Structural adhesives are materials that are capable of bearing significant loads in shear, and sometimes tension, over a range of strains and strain rates. Adhesively bonded structures can dissipate large amounts of mechanical energy and can be lighter and more efficient than many bolted or vibration welded parts. The largest barrier to using structural adhesives in more applications is the many challenges engineers are presented with when designing and analyzing adhesively bonded structures. This study develops, characterizes and compares several material models for use in finite element analysis of adhesively bonded structures, in general, and a bonded tongue and groove (TNG) joint in particular. The results indicate that it is possible to develop a general material model for ductile adhesives used in structural applications under quasi-static conditions. Furthermore, the results also show that it is also possible to take bulk material data and apply it to an adhesively bonded specimen provided that the mode of failure of the bulk test specimen closely approximates the mode of failure of the bonded joint. / Master of Science

Modeling and simulation have played an essential role in understanding the effects of blast waves. However, a broad area of engineering problems, such as vehicle structures, buildings, bridges, or even the human body, can benefit by accurately predicting the response to blasts with little need for test or event data. This thesis reviews fundamental concepts of blast waves and explosives and discusses research in blast scaling. Blast scaling is a method that reduces the computational costs associated with modeling blasts by using empirical data and numerically calculating blast field parameters over time for various types and sizes of explosives. This computational efficiency is critical in studying blast waves' near and far-field effects. This thesis also reviews research to differentiate between free-air blasts and gun muzzle blasts and the progress of modeling the muzzle blast-structure interaction. The main focus of this thesis covers an investigation of different numerical and analytical solutions to a simple aerospace structure subjected to blast pressure. The thesis finally presents a tool that creates finite element loads utilizing muzzle blast scaling methods. This tool reduces modeling complexity and the need for multiple domains such as coupled computational fluid dynamics and finite element models by coupling blast scaling methods to a finite element model. / Master of Science / {Numerical integration methods have helped solve many complex problems in engineering and science due to their ability to solve non-linear equations that describe many phenomena. These methods are beneficial because of how well they lend to programming into a computer, and their impact has grown with the increases in computing power. In this thesis, ``modeling and simulation" refers to the characterization and prediction of an event's outcome through the use of computers and numerical techniques. Modeling and simulation play important roles in studying the effects of blast waves in many areas of engineering research such as aerospace, biomedical, naval, and civil. Their capability to predict the outcome of the interaction of a blast wave to vehicle structures, buildings, bridges, or even the human body while requiring limited experimental data has the chance to benefit a wide area of engineering problems. This thesis reviews fundamental concepts of blast waves, explosives, and research that has applied blast loading in modeling and simulation. This thesis describes the complexity of modeling an axially symmetric blast wave interaction by comparing the numerical and theoretical response blast loaded structure.

Deployable structures are used for many different spacecraft applications like solar arrays, antennas, and booms. They allow spacecraft with large structural components to comply with the volume restrictions of launch platforms. This research optimizes the shape and size of these structural components with both the stowed and deployed configurations in mind. HEEDS, a commercial optimization software, and ABAQUS, a commercial finite element analysis software, are used to evaluate and alter the structure using a single simulation. This makes the design process more efficient than running many different simulations individually. The optimization objectives, design variables, and constraints are chosen to fit the mission requirements of the structure. The structure analyzed in this research is a composite tube with a compressible cross-section wrapped around a cylinder. The change in cross-section reduces the bending stiffness of the tube and allows it to be wrapped without damaging the material. The dimensions controlling cross-section shape and the thickness of the composite layers are the design variables for the optimization. The maximum strain energy stored in the wrapped tube, the minimum volume of the structure, and the minimum weight of the tube are the objectives for the optimization. The strain energy is maximized to get the stiffest possible structure and satisfy the minimum natural frequency constraint. The weight and volume of the tube are minimized because reducing weight and volume is important for any spacecraft structure. Constraints are placed on the design variables and objectives and the Hashin damage criteria are used to ensure wrapping does not cause material failure. Three optimization runs from different initial designs are completed using SHERPA and genetic algorithm optimization methods. The results are compared to determine which optimization method performs best and how the different starting points affect the final results. After the optimized design is found, the full wrapping and deployment simulation is completed to analyze the behavior of the optimized design. / Master of Science / Spacecraft are launched into space using launch vehicles. There is limited room inside the launch vehicle for the spacecraft, but the spacecraft often needs large components like solar panels, antennas, and booms to complete the mission. These components must be designed in a way that allows them to be stowed in a smaller space. This can be accomplished by designing a system that can change the configuration of the component once the spacecraft is in orbit. This is referred to as a deployable structure, and the objective of this research is to create an optimization method for designing this type of structure. This is challenging because both the stowed and deployed configurations must be considered during the optimization. HEEDS, a commercial optimization software, and ABAQUS, a commercial structural analysis software, are used to evaluate and optimize the structure in a single simulation. The optimization objectives, design variables, and constraints are chosen to fit the mission requirements of the structure. The structure examined in this research is a composite tube with a compressible cross-section wrapped around a cylinder. As the tube is wrapped, it flattens, reducing the bending stiffness so the tube can be wrapped without damaging the material. The variables controlling cross-section shape and the thickness of the composite material layers will be altered during the optimization. The maximum strain energy stored in the wrapped tube, the volume of the tube, and the minimum weight of the tube are the objectives for the optimization. The strain energy is maximized to get the stiffest possible tube when it is unwrapped to ensure there is enough stored energy to facilitate the full deployment and to satisfy the minimum natural frequency constraint. The weight and volume of the tube are minimized because reducing weight and volume is important for any spacecraft structure. Constraints are placed on the design variables and objectives and the Hashin damage criteria are used to ensure wrapping does not cause material failure. The Hashin damage criteria use the strength of the material and the stresses on the material to determine if it is likely to fail. Three optimization runs with different starting points are completed for both the SHERPA and genetic algorithm optimization methods. The results are compared to determine which optimization method performs best and how the different starting points affect the final results. After the optimized design is found, the full wrapping and deployment simulation is completed to analyze the behavior of the optimized design.

A new particle-in-cell algorithm was developed for plasma simulations involving complex boundary conditions. The new algorithm is based on the three-dimensional immersed finite element method which is developed in this thesis, and a modified legacy particle-in-cell code. The model also applies a new meshing technique that separates the field solution mesh from the particle pushing mesh in order to increase the computational eciency of the model. The new simulation model is used in two applications of great importance to the development of ion propulsion technology: the ion optics performance and the interaction between spacecraft and the ion thruster. The first application is ion optics simulations. Simulations are performed to investigate ion optics plasma flow for a whole subscale NEXT ion optics. The operating conditions modeled cover the entire cross-over to perveance limit range. The results of the ion optics simulations demonstrated good agreement with the available experimental data. The second application is ion thruster plume simulations. Simulations are performed to investigate ion thruster plume - spacecraft interactions for the Dawn spacecraft. Plume induced contaminations on the solar array are studied for a variety of ion thruster configurations including multiple thruster firings. / Ph. D.

In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows pressurized, non-circular fuselage designs to be implemented, enabling the feasibility of Hybrid Wing Body (HWB) aircraft. In addition to its unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted at the coupon level. Therefore, a computational methodology is progressively developed based on physically realistic concepts without the use of tuning parameters. A thorough verification study is performed to identify the most effective approach to model PRSEUS, including the effect of element type, boundary conditions, bonding properties, and model fidelity. Using the results of this baseline study, a high fidelity stringer model is created at the component scale and validated against the existing experiments. Finally, the validated model is extended to larger scales to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is demonstrated to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available predictive tools, the methodology developed herein can accurately predict local behavior up to and beyond failure for stitched structures such as PRSEUS for the first time. Additionally, by extending the methodology to a large scale fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development. / PHD / In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows a new type of fuselage design to be implemented, known as Hybrid Wing Body (HWB) aircraft. PRSEUS unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted for small specimens. Therefore, a computational predictive methodology is developed to accurately model the response of PRSEUS. A thorough analysis is performed to identify what needs to be considered in the model to predict an accurate result while remaining computationally efficient. From the baseline analysis, realistic models of the PRSEUS structure are created numerically and validated against the existing experiments. Finally, the validated approach is extended to panel and a fuselage section to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is shown to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available softwares, the methodology developed herein can accurately predict local behavior up to and beyond failure for structures such as PRSEUS for the first time. Additionally, by applying the methodology to a fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development.

Computational fluid dynamics and finite element analysis were employed to study vestibular hair cell bundle mechanics under physiologic stimulus conditions. CFD was performed using ANSYS CFX and FEA utilized a custom MATLAB model. Nine varieties of hair cell bundles were modeled using tip-forcing only (commonly used experimentally), fluid-flow only (physiologic for free-standing bundles), and combined loading (physiologic for bundles with tip attachments) conditions to determine how the bundles behaved in each case. The bundles differed in the heights of their components, their length and width, and their number of steriocilia. Tip links were modeled to determine ion-channel opening behavior. Results show that positive pressures, negative pressures, and shear stresses on the exterior of the bundles are of comparable magnitude. Under combined loading, some bundles experienced very high suction pressures on their interior. The bundles with tall steriocilia are hindered by the endolymph while those with short steriocilia and much taller kinocilia are assisted by the fluid flow. Each bundle type has a different range over which it is most sensitive so that the bundles cumulatively cover a very large range of stimuli; the order in which bundles respond from smallest stimulus magnitude to largest is free-standing extrastriolar bundles, attached striolar bundles, attached extrastriolar bundles, and free-standing extrastriolar bundles. A short examination of off-axis loading shows that the prevailing theory suggesting that bundle response is proportional to the cosine of the angle between the stimulus direction and the bundle's direction of maximum excitation is incorrect. / Ph. D.

Each year roughly there are roughly 40,000 traffic-related fatalities. Common roadside objects such as trees, poles, guardrails, embankments, culverts, and fences result account for roughly 46% of these fatalities. Efforts to reduce to injury risk and risk exposure in these crashes have been hampered by the difficulty in performing reconstructions. To address the need for accurate reconstructions in order to assess injury risk, a vehicle-specific stiffness database was added to the WinSmash reconstruction program. This single modification increased the average estimated delta-V by 8% and reduced error from 23% to 13% on average. A method to extend the WinSmash energy-based reconstruction approach to poles and trees that were damaged or broken was implemented to provide delta-V estimates for these crashes. The error of the pole and tree reconstruction component was roughly 44% but still represented a significant step forward for these crashes which previously could not be reconstructed. The use of strong-post w-beam guardrail along roadsides is the primary method by which exposure to risk is reduced. Efforts to model guardrails using finite element methods were hampered by the large number of unknowns and lack of knowledge about the sensitivity of the crash outcome to each variable. Through a parametric study the soil properties and rail mesh density were identified as the most significant influences in simulation outcome. This knowledge was applied to finite element models of damaged guardrail to identify when the damage compromises the guardrail ability to prevent risk exposure. Models of guardrail with rail deflection, missing posts, and missing blockouts identified rail deflection over 6 inches and any number of missing posts as hazardous conditions. The removal of a single blockout was found to be acceptable if not desirable. These findings have far-reaching implications. The enhanced WinSmash reconstruction program has been adopted by NASS/CDS to generate delta-V estimates used by researchers in all areas of transportation research. The identification of hazardous guardrail was of great interest to transportation agencies responsible for prioritizing and performing repairs of damaged guardrail. / Ph. D.

A comparison of rectangular finite elements based on a first order shear deformation plate theory and a refined higher order plate theory is presented. Special attention is given to the representation of transverse shear strain, the phenomenon of "shear locking", and the selection of the interpolating polynomial. Both C⁰ and C¹ continuity elements are represented; the elements range from: 3 or 5 DOF per node, and 12 - 27 DOF per element. Static and free vibration analysis of isotropic and laminated plates with thicknesses ranging from extremely thin to very thick are presented, along with a convergence study. The finite element results are compared with the exact plate theory solutions. Of the elements investigated, the modified refined higher order theory element exhibits the best overall behavior. / Master of Science

A plane frame element based on linear, elastic theory is developed and implemented into the MESS finite element program. Post-processed results include nodal displacements, end reactions, maximum tensile and average shear stress, and a deformed geometry plot. The element is tested for accuracy relative to simple beam theory and by comparison with results generated using another finite element program. In both cases agreement to within 6 significant figures was achieved. Because the intended use is educational, a survey of its benefit as a design aid in undergraduate instruction is included. These benefits are based on test cases from senior design class projects. Results generated using analysis techniques presently available are contrasted with those using the plane frame element. Students' work that was examined contained mistakes resulting from large amounts of hand calculations. Conversely, results generated using the finite element method proved to be easily obtained and to have a higher degree of accuracy. A recommendation for further improvements in program capability is provided at the end of the study. / Master of Science

An eight-node isoparametric hybrid stress element is developed for two dimensional plane stress and plane strain analyses. The assumed stresses are represented by 18 unknown parameters. An eight-node isoparametric displacement element is implemented in an existing finite element program [10]. Several example problems are solved to compare the results of the hybrid and the displacement elements. / Master of Science