth, the material system is expanded to a five-component CoCrFeNiTi HEA alloy. Three CoCrFeNiTi HEA alloys with different compositions are designed and synthesized from premixed elemental powders via the DED process. Through a delicate design of composition and powder preparation, different microstructures are formed. H3-Co24.4Cr17.4Fe17.5Ni24.2Ti16.5 is mainly composed of a soft face-centered cubic (FCC)-γ phase while σ-FeCr, δ-NiTi2, and a small amount of Ni3Ti2 are precipitated and uniformed distributed in the FCC matrix for H1-Co22.2Cr16.1Fe19Ni21.8Ti20.9 and H2-Co25.9Cr15Fe17Ni20.8Ti21.3. With a large percent of the secondary phases, H1 exhibits a hardness value of about 853 HV0.5. These HEA alloys display a high oxidation resistance comparable to Inconel 625 superalloy. A detailed evaluation of the hardness, oxidation resistance, and wear resistance of these HEAs are conducted as compared with those of a reference HEA and two popular anti-wear steels.</div><div>Finally, a novel 3D Cellular Automata-Phase Field (CA-PF) model that can accurately predict the dendrite formation in a large domain, which combines a 3D CA model with a 1D PF component, is developed. In this integrated model, the PF component reformulated in a spherical coordinate is employed to accurately calculate the local growth kinetics including the growth velocity and solute partition at the solidification front while the 3D CA component uses the growth kinetics as inputs to update the dendritic morphology variation and composition redistribution throughout the entire domain. Taking advantage of the high efficiency of the CA model and the high fidelity of the PF model, the 3D CA-PF model saves the computational cost more than five orders of magnitude compared to the 3D PF models without losing much accuracy. By coupling the thermodynamic and kinetic calculations into the PF component, the CA-PF model is capable of handling the microstructure evolution of any complex multi-component alloys. Al-Cu binary alloys with 2 wt.% and 4 wt.% Cu are first used to validate the 3D CA-PF model against the Lipton-Glicksman-Kurz analytical model and a 3D PF model. Then, the 3D CA-PF model is applied to predicting the dendrite growth during large-scale solidification processes of directional solidification of Al-30wt.%Cu and laser welding of Al-Cu-Mg and Al-Si-Mg alloys. </div>

Cellular metals made from aluminum, titanium, or other metals are becoming increasingly popular for use in structural components of automobiles, aircraft, and orthopaedic implants. Civil engineering applications remain largely absent, primarily due to poor understanding of the material and its structural properties. However, the material features a high stiffness to weight ratio, excellent energy dissipation, and low thermal conductivity, suggesting that it could become a highly valuable new material in structural engineering. Previous attempts to characterize the mechanical properties of steel foam have focused almost exclusively upon uniaxial compression tests, both in experimental research and in computational simulations. Further, computational simulations have rarely taken the randomness of the material’s microstructure into account and have instead simplified the material to a regular structure. Experimental tests have therefore been performed upon both hollow spheres and PCM steel foams to determine compressive, tensile, and shear properties. Computational simulations which accurately represent the randomness within the microstructure have been validated against these experimental results and then used to simulate other material scale tests. Simulated test matrices have determined macroscopic system sensitivity to various material and geometrical parameters.