Browsing by Author "Akin, John E."

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    Evaluation of finite element modeling methods for predicting compression screw failure in a custom pelvic implant
    (Frontiers Media S.A., 2024) Zhu, Yuhui; Babazadeh-Naseri, Ata; Brake, Matthew R. W.; Akin, John E.; Li, Geng; Lewis, Valerae O.; Fregly, Benjamin J.
    Introduction: Three-dimensional (3D)-printed custom pelvic implants have become a clinically viable option for patients undergoing pelvic cancer surgery with resection of the hip joint. However, increased clinical utilization has also necessitated improved implant durability, especially with regard to the compression screws used to secure the implant to remaining pelvic bone. This study evaluated six different finite element (FE) screw modeling methods for predicting compression screw pullout and fatigue failure in a custom pelvic implant secured to bone using nine compression screws. Methods: Three modeling methods (tied constraints (TIE), bolt load with constant force (BL-CF), and bolt load with constant length (BL-CL)) generated screw axial forces using functionality built into Abaqus FE software; while the remaining three modeling methods (isotropic pseudo-thermal field (ISO), orthotropic pseudo-thermal field (ORT), and equal-and-opposite force field (FOR)) generated screw axial forces using iterative physics-based relationships that can be implemented in any FE software. The ability of all six modeling methods to match specified screw pretension forces and predict screw pullout and fatigue failure was evaluated using an FE model of a custom pelvic implant with total hip replacement. The applied hip contact forces in the FE model were estimated at two locations in a gait cycle. For each of the nine screws in the custom implant FE model, likelihood of screw pullout failure was predicted using maximum screw axial force, while likelihood of screw fatigue failure was predicted using maximum von Mises stress. Results: The three iterative physics-based modeling methods and the non-iterative Abaqus BL-CL method produced nearly identical predictions for likelihood of screw pullout and fatigue failure, while the other two built-in Abaqus modeling methods yielded vastly different predictions. However, the Abaqus BL-CL method required the least computation time, largely because an iterative process was not needed to induce specified screw pretension forces. Of the three iterative methods, FOR required the fewest iterations and thus the least computation time. Discussion: These findings suggest that the BL-CL screw modeling method is the best option when Abaqus is used for predicting screw pullout and fatigue failure in custom pelvis prostheses, while the iterative physics-based FOR method is the best option if FE software other than Abaqus is used.
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    Heterogeneous material mapping methods for patient-specific finite element models of pelvic trabecular bone: A convergence study
    (Elsevier, 2021) Babazadeh Naseri, Ata; Dunbar, Nicholas J.; Baines, Andrew J.; Akin, John E.; Higgs, C. Fred III; Fregly, Benjamin J.
    Patient-specific finite element (FE) models of bone require the assignment of heterogeneous material properties extracted from the subject's computed tomography (CT) images. Though node-based (NB) and element-based (EB) material mapping methods (MMMs) have been proposed, the sensitivity and convergence of FE models to MMM for varying mesh sizes are not well understood. In this work, CT-derived and synthetic bone material data were used to evaluate the effect of MMM on results from FE analyses. Pelvic trabecular bone data was extracted from CT images of six subjects, while synthetic data were created to resemble trabecular bone properties. The numerical convergence of FE bone models using different MMMs were evaluated for strain energy, von-Mises stress, and strain. NB and EB MMMs both demonstrated good convergence regarding total strain energy, with the EB method having a slight edge over the NB. However, at the local level (e.g., maximum stress and strain), FE results were sensitive to the field type, MMM, and the FE mesh size. The EB method exhibited superior performance in finer meshes relative to the voxel size. The NB method converged better than did the EB method for coarser meshes. These findings may lead to higher-fidelity patient-specific FE bone models.
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    Inclusion of Muscle Forces Affects Finite Element Prediction of Compression Screw Pullout but Not Fatigue Failure in a Custom Pelvic Implant
    (MDPI, 2024) Zhu, Yuhui; Babazadeh-Naseri, Ata; Brake, Matthew R. W.; Akin, John E.; Li, Geng; Lewis, Valerae O.; Fregly, Benjamin J.
    Custom implants used for pelvic reconstruction in pelvic sarcoma surgery face a high complication rate due to mechanical failures of fixation screws. Consequently, patient-specific finite element (FE) models have been employed to analyze custom pelvic implant durability. However, muscle forces have often been omitted from FE studies of the post-operative pelvis with a custom implant, despite the lack of evidence that this omission has minimal impact on predicted bone, implant, and fixation screw stress distributions. This study investigated the influence of muscle forces on FE predictions of fixation screw pullout and fatigue failure in a custom pelvic implant. Specifically, FE analyses were conducted using a patient-specific FE model loaded with seven sets of personalized muscle and hip joint contact force loading conditions estimated using a personalized neuromusculoskeletal (NMS) model. Predictions of fixation screw pullout and fatigue failure—quantified by simulated screw axial forces and von Mises stresses, respectively—were compared between analyses with and without personalized muscle forces. The study found that muscle forces had a considerable influence on predicted screw pullout but not fatigue failure. However, it remains unclear whether including or excluding muscle forces would yield more conservative predictions of screw failures. Furthermore, while the effect of muscle forces on predicted screw failures was location-dependent for cortical screws, no clear location dependency was observed for cancellous screws. These findings support the combined use of patient-specific FE and NMS models, including loading from muscle forces, when predicting screw pullout but not fatigue failure in custom pelvic implants.
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    Lattice-Boltzmann Modeling of Potential Fluid Flow Impairment Caused by Asphaltene Deposition in Porous Media
    (2018-11-30) Lin, Pei-Hsuan; Vargas, Francisco; Biswal, Sibani L.; Akin, John E.
    Asphaltene deposition in porous media has significant effects on oil flow during primary and secondary oil production. When asphaltenes deposit in porous media, the pore throats become plugged, leading to the impairment of permeability, and in turn causes several difficulties for oil production. In order to solve the problem of asphaltene deposition in porous media, it is important to understand the mechanism of asphaltene deposition in order to find measures to mitigate this problem. Moreover, a predictive model would greatly help control and reduce asphaltene deposition at the early stage. However, the mechanism of asphaltene deposition in porous media is still unclear. Only a few deposition models have been proposed and most of them have too many fitting parameters. Hence, the objective of this study is to propose a new model which has a fewer number of fitting parameters but still effectively predicts asphaltene deposition. Furthermore, the validity of the proposed model will be verified by microchannel and core flooding experiments. Instead of using the traditional Computational Fluid Dynamics (CFD) methods, such as the Finite Element Method, Finite Difference Method, and Finite Volume Method, the Lattice Boltzmann Method (LBM) is employed to model asphaltene deposition in porous media. The benefit of using LBM for simulating fluid flow in complex geometric settings is that it is comparatively easier for computational implementation and parallel computing due to being a meshless technique. Once the mechanism of asphaltene deposition in porous media has been understood clearly, mitigation methods can be then applied to prevent asphaltene deposition. Also, the simulation tool that is developed using LBM can help reduce the cost involved in experiments, which is extremely important in the petroleum industry.
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    Modeling and Analysis of an In-line Pump Jet Thruster for Swimming Robots
    (2015-04-24) Kirmizi, Mehmet; Ghorbel, Fathi Hassan; Akin, John E.; Dick, Andrew J
    Nowadays, underwater robots play a significant role in many aspects of industry such as mine hunting, sea floor mapping, sub-sea pipeline construction and pipe surveys. Ever increasing demand on better control systems of underwater robots ignites engineers on modeling the systems for perfection. Since they are the lowest chain of the control problem, yet the most important ones. Thrusters are the most crucial parts of the underwater robot actuation. The need for maneuvering precision, for small and fast dynamic positioning, for precise motion and position control and for station keeping made developments of thruster modeling inevitable. The fundamental difficulty in modeling of a thruster is that axial flow velocity is an unmeasurable state, yet the model depends on that variable. This work addresses this difficulty. It explores the dynamics of in-line pump jet thrusters which are used for underwater robotic actuation. This work uses impeller geometry characterization, fluid flow analysis through impeller blades and through pump jet, thrust modeling and DC motor modeling. The main contribution of this work is incorporation of axial flow velocity into thruster modeling in order to resolve the unmeasurable state problem, thanks to the dynamical properties of impeller geometry and flow conditions through impeller blades.
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    Tectonic and gravitational fold and thrust belts: Insights from discrete element simulations and Galicia ocean-continent transition zone: New seismic reflection constraints
    (2014-07-21) Dean, Sarah; Morgan, Julia K.; Dugan, Brandon; Sawyer, Dale; Anderson, John; Akin, John E.
    The evolution and formation of tectonically and gravitationally driven fold and thrust belts were investigated with 2D numerical simulations using the discrete element method (DEM). In the tectonic study, the occurrence of triangle zones at the front of thrust belts was investigated, specifically how mechanical stratigraphy affects their formation. Simulations with homogenous stratigraphy deformed predominantly along forethrusts. Adding a weak upper unit caused more forethrusts and popup structures in the upper unit relative to the lower unit. Thicker décollement surfaces and strong upper and lower units enhanced decoupling and formed triangle zones. Results compared favorably with triangle zones in Alberta. Simulations of gravitationally driven thrust belts consisted of an updip extensional zone, syntectonically loaded with sediments, which is connected at depth to a downdip contractional zone. The whole system overlies a mobile shale unit. Our simulations show more diffuse décollements connecting the normal faults in the extensional zone with toe thrusts in the contractional zone, then are interpreted on seismic profiles. We also look at the distributions of stress and strain within out simulations, relating the distributions of σ1 to the vergence of thrust faults. We compare out models to the Niger Delta type locale for shale tectonics. The West Iberia continental margin is a type locale for magma-poor rifting, and has been instrumental in changing the classical view of the ocean-continent transition (OCT) from a discrete boundary juxtaposing continental and oceanic crust, into a more complicated zone of varying width that can include exhumed mantle. This study examines two new seismic lines in the Galicia Bank area extending west of the Peridotite Ridge, showing high resolution images of five new ridges. These ridges could be hyperextended continental crust, exhumed continental mantle, or rough ultra-slow spreading oceanic crust. There are no tilted fault blocks with pre-syn rift stratigraphy that would indicate continental crust. There are also no faults indicating mid-ocean spreading with seismic layer stratigraphy indicating normal oceanic crust. Therefore, it is likely the western ridges are also made of serpentinized mantle, with a wide OCT similar to the Southern Iberia Abyssal Plain.