A finite element program post-processor is developed to help synthesize a near-optimum mesh. This post-processor displays, interactively, six different energy functions, along with the usual stress functions. The six energy functions are functions of the energy within each element of the mesh. Since it is intended to be an educational package, the post-processor does not automatically generate the new mesh for the user. Instead, guidelines for near-optimum mesh generation are presented. The guidelines are based on the test cases included in the study. These test cases were also used to select the combination of functions that seemed to be more desirable for optimization. A list of possible future work is included. / Master of Science

A simple, user friendly, mesh generator designed as a preprocessor for two-dimensional finite element codes is presented. The program generates both triangular and quadrilateral two-dimensional elements and has automatic transitioning capability. The required input is the definition of the boundary of the structure, a measure of the relative size of the mesh at the key geometry points, and loading information. Both boundary and interior nodal points are generated. The user is not required to develop the overall node and element plan and is not limited to quadrilateral regions with equal number of nodes on opposing sides. The generated mesh density as controlled by the user will automatically transition from low to high density areas. A bandwidth reduction routine is included to renumber the generated nodes most efficiently. An output file is created of node, element, and nodal load definitions which is organized for input to a finite element program. / Master of Science

A study of the bending stresses in a pair of meshed spur gears using the finite element method is presented. The models analyzed were in the shape of a circular gear with five teeth or a five-tooth rack. A unit torque (1 lbf-ft) was applied as the form of nodal forces on the nodes around the bore hole of the driver pinion. The nodes around the bore hole of the driven gear (or the nodes along the back of the driven rack) were fixed. In order to transmit the power from the driver pinion to the driven gear (or rack), the points in contact were made coincident. Seven model groups with same diametral pitch (1.0), addendum (1.0 in.), dedendum (1.3 in.), pressure angle (20°) and hob tip radius (0.35 in.) but with varying numbers of teeth on the pinion and gear were analyzed to compute the tensile stress variation in the root fillet during the duration of contact. A model for predicting the tensile stress variation at the root fillet during the duration of contact has been created. The results were compared with AGMA and other results with agreement for the peak within 3%. / M.S.

This thesis details several modifications to the mechanical design of the Orthotic Lower Limb Exoskeleton (OLL-E) that improve upon the functionality and manufacturability of parts and their assemblies. The changes made to these parts maintain or improve the factor of safety against yield and fatigue failure as compared to the original designs. Design changes are verified by FEA simulations and hand calculations. The changes included in this thesis also allowed parts that were previously difficult or impossible to manufacture using traditional methods to be made in house or outsourced to another machine shop. In addition to the mechanical design changes, this thesis also details the design and implementation of a six axis force-torque sensor built into the foot of OLL-E. The purpose of this sensor is to provide feedback to the central control system and allow OLL-E to be self-balancing. This foot sensor design is calibrated and initial results are discussed and shown to be favorable. / Master of Science / Recent developments in the fields of robotics and exoskeleton design have increased their feasibility for use in medical rehabilitation and mobility enhancement for persons with limited mobility. The Orthotic Lower Limb Exoskeleton (OLL-E) is an exoskeleton specifically designed for enhancing mobility by allowing users with lower limb disabilities such as spinal cord injuries or paraplegia to walk. The research detailed in this thesis explains the design and manufacturing processes used to make OLL-E as well as providing design details for a force sensor built into the exoskeleton foot. Before manufacturing could take place some parts needed to be redesigned and this thesis provides insight into the reasons for these changes. After the manufacturing and design process was completed the OLL-E was assembled and the project can now move forward with physical testing.

This study evaluates the seismically induced displacements of a slope using the Finite Element Method (FEM) in comparison to the results of twelve empirical predictive approaches. First, the existing methods to analyze the stability of slopes subjected to seismic loads are presented and their capabilities to predict the onset of failure and post-failure behavior are discussed. These methods include the pseudostatic method, the Newmark method, and stress-deformation numerical methods. Whereas the pseudostatic method defines a seismic coefficient for the analysis and provides a safety factor, the Newmark method incorporates a yield coefficient and the actual acceleration time history to estimate permanent displacements. Numerical methods incorporate advanced constitutive models to simulate the coupled stress-strain soil behavior, making the process computationally more costly. In this study, a model slope previously studied at laboratory scale is selected and scaled up to prototype dimensions. Then, the slope is subjected to 88 different input motions, and the seismic displacements obtained from the numerical and empirical approaches are compared statistically. From correlation analyses between seven ground motion parameters and the numerical results, new empirical predictive equations are developed for slope displacements. The results show that overall the FEM displacements are generally in agreement with the numerically developed methods by Fotopoulou and Pitilakis (2015) labelled "Method 2" and "Method 3", and the Newmark-type Makdisi and Seed (1978) and Bray and Travasarou (2007) methods for rigid slopes. Finally, functional forms for seismic slope displacement are proposed as a function of peak ground acceleration (PGA), Arias intensity (Ia), and yield acceleration ratio (Ay/PGA). These functions are expected to be valid for granular slopes such as earth dams, embankments, or landfills built on a rigid base and with low fundamental periods (Ts<0.2). / Master of Science / A landslide is a displacement on a sloped ground that can be triggered by earthquake shaking. Several authors have investigated the failure mechanisms that lead to landslide initiation and subsequent mass displacement and proposed methodologies to assess the stability of slopes subjected to seismic loads. The development of these methodologies has to rely on field data that in most of the cases are difficult to obtain because identifying the location of future earthquakes involves too many uncertainties to justify investments in field instrumentation (Kutter, 1995). Nevertheless, the use of scale models and numerical techniques have helped in the investigation of these geotechnical hazards and has led to development of equations that predict seismic displacements as function of different ground motion parameters. In this study, the capabilities and limitations of the most recognized approaches to assess seismic slope stability are reviewed and explained. In addition, a previous shaking-table model is used for reference and scaled up to realistic proportions to calculate its seismic displacement using different methods, including a Finite Element model in the commercial software Plaxis2D. These displacements are compared statistically and used to develop new predictive equations. This study is relevant to understand the capabilities of newer numerical approaches in comparison to classical empirical methods.

Since the last century a lot of effort has been spent trying to analyze damage and crack evolution in solids. This field is of interest because of the many applications that require the study of the behavior of materials at the micro- or nanoscale, i.e. modeling of composites and advanced aerospace applications. Peridynamics is a recently developed theory that substitutes the differential equations that constitute classical continuum mechanics with integral equations. Since integral equations are valid at discontinuities and cracks, peridynamics is able to model fracture and damage in a more natural way, without having to work around mathematical singularities present in the classical continuum mechanics theory. The objective of the present work is to show how peridynamics can be implemented in finite element analysis (FEA) using a mesh of one-dimensional truss elements instead of 2-D surface elements. The truss elements can be taken as a representation of the bonds between molecules or particles in the body and their strength is found according to the physical properties of the material. The possibility implementing peridynamics in a finite element framework, the most used method for structural analysis, is critical for expanding the range of problems that can be analyzed, simplifying the verification of the code and for making fracture analysis computationally cheaper. The creation of an in-house code allows for easier modifications, customization and enrichment if more complex cases (such as multiscale modeling of composites or piezoresistive materials) are to be analyzed. The problems discussed in the present thesis involve plates with holes and inclusions subjected to tension. Displacement boundary conditions are applied in all cases. The results show good agreement with theory as well as with empirical observation. Stress concentrations reflect the behavior of materials in real life, cracks spontaneously initiate and debonding naturally happens at the right locations. Several examples clearly show this behavior and prove that peridynamics is a promising tool for stress and fracture analysis. / Master of Science

The transient response of various plate structures subject to blast loads is analyzed. In particular, simply supported isotropic and laminated composite plates are modeled using the commercial finite element code NASTRAN and the method of modal superposition. Both analysis procedures are used to quantify the linear transient response of such plates subject to uniform and patch blast loads. Furthermore, NASTRAN is used to study the nonlinear response of plates subject to close proximity explosions. Also considered here is the case for which a blast loaded plate impacts another closely neighboring plate. The NASTRAN solution used here accounts for nonlinearities due to large plate deflections, plasticity, and plate-to-plate contact. Many studies are currently available in which the blast load is considered to be spatially uniform across the plate; with a temporal distribution described by step, N-pulse, or Friedlander equations. The novel aspect considered here is the case for which the blast pressure is due to a close proximity explosion, and it is therefore taken to be both spatially and temporally varying. A FORTRAN program is described which automates the application of an arbitrary blast load to a generic finite element mesh. The results presented here are a collection of analyses performed for a variety of parameters important to the dynamic response of blast loaded contacting plates. Conclusions are drawn concerning the influence of the various parameters on the nature of the plate response and the quality of the solution. / Master of Science

The research conducted was sponsored by JVI of Pittsfield, Massachusetts. JVI has developed a steel embedded connection, referred to as a shooter. The shooter is provided in capacities, a 40 kip capacity shooter and a 50 kip capacity shooter. This steel connection is embedded near the end of a double-tee prestressed girder. Double-tee prestressed girders are a primary component used in the construction of parking garages. Typical double-tee lengths are 60 to 75 feet; however, for this research 20 ft long segments were cast and tested with the shooter installed. This project had three main goals. The first goal was to develop a preliminary design for the reinforcement around the shooter and test the shooters' capacity in the laboratory to determine if the stated capacity was correct. Four different designs were created, two designs for the 40 kip capacity shooter and 2 designs for the 50 kip capacity shooter. Each design was placed in one stem of the double-tee and tested at the laboratory. Results from these tests indicate that that each specimen achieved the stated capacity. However, failure was not a connection failure but a shear bond failure. The second goal was to take the information gathered from testing and develop a design model that could be used for other situations for this type of connection. The design model was created according to the guidelines in the ACI 318-08 code. Two different methods were used, a strut-and-tie model and a modified ACI code approach. From these designs it was determined that the strut-and-tie model should be used for the design of these connections; however, more research needs to be done before using the modified ACI code approach. The final goal was to determine if finite element analysis could be used to determine if the load at which large cracks that immediately proceed failure occur could be predicted. From this analysis it was determined that a load range could be predicted in which a crack could form as well as a range of what the transfer length of the strands could be. / Master of Science

The majority of the physical properties of tissue depend directly on the interstitial or intracellular concentration of water within the epidermal and dermal layers. The relationship between skin constituent concentrations, such as water and protein, and the mechanical and optical properties of human skin is important to understand its complex nature. Localized mechanical loading has been proven to alter optical properties of tissue, but the mechanisms by which it is accomplished have not been studied in depth. In this thesis, skin's complex nature is investigated experimentally and computationally to give us better insight on how localized mechanical loading changes tissues water content and its optical properties. Load-based compression and subsequent increased optical power transmission through tissue is accomplished to explore a relationship between localized mechanical loading and tissue optical and mechanical properties. Using Optical Coherence Tomography (OCT), modification of optical properties, such as refractive index, are observed to deduce water concentration changes in tissue due to mechanical compression. A computational finite element model is developed to correlate applied mechanical force to tissue strain and water transport. Comprehensive understanding of the underlying physical principles governing the optical property changes within skin due to water concentration variation will enable future development of applications in the engineered tissue optics field. / Master of Science

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