Browsing by Subject "Finite Element Analysis"

Now showing 1 - 6 of 6
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    A detailed design analysis of a lumenally delivered, flexible, balloon-assisted, sterile endoscopic overtube.
    (2010-09) Buesseler, Ryan Kenneth
    The goal of this project was to develop a sterile surgical device for gaining peritoneal cavity access without external incisions via the body's major natural orifices. Such Natural Orifice Translumenal Endoscopic Surgery (NOTES) interventions have recently received innovative attention as the next step in minimally-invasive surgery, however, the safety and efficacy depends greatly on sterility. The research for this thesis focused primarily on gastric interventions is divided into four main foci: tensile testing and determination of the most accurate material model for human gastric tissue, FEA analysis of the balloon dilation, experimentally validating these results using a silicone tissue phantom, and the paper design of a theoretical prototype. Tensile testing the human gastric tissue provided the only material properties for the entire stress-strain curve known to the literature to be accurate. A series of tests were conducted on several different freshly donated organs. Statistical analyses were performed comparing the inner and outer layers, and the 0° and 90° orientation. These results showed that while visually different, the elastic portion of the stress-strain curve showed no statistical differences between layers or orientation. A FE model was created in 2D axisymmetric and 3D to determine the minimum size incision needed to dilate large enough to allow passage for the device and endoscope without inducing irreversible damage to the tissue. Conclusions from tensile testing led to the material model being hyperelastic, homogeneous, and isotropic. ABAQUS Explicit was used to model the quasi-steady state problem and to more effectively manage contact definitions in the 3D simulation. The models were also simulated with a silicone material for experimental validation. Using the same assumptions from the ABAQUS model, a physical experiment was performed with the silicone tissue phantom. From a circular incision of 10mm, several final diameters were tested. Rigid objects were used to dilate rather than balloons for ease of visualization. The surface had nodal coordinates drawn on the material and digital images taken before and after dilation. Coordinates were extracted using xyExtract.exe, and strains calculated with a user defined MATLAB program. Taking the above work into consideration, three different prototype designs were proposed. All three incorporate dual-donut balloons as the primary means of dilating the initial incision, holding the device in place, and providing a mechanical means for ensuring sterility while maintaining insufflation. The first embodiment simply incorporates dual balloons on the end of a PTFE sheath. The second utilizes a corkscrew-shaped singular balloon crimped in the middle to form the dual expansion zones. Finally, the last prototype design uses a second outer sheath to encapsulate both balloons which provides the dilatory force to the tissue, and acts as a long cylindrical balloon to stabilize the length of the sheath.
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    Finite element modeling of articular cartilage at different length scales
    (2012-04) Chiravarambath, Sidharth Saktan
    The composition and structure of articular cartilage (AC) are inhomogeneous within the tissue and vary throughout its depth. Its extracellular matrix can be considered as a fiber-reinforced composite solid consisting of a dense stable network of collagen fibers embedded in a proteoglycan (PG) gel. Several studies have shown that this specialized structure plays a vital role in the mechanical function of AC. In pathological conditions, such as osteoarthritis (OA), degeneration of cartilage due to changes in mechanical properties is observed. Osteoarthritis is the most common cause of disability in the elderly and affects more than 20 million people in the USA alone. The focus of this work is to understand the mechanical response of AC using finite element models using ABAQUS, a commercial FEA package that is widely used in the field of cartilage mechanics. This is done at two different scales - the macroscale and the mesoscale. At the macroscale, AC is considered as a homogeneous isotropic poroviscoelastic (PVE) material saturated by the interstitial fluid (water). Indentation tests are performed on cartilage from the mouse tibia plateau using two different sized flat-ended conical indenters with flat-end diameters of 15 μm and 170 μm. A finite element (FE) model of the test is developed and the PVE parameters identified by using inverse methods to minimize the errors between FE simulated and test data. Data from the smaller indenter is first used to fit the viscoelastic (VE) parameters, on the basis that for this tip size the gel diffusion time (approximate time constant of the poroelastic (PE) response) is of the order of 0.1 s, so that the PE response is negligible. These parameters are then used to fit the data from the larger indenter for the PE parameters, using the VE parameters extracted from the data from the smaller indenter. At the mesoscale the inhomogeneities of AC need to be addressed to understand the microstructural behavior of AC. The problem of interest in this part of the work is to understand the mechanical role of interfibrillar cross-links (IFLs), if they exist, suspected in AC and most collagenous tissues. A 3D FE model of AC meso-structure motivated by the parallel fibril geometry of the mid and deep zones of the patella is developed consisting of a PE matrix, unidirectional, bilinear fibrils (different stiffness in tension and compression), and the IFLs. Parametric studies are then performed for the model in simulated compression tests along the fibril direction and the effect of the IFLs and matrix are predicted and compared. Results suggest presence of IFLs would increase the effective modulus in compression. This is due to maintaining organization of the fibrils into a network due to IFLs imparting stability to the network by preventing early bending of fibrils and effectively reducing the Poisson effect. Finally, with a set of literature based parameters, compression tests for AC using the mesomodel show that removing the cross-links results in a significant (43%) drop in the effective compressive modulus, suggesting resolution necessary to experimentally detect the IFLs. At the mesoscale, the IFLs would play the mechanical role of stabilizing the fibril network and enhancing its stiffness.
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    Finite Element Modeling of Cornea for Penetrating Keratoplasty: Development of a New Wound Geometry
    (2018-06) Joseph, Soumya
    Cornea transplant surgery, also known as Keratoplasty, is a procedure in which a part of a patient’s cornea is replaced with corneal tissue from a healthy donor eye obtained from an eye bank. The cornea being replaced is referred to as the host while the donor cornea is referred to as the graft. Presently, the procedure utilizes 16 to 24 interrupted sutures in a radial pattern to ensure the graft-host wound closure post operation. Suture placement needs to be performed with a high level of skill, ensuring all sutures are near identical in their pass length, depth, tension, distance from the optical center of the cornea and radiality. Problems with any of the mentioned factors could result in the induction of astigmatism, requiring multiple post-operative visits – usually over the course of a year – to assess and mitigate it. The procedure in itself is time intensive, taking between 45 and 75 minutes for a full transplant by an experienced surgeon. Therefore, the cornea transplant surgeon needs a method of graft-host wound closure that changes the way current sutures are used or the way their forces are distributed, minimizes the number of sutures used in the current manner, and/or abandons sutures in lieu of another technology because sutures as used in the current manner create astigmatism that is difficult to manage (for the reasons listed above, and probably others). Finite Element Analysis was used to predict the behavior of the cornea under different loading conditions. One of the solutions proposed in this work attempt to limit the number of sutures required to a maximum of 12 by utilizing stopping forces created due to the geometry of the graft-host wound interface that minimize the separation of the graft from the host under the influence of intra-ocular pressure. The other solution is the introduction of an intra-stromal corneal ring segment which enables suturing force to be taken by the ring rather than the central cornea leading to less central cornea distortion.
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    Insights into Biomechanics of Pelvic Organs: A 3D Finite Element Analysis Investigating Vaginal Vault Prolapse Mechanisms and Surgical Intervention
    (2024-01) Togaru, Lavanith
    Pelvic Floor Disorders (PFDs), including Vaginal Vault Prolapse (VVP), pose significant health concerns for women, affecting urinary, rectal, and sexual functions. This study addresses the lack of understanding in prolapse mechanisms and pelvic organ dynamics through a precise 3D Finite Element Analysis (FEA) model. Developed with data from the visible human project, this model integrates diverse material properties, ensuring accurate representation of pelvic organ behavior. Investigating nine simulated cases, the study unveils subtle variations in pelvic dynamics, emphasizing the link between changes in pelvic organ stiffness and prolapse. A simulated surgical Y-mesh demonstrates efficacy in rectifying VVP and pelvic floor issues. This research contributes substantially to VVP biomechanics, emphasizing the integral nature of pelvic organ interactions and urging further exploration of long-term surgical effects. The model not only enriches discussions on women's health but also lays the groundwork for advanced models to understand and manage Vaginal Vault Prolapse
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    A Novel Computational Framework Integrating Different Space Discretization and Time Discretization Methods with Multiple Subdomains and Reduced Order Modeling
    (2020-09) Tae, David
    This thesis presents advances and developments in the field of spatial discretization and time integration. Along with the growth of the FEM, there has been a steady development of particle discretization methods such as Moving Particle Semi-implicit method or Smoothed Particle Hydrodynamics method. We propose a novel generalized approach to describe numerous existing particle methods by exploiting Taylor series expansion and the weighted residual method. The method is then validated through various problems in first and second order systems. The FEM and the particle methods have their own strengths and weaknesses. With the concept of subdomains and Differential Algebraic Equations (DAE) framework, we can divide a body and implement different methods in different regions of the body targeting an area with a specific method which can fully utilize its best features. We propose an implementation of multi-spatial method, multi-time scheme subdomain DAE framework allowing a mix of different space discretization methods and different time schemes on a single body analysis. This is not possible in the current state of the technology as it shows limitations in order of accuracy, and consistency. Various combinations of spatial methods and time schemes between subdomains are tested in linear and nonlinear problems for first and second order systems. Lastly, we introduced reduced order modeling via Proper Orthogonal Decomposition (POD) which decreases the size of the system based on its eigenvalues. The snapshot data are used to establish the reduced order basis. We additionally propose the integration of POD into the subdomain DAE framework. As the required amount of snapshot data are unknown and problem specific, we present an iterative process to ensure the snapshot data to accurately capture the physics of the system. In addition, the iteration approach is extended to include the convergence check in time on the solution for implicit time schemes. The proposed DAE POD framework is tested on numerous linear and nonlinear problems for first and second order systems. In all cases, we see time savings in computational effort.
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    Strength and Stability of Prestressed Concrete Through-Girder Pedestrian Bridges Subjected to Vehicular Impact
    (Minnesota Department of Transportation, Research Services Section, 2007-01) Baran, Eray; Schultz, Arturo; French, Catherine
    Two issues regarding the prestressed concrete through-girder pedestrian bridge system are investigated. The first issue concerns the ductility of prestressed concrete girders in these bridges because the section that is typically used may be considered to be over-reinforced according to AASHTO LRFD Bridge Specifications. Response of the section, including neutral axis location, strand stress at ultimate capacity, and moment capacity, predicted by AASHTO Standard and AASHTO LRFD Specifications are compared with the sectional response determined from nonlinear strain compatibility analyses. Modifications are proposed to the AASHTO LRFD procedure to rectify the errors in predicting sectional response. The second issue that was investigated concerns the strength and stability of prestressed concrete through-girder pedestrian bridges when subjected to impact by over-height vehicles. Three-dimensional finite element models of entire bridges and subassemblages were used to evaluate the strength, stiffness, and ductility characteristics of the bridge system and connection details. Accurate representation of the bridge details in the finite element models were assured by utilizing experimentally determined load-deformation characteristics for the connections. Results showed that significant improvements in the lateral load-deflection behavior of the bridge system could be obtained by implementing alternate connection schemes, and that concrete side-walls should be provided at girder ends.