Mechanical Engineering Publicationshttp://hdl.handle.net/1911/648782019-06-16T19:22:24Z2019-06-16T19:22:24ZThe effect of plate design, bridging span, and fracture healing on the performance of high tibial osteotomy plates: An experimental and finite element studyMacLeod, A.R.Serrancoli, G.Fregly, B.J.Toms, A.D.Gill, H.S.http://hdl.handle.net/1911/1051302019-05-16T17:06:14Z2018-01-01T00:00:00ZThe effect of plate design, bridging span, and fracture healing on the performance of high tibial osteotomy plates: An experimental and finite element study
MacLeod, A.R.; Serrancoli, G.; Fregly, B.J.; Toms, A.D.; Gill, H.S.
Objectives: Opening wedge high tibial osteotomy (HTO) is an established surgical procedure for the treatment of early-stage knee arthritis. Other than infection, the majority of complications are related to mechanical factors – in particular, stimulation of healing at the osteotomy site. This study used finite element (FE) analysis to investigate the effect of plate design and bridging span on interfragmentary movement (IFM) and the influence of fracture healing on plate stress and potential failure. Materials and Methods: A 10° opening wedge HTO was created in a composite tibia. Imaging and strain gauge data were used to create and validate FE models. Models of an intact tibia and a tibia implanted with a custom HTO plate using two different bridging spans were validated against experimental data. Physiological muscle forces and different stages of osteotomy gap healing simulating up to six weeks postoperatively were then incorporated. Predictions of plate stress and IFM for the custom plate were compared against predictions for an industry standard plate (TomoFix). Results: For both plate types, long spans increased IFM but did not substantially alter peak plate stress. The custom plate increased axial and shear IFM values by up to 24% and 47%, respectively, compared with the TomoFix. In all cases, a callus stiffness of 528 MPa was required to reduce plate stress below the fatigue strength of titanium alloy. Conclusion: We demonstrate that larger bridging spans in opening wedge HTO increase IFM without substantially increasing plate stress. The results indicate, however, that callus healing is required to prevent fatigue failure.
2018-01-01T00:00:00ZA finite element model for magnetohydrodynamic squeeze-film flowsWagner, Jordan R.Higgs, C. Fred IIIhttp://hdl.handle.net/1911/1051152019-05-16T17:06:14Z2018-01-01T00:00:00ZA finite element model for magnetohydrodynamic squeeze-film flows
Wagner, Jordan R.; Higgs, C. Fred III
A computational model is developed to analyze magnetohydrodynamic (MHD) squeeze-film flows featuring an electrically conducting fluid subjected to imposed magnetic and electric fields. The model is based on the so-called MHD Reynolds equation for squeeze-films—an extension of the classical hydrodynamic Reynolds equation. A complete derivation of the MHD Reynolds equation is performed by applying thin-film and quasi-steady assumptions to the Maxwell/Navier-Stokes system coupled by the Lorentz force. The resulting equation is a two-dimensional and variable-coefficient Poisson equation for pressure, which reduces to the purely hydrodynamic form in the limit of vanishing Hartmann number. A geometric calculus formulation facilitates the reduction of the mathematical system into two dimensions, which is a challenge in standard vector calculus due to the cross product. The model permits realistic geometrical representations of the constraining squeeze-surfaces, and we demonstrate the use of a multi-variate Weierstrass-Mandelbrot fractal to numerically generate scale-invariant surface roughness profiles. Ultimately, the governing equation is solved with the Galerkin finite element method. Several numerical examples are conducted to highlight some of the model’s capabilities. MHD forces—as well as the roughness, geometry, and topology of the squeeze surfaces—are shown to significantly influence flow characteristics.
2018-01-01T00:00:00ZAorta zero-stress state modeling with T-spline discretizationSasaki, TakafumiTakizawa, KenjiTezduyar, Tayfun E.http://hdl.handle.net/1911/1049772019-05-16T17:06:14Z2018-01-01T00:00:00ZAorta zero-stress state modeling with T-spline discretization
Sasaki, Takafumi; Takizawa, Kenji; Tezduyar, Tayfun E.
The image-based arterial geometries used in patient-specific arterial fluid–structure interaction (FSI) computations, such as aorta FSI computations, do not come from the zero-stress state (ZSS) of the artery. We propose a method for estimating the ZSS required in the computations. Our estimate is based on T-spline discretization of the arterial wall and is in the form of integration-point-based ZSS (IPBZSS). The method has two main components. (1) An iterative method, which starts with a calculated initial guess, is used for computing the IPBZSS such that when a given pressure load is applied, the image-based target shape is matched. (2) A method, which is based on the shell model of the artery, is used for calculating the initial guess. The T-spline discretization enables dealing with complex arterial geometries, such as an aorta model with branches, while retaining the desirable features of isogeometric discretization. With higher-order basis functions of the isogeometric discretization, we may be able to achieve a similar level of accuracy as with the linear basis functions, but using larger-size and much fewer elements. In addition, the higher-order basis functions allow representation of more complex shapes within an element. The IPBZSS is a convenient representation of the ZSS because with isogeometric discretization, especially with T-spline discretization, specifying conditions at integration points is more straightforward than imposing conditions on control points. Calculating the initial guess based on the shell model of the artery results in a more realistic initial guess. To show how the new ZSS estimation method performs, we first present 3D test computations with a Y-shaped tube. Then we show a 3D computation where the target geometry is coming from medical image of a human aorta, and we include the branches in our model.
2018-01-01T00:00:00ZStrain Hardening for Elastic-Perfectly Plastic to Perfectly Elastic Flattening Single Asperity ContactGhaednia, HamidBrake, Matthew R.W.Berryhill, MichaelJackson, Robert L.http://hdl.handle.net/1911/1032582019-05-16T17:06:14Z2018-01-01T00:00:00ZStrain Hardening for Elastic-Perfectly Plastic to Perfectly Elastic Flattening Single Asperity Contact
Ghaednia, Hamid; Brake, Matthew R.W.; Berryhill, Michael; Jackson, Robert L.
For elastic contact, an exact analytical solution for the stresses and strains within two contacting bodies has been known since the 1880s. Despite this, there is no similar solution for elastic-plastic contact due to the integral nature of plastic deformations, and the few models that do exist develop approximate solutions for the elastic-perfectly plastic material model. In this work, the full transition from elastic-perfectly plastic to elastic materials in contact is studied using a bilinear material model in a finite element environment for a frictionless dry flattening contact. Even though the contact is considered flattening, elastic deformations are allowed to happen on the flat. The real contact radius is found to converge to the elastic contact limit at a tangent modulus of elasticity around 20 %. For the contact force, results show a different trend in which there is a continual variation in forces across the entire range of material models studied. A new formulation has been developed based on finite element results to predict the deformations, the real contact area, and contact force. A second approach has been introduced to calculate the contact force based on the approximation of the Hertzian solution for the elastic deformations on the flat. The proposed formulation is verified for five different materials sets.
2018-01-01T00:00:00Z