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- All Subjects: Mechanical Engineering
- Creators: Oswald, Jay
Description
Gels are three-dimensional polymer networks with entrapped solvent (water etc.). They bear amazing features such as stimuli-responsive (temperature, PH, electric field etc.), high water content and biocompatibility and thus find a lot of applications. To understand the complex physics behind gel's swelling phenomenon, it is important to build up fundamental mechanical model and extend to complicated cases. In this dissertation, a coupled large deformation and diffusion model regarding gel's swelling behavior is presented. In this model, free-energy of the total gel is constituted by polymer stretching energy and polymer-solvent mixing energy. In-house nonlinear finite element code is implemented with fast computational capability. Complex phenomenon such as buckling and healing of cracked gel by swelling are studied. Due to the wide coverage of polymeric materials and solvents, solvent diffusion in gels not only follows Fickian diffusion law where concentration map is continuous but also follows non-Fickian diffusion law where concentration map shows high gradient. Phenomenological model with viscoelastic polymer constitutive and concentration dependent diffusivity is created. The model well captures this special diffusion phenomenon such as sharp diffusion front and distinctive swollen and unswollen region.
ContributorsZhang, Jiaping (Author) / Jiang, Hanqing (Thesis advisor) / Peralta, Pedro (Committee member) / Dai, Lenore (Committee member) / Rajan, Subramaniam D. (Committee member) / Chawla, Nikhilesh (Committee member) / Arizona State University (Publisher)
Created2012
Description
The objective of this research is to develop robust, accurate, and adaptive algorithms in the framework of the extended finite element method (XFEM) for fracture analysis of highly heterogeneous materials with complex internal geometries. A key contribution of this work is the creation of novel methods designed to automate the incorporation of high-resolution data, e.g. from X-ray tomography, that can be used to better interpret the enormous volume of data generated in modern in-situ experimental testing. Thus new algorithms were developed for automating analysis of complex microstructures characterized by segmented tomographic images.
A centrality-based geometry segmentation algorithm was developed to accurately identify discrete inclusions and particles in composite materials where limitations in imaging resolution leads to spurious connections between particles in close contact.To allow for this algorithm to successfully segment geometry independently of particle size and shape, a relative centrality metric was defined to allow for a threshold centrality criterion for removal of voxels that spuriously connect distinct geometries.
To automate incorporation of microstructural information from high-resolution images, two methods were developed that initialize signed distance fields on adaptively-refined finite element meshes. The first method utilizes a level set evolution equation that is directly solved on the finite element mesh through Galerkins method. The evolution equation is formulated to produce a signed distance field that matches geometry defined by a set of voxels segmented from tomographic images. The method achieves optimal convergence for the order of elements used. In a second approach, the fast marching method is employed to initialize a distance field on a uniform grid which is then projected by least squares onto a finite element mesh. This latter approach is shown to be superior in speed and accuracy.
Lastly, extended finite element method simulations are performed for the analysis of particle fracture in metal matrix composites with realistic particle geometries initialized from X-ray tomographic data. In the simulations, particles fracture probabilistically through a Weibull strength distribution. The model is verified through comparisons with the experimentally-measured stress-strain response of the material as well as analysis of the fracture. Further, simulations are then performed to analyze the effect of mesh sensitivity, the effect of fracture of particles on their neighbors, and the role of a particles shape on its fracture probability.
A centrality-based geometry segmentation algorithm was developed to accurately identify discrete inclusions and particles in composite materials where limitations in imaging resolution leads to spurious connections between particles in close contact.To allow for this algorithm to successfully segment geometry independently of particle size and shape, a relative centrality metric was defined to allow for a threshold centrality criterion for removal of voxels that spuriously connect distinct geometries.
To automate incorporation of microstructural information from high-resolution images, two methods were developed that initialize signed distance fields on adaptively-refined finite element meshes. The first method utilizes a level set evolution equation that is directly solved on the finite element mesh through Galerkins method. The evolution equation is formulated to produce a signed distance field that matches geometry defined by a set of voxels segmented from tomographic images. The method achieves optimal convergence for the order of elements used. In a second approach, the fast marching method is employed to initialize a distance field on a uniform grid which is then projected by least squares onto a finite element mesh. This latter approach is shown to be superior in speed and accuracy.
Lastly, extended finite element method simulations are performed for the analysis of particle fracture in metal matrix composites with realistic particle geometries initialized from X-ray tomographic data. In the simulations, particles fracture probabilistically through a Weibull strength distribution. The model is verified through comparisons with the experimentally-measured stress-strain response of the material as well as analysis of the fracture. Further, simulations are then performed to analyze the effect of mesh sensitivity, the effect of fracture of particles on their neighbors, and the role of a particles shape on its fracture probability.
ContributorsYuan, Rui (Author) / Oswald, Jay (Thesis advisor) / Chawla, Nikhilesh (Committee member) / Liu, Yongming (Committee member) / Solanki, Kiran (Committee member) / Chen, Kangping (Committee member) / Arizona State University (Publisher)
Created2015
Description
Uranium Dioxide (UO2) is a significant nuclear fission fuel, which is widely used
in nuclear reactors. Understanding the influence of microstructure on thermo-mechanical behavior of UO2 is extremely important to predict its performance. In particular, evaluating mechanical properties, such as elasticity, plasticity and creep at sub-grain length scales is key to developing this understanding as well as building multi-scale models of fuel behavior with predicting capabilities. In this work, modeling techniques were developed to study effects of microstructure on Young’s modulus, which was selected as a key representative property that affects overall mechanical behavior, using experimental data obtained from micro-cantilever bending testing as benchmarks. Beam theory was firstly introduced to calculate Young's modulus of UO2 from the experimental data and then three-dimensional finite element models of the micro-cantilever beams were constructed to simulate bending tests in UO2 at room temperature. The influence of the pore distribution was studied to explain the discrepancy between predicted values and experimental results. Results indicate that results of tests are significantly affected by porosity given that both pore size and spacing in the samples are of the order of the micro-beam dimensions. Microstructure reconstruction was conducted with images collected from three-dimensional serial sectioning using focused ion beam (FIB) and electron backscattering diffraction (EBSD) and pore clusters were placed at different locations along the length of the beam. Results indicate that the presence of pore clusters close to the substrate, i.e., the clamp of the micro-cantilever beam, has the strongest effect on load-deflection behavior, leading to a reduction of stiffness that is the largest for any location of the pore cluster. Furthermore, it was also found from both numerical and i
analytical models that pore clusters located towards the middle of the span and close to the end of the beam only have a very small effect on the load-deflection behavior, and it is concluded that better estimates of Young's modulus can be obtained from micro- cantilever experiments by using microstructurally explicit models that account for porosity in about one half of the beam length close to the clamp. This, in turn, provides an avenue to simplify micro-scale experiments and their analysis.
in nuclear reactors. Understanding the influence of microstructure on thermo-mechanical behavior of UO2 is extremely important to predict its performance. In particular, evaluating mechanical properties, such as elasticity, plasticity and creep at sub-grain length scales is key to developing this understanding as well as building multi-scale models of fuel behavior with predicting capabilities. In this work, modeling techniques were developed to study effects of microstructure on Young’s modulus, which was selected as a key representative property that affects overall mechanical behavior, using experimental data obtained from micro-cantilever bending testing as benchmarks. Beam theory was firstly introduced to calculate Young's modulus of UO2 from the experimental data and then three-dimensional finite element models of the micro-cantilever beams were constructed to simulate bending tests in UO2 at room temperature. The influence of the pore distribution was studied to explain the discrepancy between predicted values and experimental results. Results indicate that results of tests are significantly affected by porosity given that both pore size and spacing in the samples are of the order of the micro-beam dimensions. Microstructure reconstruction was conducted with images collected from three-dimensional serial sectioning using focused ion beam (FIB) and electron backscattering diffraction (EBSD) and pore clusters were placed at different locations along the length of the beam. Results indicate that the presence of pore clusters close to the substrate, i.e., the clamp of the micro-cantilever beam, has the strongest effect on load-deflection behavior, leading to a reduction of stiffness that is the largest for any location of the pore cluster. Furthermore, it was also found from both numerical and i
analytical models that pore clusters located towards the middle of the span and close to the end of the beam only have a very small effect on the load-deflection behavior, and it is concluded that better estimates of Young's modulus can be obtained from micro- cantilever experiments by using microstructurally explicit models that account for porosity in about one half of the beam length close to the clamp. This, in turn, provides an avenue to simplify micro-scale experiments and their analysis.
ContributorsGong, Bowen (Author) / Peralta, Pedro (Thesis advisor) / Rajagopalan, Jagannathan (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2015
Description
The stability of nanocrystalline microstructural features allows structural materials to be synthesized and tested in ways that have heretofore been pursued only on a limited basis, especially under dynamic loading combined with temperature effects. Thus, a recently developed, stable nanocrystalline alloy is analyzed here for quasi-static (<100 s-1) and dynamic loading (103 to 104 s-1) under uniaxial compression and tension at multiple temperatures ranging from 298-1073 K. After mechanical tests, microstructures are analyzed and possible deformation mechanisms are proposed. Following this, strain and strain rate history effects on mechanical behavior are analyzed using a combination of quasi-static and dynamic strain rate Bauschinger testing. The stable nanocrystalline material is found to exhibit limited flow stress increase with increasing strain rate as compared to that of both pure, coarse grained and nanocrystalline Cu. Further, the material microstructural features, which includes Ta nano-dispersions, is seen to pin dislocation at quasi-static strain rates, but the deformation becomes dominated by twin nucleation at high strain rates. These twins are pinned from further growth past nucleation by the Ta nano-dispersions. Testing of thermal and load history effects on the mechanical behavior reveals that when thermal energy is increased beyond 200 °C, an upturn in flow stress is present at strain rates below 104 s-1. However, in this study, this simple assumption, established 50-years ago, is shown to break-down when the average grain size and microstructural length-scale is decreased and stabilized below 100nm. This divergent strain-rate behavior is attributed to a unique microstructure that alters slip-processes and their interactions with phonons; thus enabling materials response with a constant flow-stress even at extreme conditions. Hence, the present study provides a pathway for designing and synthesizing a new-level of tough and high-energy absorbing materials.
ContributorsTurnage, Scott Andrew (Author) / Solanki, Kiran N (Thesis advisor) / Rajagopalan, Jagannathan (Committee member) / Peralta, Pedro (Committee member) / Darling, Kristopher A (Committee member) / Mignolet, Marc (Committee member) / Arizona State University (Publisher)
Created2017
Description
The exceptional mechanical properties of polymers with heterogeneous structure, such as the high toughness of polyethylene and the excellent blast-protection capability of polyurea, are strongly related to their morphology and nanoscale structure. Different polymer microstructures, such as semicrystalline morphology and segregated nanophases, lead to coordinated molecular motions during deformation in order to preserve compatibility between the different material phases. To study molecular relaxation in polyethylene, a coarse-grained model of polyethylene was calibrated to match the local structural variable distributions sampled from supercooled atomistic melts. The coarse-grained model accurately reproduces structural properties, e.g., the local structure of both the amorphous and crystalline phases, and thermal properties, e.g., glass transition and melt temperatures, and dynamic properties: including the vastly different relaxation time scales of the amorphous and crystalline phases. A hybrid Monte Carlo routine was developed to generate realistic semicrystalline configurations of polyethylene. The generated systems accurately predict the activation energy of the alpha relaxation process within the crystalline phase. Furthermore, the models show that connectivity to long chain segments in the amorphous phase increases the energy barrier for chain slip within crystalline phase. This prediction can guide the development of tougher semicrystalline polymers by providing a fundamental understanding of how nanoscale morphology contributes to chain mobility. In a different study, the macroscopic shock response of polyurea, a phase segregated copolymer, was analyzed using density functional theory (DFT) molecular dynamics (MD) simulations and classical MD simulations. The two models predict the shock response consistently up to shock pressures of 15 GPa, beyond which the DFT-based simulations predict a softer response. From the DFT simulations, an analysis of bond scission was performed as a first step in developing a more fundamental understanding of how shock induced material transformations effect the shock response and pressure dependent strength of polyurea subjected to extreme shocks.
ContributorsLi, Yiyang (Author) / Oswald, Jay (Thesis advisor) / Rajan, Subramaniam D. (Committee member) / Solanki, Kiran (Committee member) / Chamberlin, Ralph (Committee member) / Rajagopalan, Jagannathan (Committee member) / Arizona State University (Publisher)
Created2017
Description
The following is a report that will evaluate the microstructure of the nickel-based superalloy Hastelloy X and its relationship to mechanical properties in different load conditions. Hastelloy X is of interest to the company AORA because its strength and oxidation resistance at high temperatures is directly applicable to their needs in a hybrid concentrated solar module. The literature review shows that the microstructure will produce different carbides at various temperatures, which can be beneficial to the strength of the alloy. These precipitates are found along the grain boundaries and act as pins that limit dislocation flow, as well as grain boundary sliding, and improve the rupture strength of the material. Over time, harmful precipitates form which counteract the strengthening effect of the carbides and reduce rupture strength, leading to failure. A combination of indentation and microstructure mapping was used in an effort to link local mechanical behavior to microstructure variability. Electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were initially used as a means to characterize the microstructure prior to testing. Then, a series of room temperature Vickers hardness tests at 50 and 500 gram-force were used to evaluate the variation in the local response as a function of indentation size. The room temperature study concluded that both the hardness and standard deviation increased at lower loads, which is consistent with the grain size distribution seen in the microstructure scan. The material was then subjected to high temperature spherical indentation. Load-displacement curves were essential in evaluating the decrease in strength of the material with increasing temperature. Through linear regression of the unloading portion of the curve, the plastic deformation was determined and compared at different temperatures as a qualitative method to evaluate local strength.
ContributorsCelaya, Andrew Jose (Author) / Peralta, Pedro (Thesis director) / Solanki, Kiran (Committee member) / Barrett, The Honors College (Contributor) / Mechanical and Aerospace Engineering Program (Contributor)
Created2015-05
Description
The study of the mechanical behavior of nanocrystalline metals using microelectromechanical systems (MEMS) devices lies at the intersection of nanotechnology, mechanical engineering and material science. The extremely small grains that make up nanocrystalline metals lead to higher strength but lower ductility as compared to bulk metals. Effects of strain-rate dependence on the mechanical behavior of nanocrystalline metals are explored. Knowing the strain rate dependence of mechanical properties would enable optimization of material selection for different applications and lead to lighter structural components and enhanced sustainability.
ContributorsHall, Andrea Paulette (Author) / Rajagopalan, Jagannathan (Thesis director) / Liao, Yabin (Committee member) / Barrett, The Honors College (Contributor) / Mechanical and Aerospace Engineering Program (Contributor)
Created2014-05
Description
Fracture phenomena have been extensively studied in the last several decades. Continuum mechanics-based approaches, such as finite element methods and extended finite element methods, are widely used for fracture simulation. One well-known issue of these approaches is the stress singularity resulted from the spatial discontinuity at the crack tip/front. The requirement of guiding criteria for various cracking behaviors, such as initiation, propagation, and branching, also poses some challenges. Comparing to the continuum based formulation, the discrete approaches, such as lattice spring method, discrete element method, and peridynamics, have certain advantages when modeling various fracture problems due to their intrinsic characteristics in modeling discontinuities.
A novel, alternative, and systematic framework based on a nonlocal lattice particle model is proposed in this study. The uniqueness of the proposed model is the inclusion of both pair-wise local and multi-body nonlocal potentials in the formulation. First, the basic ideas of the proposed framework for 2D isotropic solid are presented. Derivations for triangular and square lattice structure are discussed in detail. Both mechanical deformation and fracture process are simulated and model verification and validation are performed with existing analytical solutions and experimental observations. Following this, the extension to general 3D isotropic solids based on the proposed local and nonlocal potentials is given. Three cubic lattice structures are discussed in detail. Failure predictions using the 3D simulation are compared with experimental testing results and very good agreement is observed. Next, a lattice rotation scheme is proposed to account for the material orientation in modeling anisotropic solids. The consistency and difference compared to the classical material tangent stiffness transformation method are discussed in detail. The implicit and explicit solution methods for the proposed lattice particle model are also discussed. Finally, some conclusions and discussions based on the current study are drawn at the end.
A novel, alternative, and systematic framework based on a nonlocal lattice particle model is proposed in this study. The uniqueness of the proposed model is the inclusion of both pair-wise local and multi-body nonlocal potentials in the formulation. First, the basic ideas of the proposed framework for 2D isotropic solid are presented. Derivations for triangular and square lattice structure are discussed in detail. Both mechanical deformation and fracture process are simulated and model verification and validation are performed with existing analytical solutions and experimental observations. Following this, the extension to general 3D isotropic solids based on the proposed local and nonlocal potentials is given. Three cubic lattice structures are discussed in detail. Failure predictions using the 3D simulation are compared with experimental testing results and very good agreement is observed. Next, a lattice rotation scheme is proposed to account for the material orientation in modeling anisotropic solids. The consistency and difference compared to the classical material tangent stiffness transformation method are discussed in detail. The implicit and explicit solution methods for the proposed lattice particle model are also discussed. Finally, some conclusions and discussions based on the current study are drawn at the end.
ContributorsChen, Hailong (Author) / Liu, Yongming (Thesis advisor) / Jiao, Yang (Committee member) / Mignolet, Marc (Committee member) / Oswald, Jay (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2015
Description
In this thesis, a FORTRAN code is rewritten in C++ with an object oriented ap-
proach. There are several reasons for this purpose. The first reason is to establish
the basis of a GPU programming. To write programs that utilize GPU hardware,
CUDA or OpenCL is used which only support C and C++. FORTRAN has a feature
that lets its programs to call C/C++ functions. FORTRAN sends relevant data to
C/C++, which in turn sends that data to OpenCL. Although this approach works,
it makes the code messy and bulky and in the end more difficult to deal with. More-
over, there is a slight performance decrease from the additional data copy. This is
the motivation to have the code entirely written in C++ to make it more uniform,
efficient and clean. The second reason is the object oriented feature of the C++. The
“abstraction”, “inheritance” and “run-time polymorphism” features of C++ provide
some form of classes and objects, the ability to build new abstractions, and some
form of run-time binding, respectively. In recent years, some of popular codes has
been rewritten in C++ which were initially in FORTRAN. One of these softwares is
LAMMPS.
In this code the level set equation is solved by RLSG method to track the interface in
two phase flow. In gas/fluid flows, the surface tension is important and only exists at
the interface. Therefore, the location and some geometric features of interface need
to be evaluated which can be achieved by solving the level set equation.
proach. There are several reasons for this purpose. The first reason is to establish
the basis of a GPU programming. To write programs that utilize GPU hardware,
CUDA or OpenCL is used which only support C and C++. FORTRAN has a feature
that lets its programs to call C/C++ functions. FORTRAN sends relevant data to
C/C++, which in turn sends that data to OpenCL. Although this approach works,
it makes the code messy and bulky and in the end more difficult to deal with. More-
over, there is a slight performance decrease from the additional data copy. This is
the motivation to have the code entirely written in C++ to make it more uniform,
efficient and clean. The second reason is the object oriented feature of the C++. The
“abstraction”, “inheritance” and “run-time polymorphism” features of C++ provide
some form of classes and objects, the ability to build new abstractions, and some
form of run-time binding, respectively. In recent years, some of popular codes has
been rewritten in C++ which were initially in FORTRAN. One of these softwares is
LAMMPS.
In this code the level set equation is solved by RLSG method to track the interface in
two phase flow. In gas/fluid flows, the surface tension is important and only exists at
the interface. Therefore, the location and some geometric features of interface need
to be evaluated which can be achieved by solving the level set equation.
ContributorsSafarkhani, Salar (Author) / Herrmann, Mrcus (Thesis advisor) / Oswald, Jay (Committee member) / Rykczewski, Konrad (Committee member) / Arizona State University (Publisher)
Created2015
Description
Shock loading is a complex phenomenon that can lead to failure mechanisms such as strain localization, void nucleation and growth, and eventually spall fracture. The length scale of damage with respect to that of the surrounding microstructure has proven to be a key aspect in determining sites of failure initiation. Studying incipient stages of spall damage is of paramount importance to accurately determine initiation sites in the material microstructure where damage will nucleate and grow and to formulate continuum models that account for the variability of the damage process due to microstructural heterogeneity, which is the focus of this research. Shock loading experiments were conducted via flyer-plate impact tests for pressures of 2-6 GPa and strain rates of 105/s on copper polycrystals of varying thermomechanical processing conditions. Serial cross sectioning of recovered target disks was performed along with electron microscopy, electron backscattering diffraction (EBSD), focused ion beam (FIB) milling, and 3-D X-ray tomogrpahy (XRT) to gain 2-D and 3-D information on the spall plane and surrounding microstructure. Statistics on grain boundaries (GB) containing damage were obtained from 2-D data and GBs of misorientations 25° and 50° were found to have the highest probability to contain damage in as-received (AR), heat treated (HT), and fully recrystallized (FR) microstructures, while {111} Σ3 GBs were globally strong. The AR microstructure’s probability peak was the most pronounced indicating GB strength is the dominant factor for damage nucleation. 3-D XRT data was used to digitally render the spall planes of the AR, HT, and FR microstructures. From shape fitting the voids to ellipsoids, it was found that the AR microstructure contained greater than 55% intergranular damage, whereas the HT and FR microstructures contained predominantly transgranular and coalesced damage modes, respectively. 3-D reconstructions of large volume damage sites in shocked Cu multicrystals showed preference for damage nucleation at GBs between adjacent grains of a high Taylor factor mismatches as well as an angle between the shock direction and the GB physical normal of ~30°-45°. 3-D FIB sectioning of individual voids led to the discovery of uniform plastic zones ~25-50% the size of the void diameter and plastic deformation directions were characterized via local average misorientation maps. Incipient transgranular voids revealed from the sectioning process were present in grains of high Taylor factors along the shock direction, which is expected as materials with a low Taylor factor along the shock direction are susceptible to growth due their accomodation of plastic deformation. Fabrication of square waves using photolithography and chemical etching was developed to study the nature of plasticity at GBs away from the spall plane. Grains oriented close to <0 1 1> had half the residual amplitudes than grains oriented close to <0 0 1>.
ContributorsBrown, Andrew (Author) / Peralta, Pedro (Committee member) / Mignolet, Marc (Committee member) / Sieradzki, Karl (Committee member) / Solanki, Kiran (Committee member) / Jiang, Hanqing (Committee member) / Arizona State University (Publisher)
Created2015