Browsing by Subject "Micromechanics"
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Item A micromechanics based ductile damage model for anisotropic titanium alloys(2009-05-15) Keralavarma, Shyam MohanThe hot-workability of Titanium (Ti) alloys is of current interest to the aerospace industry due to its widespread application in the design of strong and light-weight aircraft structural components and engine parts. Motivated by the need for accurate simulation of large scale plastic deformation in metals that exhibit macroscopic plastic anisotropy, such as Ti, a constitutive model is developed for anisotropic materials undergoing plastic deformation coupled with ductile damage in the form of internal cavitation. The model is developed from a rigorous micromechanical basis, following well-known previous works in the field. The model incorporates the porosity and void aspect ratio as internal damage variables, and seeks to provide a more accurate prediction of damage growth compared to previous existing models. A closed form expression for the macroscopic yield locus is derived using a Hill-Mandel homogenization and limit analysis of a porous representative volume element. Analytical expressions are also developed for the evolution of the internal variables, porosity and void shape. The developed yield criterion is validated by comparison to numerically determined yield loci for specific anisotropic materials, using a numerical limit analysis technique developed herein. The evolution laws for the internal variables are validated by comparison with direct finite element simulations of porous unit cells. Comparison with previously published results in the literature indicates that the new model yields better agreement with the numerically determined yield loci for a wide range of loading paths. Use of the new model in continuum finite element simulations of ductile fracture may be expected to lead to improved predictions for damage evolution and fracture modes in plastically anisotropic materials.Item A Multiscale Model for Coupled Heat Conduction and Deformations of Viscoelastic Composites(2012-07-16) Khan, Kamran AhmedThis study introduces a multiscale model for analyzing nonlinear thermo-viscoelastic responses of particulate composites. A simplified micromechanical model consisting of four sub-cells, i.e., one particle and three matrix sub-cells is formulated to obtain the effective thermal and mechanical properties and time-dependent response of the composites. The particle and matrix constituents are made of isotropic homogeneous viscoelastic bodies undergoing small deformation gradients. Perfect bonds are assumed along the sub-cell???s interfaces. The coupling between the thermal and mechanical response is attributed to the dissipation of energy due to the viscoelastic deformation and temperature dependent material parameters in the viscoelastic constitutive model. The micromechanical relations are formulated in terms of incremental average field quantities, i.e., stress, strain, heat flux and temperature gradient, in the sub-cells. The effective mechanical properties and coefficient of thermal expansion are derived by satisfying displacement- and traction continuities at the interfaces during the thermo-viscoelastic deformations. The effective thermal conductivity is formulated by imposing heat flux- and temperature continuities at the subcells??? interfaces. The expression of the effective specific heat at a constant stress is also established. A time integration algorithm for simultaneously solving the equations that govern heat conduction and thermoviscoelastic deformations of isotropic materials is developed. The algorithm is then incorporated within each sub-cell of the micromechanical model together with the macroscopic energy equation to determine the effective coupled thermoviscoelastic response of the particulate composite. The numerical formulation is implemented within the ABAQUS, general purpose displacement based FE software, allowing for analyzing coupled heat conduction and deformations of composite structures. Experimental data on the effective thermal properties and time dependent responses of particulate composites available in the literature are used to verify the micromechanical model formulation. The multiscale model capability is also examined by comparing the field variables, i.e., temperature, displacement, stresses and strains, obtained from heterogeneous and homogeneous composite structures, during the transient heat conduction and deformations. Examples of coupled thermoviscoelastic analyses of particulate composites and functionally graded structures are also presented. The present micromechanical modeling approach is found to be computationally efficient and shows good agreement with experiments in predicting the effective thermo-mechanical response of particulate composites and functionally graded materials. Our analyses forecast a better design for creep resistant and less dissipative structures using particulate composites and functionally graded materials.Item Development of microfluidic systems for biological applications and their transport issues(2004) Li, Shifeng; Chen, Shaozhen.Microfluidic systems have been an exciting research area with a wide variety of promising biomedical applications. However, many challenging issues are still facing the research community with regard to mechanical as well as biological issues. The goal of this dissertation is to develop novel microfluidic systems targeting biomedical and life sciences applications with detailed investigation of thermal/fluid transport, materials and mechanics, and micromanufacturing processes. For a valveless micropump used for fluid delivery, a theoretical model is derived to analyze a lead zirconate titanate (PZT) microactuator and a system level analytical analysis is carried out for the PZT-actuated micropump. The effects of several important parameters and nondimensional variable groups on the actuator performances are also investigated. For rapid DNA analysis, a continuous-flow polymerase chain reaction (PCR) microchip with regional velocity control is developed. Finite element analysis is conducted to study the temperature uniformity of each reaction zone. A semi-analytical heat transfer model is developed to study heat transfer inside the PCR chip. Fluid velocity in the microchannel is measured using micro-particle image velocimetry (µ-PIV). The PCR chip is successfully amplified 90 base pair DNA sample. To connect a microfluidic device with other devices, novel polydimethysiloxane (PDMS) based interconnects have been fabricated. Microfabrication processes for through-hole type and “┌” type PDMS interconnects of glass and plastic capillary tubing have been developed. Leakage pressure, leakage flow rate, and pull-out force are characterized for these PDMS interconnects bonded to a variety of substrates. Two disposable analysis microchips in PDMS have been developed for protein/DNA detection. An established bead-based fluorescent assays for C-reactive protein (CRP) is used to characterize these chips. The detection limit of a single chamber chip is found to be as low as 0.1ng/ml. To increase detection capacity, a multiplechamber PDMS chip has also been developed. Fluid flow through the multiple-chamber microchip is improved by a back pressure compensation method. This has significantly improved the performances of the microchip.Item Extreme energy absorption : the design, modeling, and testing of negative stiffness metamaterial inclusions(2013-08) Klatt, Timothy Daniel; Seepersad, Carolyn C.; Haberman, Michael R.A persistent challenge in the design of composite materials is the ability to fabricate materials that simultaneously display high stiffness and high loss factors for the creation of structural elements capable of passively suppressing vibro-acoustic energy. Relevant recent research has shown that it is possible to produce composite materials whose macroscopic mechanical stiffness and loss properties surpass those of conventional composites through the addition of trace amounts of materials displaying negative stiffness (NS) induced by phase transformation [R. S. Lakes, et al., Nature, 410, pp. 565-567, (2001)]. The present work investigates the ability to elicit NS behavior without employing physical phenomena such as inherent nonlinear material behavior (e.g., phase change or plastic deformation) or dynamic effects, but rather the controlled buckling of small-scale structural elements, metamaterials, embedded in a continuous viscoelastic matrix. To illustrate the effect of these buckled elements, a nonlinear hierarchical multiscale material model is derived which estimates the macroscopic stiffness and loss of a composite material containing pre-strained microscale structured inclusions. The nonlinear multiscale model is then utilized in a set-based hierarchical design approach to explore the design space over a wide range of inclusion geometries. Finally, prototype NS inclusions are fabricated using an additive manufacturing technique and tested to determine quasi-static inclusion stiffness which is compared with analytical predictions.Item Optical deformability : micromechanics from cell research to biomedicine(2001-12) Guck, Jochen Reinhold; Käs, Josef A.Item Organic transistor based circuits as drivers for planar microfluidic devices(2007-12) Nadkarni, Suvid Vikas, 1981-; Dodabalapur, Ananth, 1963-The work presented in this dissertation is focused on integrating organic transistor based circuits with planar microfluidic devices for discrete droplet handling. Discrete droplet based microfluidic systems are being increasingly investigated for lab-on-a-chip type applications. An essential component of a lab-on-a-chip system is the drive circuitry that runs the system. Conventionally, a variety of schemes have been implemented for acting as drivers for microfluidic devices. Organic transistor based circuits offer a viable and cost-effective option for serving as drivers for planar microfluidic devices. The magnitudes of voltages and the time scales involved in implementing these discrete droplet based systems are in good agreement with the values of voltages that can be reliably generated using organic transistor based circuits. Thus, the union of two cost-effective technologies with the ability to perform a wide variety of functions in a lab-on-a-chip type system would be highly desirable. A simple, planar microfluidic device with an open structure is implemented on a glass substrate. The device is optimized for reliable and repeatable performance using Cytop as the insulating dielectric. Cytop provides a highly hydrophobic surface for reversible wetting to take place on the application of electrical voltage. Various organic transistor based circuits are fabricated using Pentacene as the p-type semiconducting material and N,N'-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN₂) as the n-type material. A top contact inverter, which is the most basic complementary metal oxide semiconductor circuit is fabricated and used as the driver for the planar microfluidic device. The output voltages generated by the inverter are used to actuate discrete water droplets over adjacent electrodes and also to perform merging of droplets, which is another basic functional operation that is performed on lab-on-a-chip type assemblies. Reliable and repeatable performance of the microfluidic device as well as the CMOS circuit is achieved. This work presents the first implementation of a discrete droplet based device driven by electrical voltages generated by an organic transistor based circuit. The physical mechanisms that are responsible for the motion of droplets have been investigated and contributions from electrowetting forces and dielectrophoretic forces have been resolved.Item Solutions of Eshelby-Type Inclusion Problems and a Related Homogenization Method Based on a Simplified Strain Gradient Elasticity Theory(2011-08-08) Ma, HemeiEshelby-type inclusion problems of an infinite or a finite homogeneous isotropic elastic body containing an arbitrary-shape inclusion prescribed with an eigenstrain and an eigenstrain gradient are analytically solved. The solutions are based on a simplified strain gradient elasticity theory (SSGET) that includes one material length scale parameter in addition to two classical elastic constants. For the infinite-domain inclusion problem, the Eshelby tensor is derived in a general form by using the Green?s function in the SSGET. This Eshelby tensor captures the inclusion size effect and recovers the classical Eshelby tensor when the strain gradient effect is ignored. By applying the general form, the explicit expressions of the Eshelby tensor for the special cases of a spherical inclusion, a cylindrical inclusion of infinite length and an ellipsoidal inclusion are obtained. Also, the volume average of the new Eshelby tensor over the inclusion in each case is analytically derived. It is quantitatively shown that the new Eshelby tensor and its average can explain the inclusion size effect, unlike its counterpart based on classical elasticity. To solve the finite-domain inclusion problem, an extended Betti?s reciprocal theorem and an extended Somigliana?s identity based on the SSGET are proposed and utilized. The solution for the disturbed displacement field incorporates the boundary effect and recovers that for the infinite-domain inclusion problem. The problem of a spherical inclusion embedded concentrically in a finite spherical body is analytically solved by applying the general solution, with the Eshelby tensor and its volume average obtained in closed forms. It is demonstrated through numerical results that the newly obtained Eshelby tensor can capture the inclusion size and boundary effects, unlike existing ones. Finally, a homogenization method is developed to predict the effective elastic properties of a heterogeneous material using the SSGET. An effective elastic stiffness tensor is analytically derived for the heterogeneous material by applying the Mori-Tanaka and Eshelby?s equivalent inclusion methods. This tensor depends on the inhomogeneity size, unlike what is predicted by existing homogenization methods based on classical elasticity. Numerical results for a two-phase composite reveal that the composite becomes stiffer when the inhomogeneities get smaller.