Browsing by Subject "finite element analysis"
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Item A Finite Element Framework for Multiscale/Multiphysics Analysis of Structures with Complex Microstructures(2010-10-12) Varghese, JulianThis research work has contributed in various ways to help develop a better understanding of textile composites and materials with complex microstructures in general. An instrumental part of this work was the development of an object-oriented framework that made it convenient to perform multiscale/multiphysics analyses of advanced materials with complex microstructures such as textile composites. In addition to the studies conducted in this work, this framework lays the groundwork for continued research of these materials. This framework enabled a detailed multiscale stress analysis of a woven DCB specimen that revealed the effect of the complex microstructure on the stress and strain energy release rate distribution along the crack front. In addition to implementing an oxidation model, the framework was also used to implement strategies that expedited the simulation of oxidation in textile composites so that it would take only a few hours. The simulation showed that the tow architecture played a significant role in the oxidation behavior in textile composites. Finally, a coupled diffusion/oxidation and damage progression analysis was implemented that was used to study the mechanical behavior of textile composites under mechanical loading as well as oxidation. A parametric study was performed to determine the effect of material properties and the number of plies in the laminate on its mechanical behavior. The analyses indicated a significant effect of the tow architecture and other parameters on the damage progression in the laminates.Item Finite Element Analysis of Three-Phase Piezoelectric Nanocomposites(2010-10-12) Maxwell, Kevin S.In recent years, traditional piezoelectric materials have been pushed to the limit in terms of performance because of countless novel applications. This has caused an increased interest in piezoelectric composites, which combine two or more constituent materials in order to create a material system that incorporates favorable attributes from each constituent. One or more of the constituents exhibits piezoelectric behavior, so that the composite has an effective electromechanical coupling. The composite material may also have enhanced properties such as stiffness, durability, and flexibility. Finite element analyses were conducted on a three-phase piezoelectric nanocomposite in order to investigate the effects of several design parameters on performance. The nanocomposite consisted of a polyimide matrix, beta-CN APB/ODPA, enhanced with single wall carbon nanotubes and PZT-5A particles. The polyimide and nan- otube phases were modeled as a single homogenized phase. This results in a two-phase nanocomposite that can be modeled entirely in the continuum domain. The material properties for the nano-reinforced matrix and PZT-5A were obtained from previous experimental efforts and from the literature. The finite element model consisted of a single representative volume element of the two-phase nanocomposite. Exact periodic boundary conditions were derived and used to minimize the analysis region. The effective mechanical, electrical, and piezoelectric properties were computed for a wide range of nanotube and PZT particle concentrations. A discrepancy was found between the experimental results from the literature and the computational results for the effective electrical properties. Several modified finite element models were developed to explore possible reasons for this discrepancy, and a hypothesis involving dispersion of the nanotubes was formulated as an attempt to explain the difference. The response of the nanocomposite under harmonic loading was also investigated using the finite element model. The effective properties were found to be highly dependent on the dielectric loss of the beta CN/SWNT matrix. It was also found that increasing the matrix loss enhanced piezoelectric performance up to a certain point. Exploiting this type of behavior could be an effective tool in designing piezoelectric composite materials.Item Improvement of a Multiscale Framework for the Analysis of Composite Materials(2014-08-11) Ballard, Michael KeithMultiscale analyses have been extensively used to virtually test how a material will respond linearly and nonlinearly, due to the initiation and evolution of damage, to a variety of loads and environmental conditions. This work improved several components of a multiscale framework. At the microscale, elastic properties were determined for four types of graphite fibers, including AS4, IM7, T300, and T650, along with a type of glass fiber, E-glass 21xK43, using an inverse method. Homogenization methods used in the inverse analyses include:finite element analysis (FEA) with a hexagonal microstructure, FEA with a random microstructure, and Mori-Tanaka averaging scheme. Fiber properties determined using FEA with a hexagonal microstructure and the Mori-Tanaka averaging scheme were very similar, while using FEA with a random arrangement of fibers resulted in significantly different properties. The predicted longitudinal shear modulus, G_(12), of the graphite fiber was observed to almost linearly depend on the minimum spacing between fibers, while the other engineering constants did not depend on the minimum space between fibers. The predicted properties for the glass fiber were shown to be insensitive to the homogenization method used. At the mesoscale, two types of continuum damage models, a cohesive zone model, and a combination of the two types were compared using a [0/90]_(s) laminate under uniaxial tension and in-plane shear loads. The volume average stress-strain response, the crack density evolution, and a metric developed using two-point correlation functions were used to quantify the similarities and differences of the progressive damage models. For a laminate under uniaxial tension, a continuum damage model that degrades the material on an element basis predicted a progression of damage similar to the cohesive zone model. A continuum damage model that degrades the material on a quadrature point basis predicted a lower applied strain for final failure and a higher crack density. Under in-plane shear, the continuum damage models predicted damage growth across fibers, which is unrealistic. Cohesive zone elements can be placed where damage is expected, but when placed in all directions, the cohesive zone model predicted the same unrealistic damage growth across fibers.Item Modeling of crack tip high inertia zone in dynamic brittle fracture(Texas A&M University, 2007-09-17) Karedla-Ravi, ShankarA phenomenological cohesive term is proposed and added to an existing cohesive constitutive law (by Roy and Dodds) to model the crack tip high inertia region proposed by Gao. The new term is attributed to fracture mechanisms that result in high energy dissipation around the crack tip and is assumed to be a function of external energy per volume input into the system. Finite element analysis is performed on PMMA with constant velocity boundary conditions and mesh discretization based on the work of Xu and Needleman. The cohesive model with the proposed dissipative term is only applied in the high inertia zone i.e., to cohesive elements very close to the crack tip and the traditional Roy and Dodds model is applied on cohesive elements in the rest of the domain. It was observed that crack propagated in three phases with a speed of 0.35cR before branching, which are in good agreement with experimental observations. Thus, modeling of high inertia zone is one of the key aspects to understanding brittle fracture.Item On simple and accurate finite element models for nonlinear bending analysis of beams and plates(Texas A&M University, 2007-09-17) Urthaler Lapeira, Yetzirah YksyaThis study is concerned with the development of simple and accurate alternative finite element models to displacement finite element models for geometrically nonlinear bending analysis of beams and plates. First, a unified corotational beam finite element that incorporates the kinematics of classical as well as refined beam theories, including the Timoshenko and Reddy beam theories, is developed in a single finite element. The governing equations are written in a "corotational" local frame that rotates with the element and with respect to which the standard linear engineering relations between strains and internal forces are valid. The element is based on Lagrange interpolation of the axial displacement, Hermite cubic interpolation of the transverse displacement, and related quadratic interpolation of the rotation, and it does not experience shear locking. The model is verified by comparisons with exact and/or approximate solutions available in the literature. Very good agreement is found in all cases. Next, a finite element model is developed using a mixed formulation of the first-order shear deformation theory of laminated composite plates. A p-type Lagrangian basis is used to approximate the nodal degrees of freedom that consist of three displacements, two rotations, and three moment resultants. The geometric nonlinearity, in the sense of the von K????arman, is included in the plate theory. The mixed plate element developed herein is employed in the linear and nonlinear bending analysis of a variety of layered composite rectangular plates. The effects of transverse shear deformation, material anisotropy, and bending-stretching coupling on deflections and stresses are investigated. The predictive capability of the present model is demonstrated by comparison with analytical, experimental, and numerical solutions available in the literature. The model provides an accurate prediction of the global bending response of thin and moderately thick plates subjected to moderate and moderately large rotations. The inclusion of the bending moments at the nodes results in increased accuracy in the computation of stresses over those determined by conventional displacement-based finite element models. The many results presented here for geometrically nonlinear bending analysis of beams and plates should serve as reference for future investigations.Item Theoretical Developments and Practical Aspects of Dynamic Systems in Wind Energy Applications(2013-11-07) Owens, Brian CThe availability of offshore wind resources in coastal regions along with a high concentration of load centers in these areas makes offshore wind energy an attractive opportunity. Infrastructure costs and operation and maintenance costs for offshore wind technology, however, are significant obstacles that need to be overcome to make offshore wind a viable option. Vertical-axis wind turbines (VAWTs) are potentially ideal candidates for large offshore wind energy applications, and may provide a means to significantly reduce life-cycle costs associated with offshore wind energy. This has motivated the development of a flexible and extensible modular analysis framework for investigating VAWT designs. The Offshore Wind Energy Simulation toolkit contains a modular analysis framework that provides a general interface to external modules such as aerodynamics, hydrodynamics/platform dynamics, and generator/drive-train modeling software. Theoretical developments in dynamic systems are also presented in this work. Implicit time integration methods are investigated for their applicability to Gyric systems (flexible systems undergoing general rotational motion). An energy conserving integration method for conventional flexible systems are considered and proven to be energy preserving for Gyric systems. A new, efficient procedure for developing linearized representation of discrete dynamic systems is also presented. Two existing approaches for developing linear representations are combined to arrive at a new, more efficient linearization procedure that overcomes the pitfalls of the individual approaches alone. Furthermore, aeroelastic stability is a known issue for large, flexible structures under aerodynamic loads, and aeroelastic analysis was considered in the development of wind energy design tools. Finally, an investigation of the structural dynamics of offshore VAWT structure is conducted. A fundamental understanding of a resonance in VAWT configurations is sought, and the effects of support conditions on dynamic response of VAWT configurations is explored.Item Using finite element analysis of retroreflective raised pavement markers to recommend testing procedures for simulating their field performance(Texas A&M University, 2006-08-16) Agrawal, Ravi PrakashRetroreflective Raised Pavement Markers (RRPMs) supplement other pavement markings to provide guidance to road users. Previous research concerning durability of the RRPMs suggests that their performance has been degrading over the years. One of the main causes for underperformance of the RRPMs is the lack of appropriate laboratory testing standards that can test the adequacy of the RRPMs to perform in field conditions. There is a need to modify the existing standards or develop new testing procedures that can better simulate field conditions. This requires identifying critical locations and magnitudes of stresses inside the markers during the tire-marker impacts that happen on roads. The goal of this research was to identify critical magnitudes and locations of the stresses in RRPMs during the tire-marker impacts by doing the finite element modeling and simulation of the impacts, and use the information to recommend laboratory testing procedures that could simulate real-world conditions. The researcher modeled and simulated the tire-marker impacts using the finite element tools Hypermesh and LS DYNA. He calibrated the material properties of the marker models to improve the tiremarker model. Based on the tire-marker impact simulations, the researcher concluded that the critical compressive stresses during impacts are located at the edge contacts of retroreflective sides with the top surface. The critical stresses may also occur at lower and upper corners of the marker. The other areas, especially the lower half of the marker, had tensile stresses. Angle of impact was found to be a critical external variable that affected the stresses inside the markers and the marker-pavement interface forces. The researcher then modeled and simulated a few laboratory-testing procedures that could simulate the field performance of the RRPMs. Based on these simulations, the researcher recommended that the ASTM compression test for evaluation of RRPMs be continued or a similar test be developed. He suggested development of one new test (named as offset compression test) that could better replicate the field conditions. He also recommended having a review of the ASTM flexural test.