Browsing by Subject "FEM"

Now showing 1 - 4 of 4
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    Explicit Crack Modeling based Approach for Structural Integrity Assessment of Brittle and Quasi-Brittle Structures
    (2015-02) Singh, Gyanender
    There is considerable variation in the fracture properties of brittle and quasi-brittle materials. Due to this large variation, probabilistic models are employed for estimating failure of brittle components/structures. However, due to limitations and shortcomings in the models, the predictions are not accurate. The shortcomings include: inability to handle stress concentrations, dependence of empirical constants on loading conditions, incorrect size-effect predictions and limited applications of the model. Although higher design margins can accommodate the inaccuracy in predictions, the cost of manufacturing increases. The work presented herein is directed towards addressing these issues. An approach based on explicit crack modeling (ECM) for accurately estimating failure in brittle/quasi-brittle components and structures is presented. Factors which govern fracture in a structure (fracture energy, strength of the material, damage behavior of the material, heterogeneity in the material microstructure) are incorporated in the ECM approach. The approach was validated by predicting the failure probability of L-shaped specimens at varying load levels followed by comparison of the predictions with published data. The study showed that the predictions from the ECM approach were not only in good agreement with the published data but were also more accurate than the Weibull model based predictions. The ECM approach can also predict size effect--the dependence of fracture properties and their statistical variation on the size of the specimen. This capability was demonstrated through failure prediction of specimens in tensile and flexural tests. Specimens of different sizes were considered and the predicted fracture properties were in good agreement with those obtained experimentally. The ECM approach for estimating failure of components/structures subjected to complex physical conditions was illustrated through the failure estimation of nuclear reactor graphite components. For modeling stresses in the graphite components subjected to high temperature and neutron irradiation, a constitutive model for evaluating the stresses was constructed and implemented through a user material (UMAT) subroutine in finite element software Abaqus. UMAT was integrated with Extended Finite Element (XFEM) technique for modeling irradiation-induced failure of the components under in-reactor conditions. Component lifetime as well as crack initiation and propagation details were predicted. This type of detailed failure information has the potential to improve design guidelines and standards of brittle components/structures.
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    Finite element modeling of thin fiber reinforced concrete pavements
    (2023-07) Sharma, Pranav
    Thin concrete pavement is an economical option for low and moderate traffic roads, where thethickness of concrete slab varies from 4-inch to 6-inch. In conventional concrete pavement, dowel bars are used to increase load transfer efficiency (LTE) and mitigate transverse joint faulting. However, dowel bars cannot be accommodated in the thin concrete pavement due to insufficient clear cover. For such pavements, structural fibers are a good option for increasing joint performance or load transfer efficiency, as well as reducing faulting. However, only limited studies are available in understanding the contribution of structural fibers to the benefits of joint performance and the behavior of fibers during the transfer of loads across the joint. In this study, finite element analysis of the thin fiber reinforced concrete (FRC) pavement was performed. A six-slab model was developed with a granular aggregate layer, replicating the actual field conditions. The effect of concrete and base layer structure, material properties, traffic and environmental loads, and joint stiffness on the transverse joint performance and critical stresses were studied. It was found that around 40% of the wheel load is transferred through the pavement foundation and the rest through the aggregate interlocking and fibers’ lateral stiffness. Critical stresses for the fatigue cracks along the wheel path were also determined in this study. This study concluded the minimum required lateral stiffness of the structural fibers for a desired level of joint performance as a function of the pavement structure.
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    Micromechanical modeling of composite materials using the finite element method for balancing discretization and material modeling error
    (2013-06) Lindberg, Sara Carol
    The goal of this research is to advance computer modeling capabilities to combine with or replace experimental testing of composite materials. To be able to achieve this goal, modeling techniques are implemented, with the aim of combining computational efficiency and accuracy. To do this the sources of error need to be mitigated when performing meso-scale numerical tests on micromechanical composite materials. The sources of error are discretization when meshing in finite elements, and material modeling error. As the refinement of a finite element mesh increases, the error decreases and the computational cost increases. In some cases it has been shown that increasing the size of a material being homogenized increases the accuracy of the prediction of the material properties, as the size of the material approaches a representative volume. Various homogenization methods have different degrees of accuracy and computational efficiency. Homogenization often requires definition of a representative volume element (RVE). This definition creates a model of a finite magnitude that represents an equivalent homogenous material. The technique is used in the investigation of several simple structures in this work. A statistical volume element (SVE) at the meso-scale defines an element on a smaller scale than the RVE but is still larger than the micro-scale. The SVE is used to statistically analyze the stiffness properties of a model on the meso-scale, where the meso-scale is defined as any scale between the micro and macro-scales. Moving window (MW) homogenization is an improved alternative to homogenizing the entire structure. Moving window homogenization is shown to increase accuracy, when to compared to benchmark results.
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    Modeling of blade cutting of viscoelastic biomaterials
    (2013-06) Peng, Yun
    The work in this thesis focuses on the modeling of blade cutting of viscoelastic materials. The blade cutting procedure is modeled in two stages. The first stage is the contact of the blade with the cutting material and the second stage is the fracture during continuous cutting. The modeling of the first stage is used to predict the initiation of the cutting fracture and the modeling of the second stage is use to characterize the cutting force during continuous fracture. Experiments that are used to determine the material parameters for the simulations and calculations of the cutting process are also carried out.The first stage is modeled as the area contact between the edge of the blade and cutting materials. It is modeled by applying the elastic-viscoelastic correspondence principle to the solutions for point load and then by performing a numerical integration scheme to extend the solutions to distributed pressure cases. The stress tensor was analytically obtained at any given point inside the viscoelastic material. The effect of slicing angles on the stress distribution is then evaluated. Using the principal stresses, the location of damage is predicted using Tresca's failure criterion. In the continuous damage stage, FEM simulation using ABAQUS is used as the modeling method. A bi-layered structure is applied to represent the tissue-bone structure which could be widely seen in a deboning process. In the simulation, the cutting force is monitored during the blade cuts through the interface. The dynamic change of the force pattern when the blade approaches the interface is analyzed in order to propose a control algorithm that prevents the blade cutting into bones. In order to provide realistic data for the simulation, several relaxation tests are designed to obtain the tensile relaxation modulus for biomaterials. Ligaments obtained from chicken wings and legs are used as specimens. The experimental data was theoretically fitted into a Burgers Model for the simulation and calculation. The model developed in this research can serve as a guideline for many applications such as the design of a surgical simulator to facilitate the training of new doctors and the intelligent control of a robot for deboning process to improve cutting yield and meat harvesting quality.