Structures and Materials Commons^™

The Role Of Slip System Activation On Ni-Based Superalloy High Temperature Fatigue, Alexandru Lopazan

Doctoral Dissertations and Master's Theses

Nickel-based superalloys are used extensively in the aviation industry, particularly in aircraft engine components. These materials are desirable due to their high strength, resistance to creep, corrosive environment, and high temperature. It has been well documented that material fatigue is characterized by material microstructure, type of loading, and other parameters. Currently, research into the effects of fatigue on complex materials like nickel-based superalloys is being pursued. Previous research has found that preconditioning of Inconel 718 samples at 700 ℃ with 1.0% strain led to an improved fatigue life. It has been suggested that this could be caused by the activation …

Characterization Of Bistability Behavior In Unsymmetric Cfrp Laminates, Evan Medora

Doctoral Dissertations and Master's Theses

Composite laminates of unsymmetric layup families often exhibit twin stable shapes at room temperature, a phenomenon known as bistability. Unsymmetric bistable composite laminates have often been suggested for applications in aerospace morphing structures, damping, and energy storage, because of their ability to easily change their shape while being homogenized with the rest of the composite structure.

This study analyzes the mechanisms responsible for bistability in unsymmetrical [0/0/90/90] laminates through a combination of experiments and modeling. Several unsymmetrical laminates with different aspect ratios were fabricated using autoclave processing. Digital image correlation (DIC) is used to monitor laminate strains in situ during …

Manufacturability And Analysis Of Topologically Optimized Continuous Fiber Reinforced Composites, Jesus A. Ferrand

Doctoral Dissertations and Master's Theses

Researchers are unlocking the potential of Continuous Fiber Reinforced Composites for producing components with greater strength-to-weight ratios than state of the art metal alloys and unidirectional composites. The key is the emerging technology of topology optimization and advances in additive manufacturing. Topology optimization can fine tune component geometry and fiber placement all while satisfying stress constraints. However, the technology cannot yet robustly guarantee manufacturability. For this reason, substantial post-processing of an optimized design consisting of manual fiber replacement and subsequent Finite Element Analysis (FEA) is still required.

To automate this post-processing in two dimensions, two (2) algorithms were developed. The …

Passive, Self-Healable, Dielectric Elastomers For Structural Health Monitoring, Nicholas Smith

Doctoral Dissertations and Master's Theses

NASA maintains the ability to track a large majority of objects in Earth’s orbit, however lack the ability to track objects smaller than five centimeters in diameter. These untrackable objects represent a significant danger to inflatable structures. This work seeks to synthesize and fabricate a self-healable, passive, dielectric elastomer impact sensor for structural health monitoring on inflatable space structures subject to impact by micrometeoroids and orbital debris. In a setting in which impact repairs can be extremely costly, the implementation of such a technology would not only alert personnel of such an event but would also serve to decrease the …

Synthetic Gecko Inspired Dry Adhesive Through Two-Photon Polymerization For Space Applications, Zefu Ren

Doctoral Dissertations and Master's Theses

This work aims to develop an advanced and cost-effective fabrication process to produce a simplified gecko-inspired microstructure with two-photon polymerization and polymer molding, aimed to improve the adhesive properties of microstructures. Such adhesive microstructures can be implemented for multi-purpose adhesive grasping devices, which have recently gained significant interest in the space exploration sector. Previous gecko-inspired microstructures were reviewed, and the new gecko-inspired microstructures have been developed with the adaptation of additive manufacturing methods for facile fabrication. The examined microstructures in this thesis were the tilted mushroom-shaped and wedge-shaped designs, which could both maximize adhesion by shearing the micropillars toward the …

In-Situ Monitoring Of Additive Manufacturing Using Digital Image Correlation, Martina Jani

Doctoral Dissertations and Master's Theses

Advances in additive manufacturing over the past decade have led to applications in multiple fields. In particular, additive manufacturing answers the need for high flexibility in designing complex structures, however, the rapid thermal fluctuations during processing lead to numerous defects and processing-induced residual stresses. In this research, an in-situ approach using Digital Image Correlation (DIC) is developed to monitor the deformation during the processing, and thereby analyze the formation of defects and residual stresses. DIC is a visual technique that compares the changes in speckle pattern in consecutive images of a specimen to determine the displacements and strains. This research …

Kinematics Of Inter-Ply Interfaces In Composite Manufacturing, Sandeep Chava

Doctoral Dissertations and Master's Theses

The ubiquitous usage of polymer matrix composites in many applications demands a comprehensive understanding of composite interfaces and processing induced residual stresses, which critically affect both the manufacturing processes and the deformation mechanisms. Processing induced residual stresses are often responsible for causing warping, delamination, and dimensional instability in composite structures. This research includes new in-situ experimental approaches, cure cycle design, interface modification, and thermomechanical modeling for developing a fundamental understanding of the cure kinetics during composite manufacturing. Strains, ply-movement, and formation of defects are observed in-situ using digital image correlation (DIC) during the autoclave cure cycle for the first time. …

Fabrication & Mechanical Properties Of Hastelloy-X Matrix Ceramic Composite, Sultan Stewart

Doctoral Dissertations and Master's Theses

In the aerospace industry, there is a desire for increased thermal and mechanical properties of sealing materials. In this thesis, porous Hastelloy-X alloy scaffolds with 80% porosity, currently used for fabricating seals in turbine applications, were used as the base material. The microstructure of the porous Hastelloy-X scaffolds was infiltrated using a room temperature curing silicate colloid (trade name -Dichtol HTWG Hydro #2506) and a novel zirconia based colloid to fabricate the metal matrix composites. Various fabrication processes, some using vacuum and pressure to assist the flow were utilized to infiltrate the scaffolds. The green composites were then cured at …

The Effect Of Part Curvature On The Formation Of Residual Stresses In Carbon Fiber Prepreg Laminates, Bradley Rogan

Doctoral Dissertations and Master's Theses

The formation of residual stresses during the processing of composite parts can lead to undesired defects such as delamination which can reduce the overall strength of a completed component. Furthermore, the addition of curvature to the part geometry tends to result in wrinkle formation which leads to stress concentrations developing throughout the curing process in the areas near these wrinkles. In this work, Digital Image Correlation (DIC) will be used to monitor the development of stresses throughout the cure cycle of carbon fiber prepreg in several different lay-ups and across tool plates with varying curvatures to determine what effect the …

Shape Optimization Of A Sandwich Composite Plate With Low-Velocity Impact Loading And Effect Of Finite Difference Sensitivity Analysis, Utsav Shah

Doctoral Dissertations and Master's Theses

The structural design of spacesuits is essential in an advancing future of both Lunar and Martian space exploration. A typical spacesuit is made of sandwich composite material and designed to withstand various pressure and loading conditions while also considering the safety and comfort of the astronauts. One of the critical load cases in spacesuit design is a low-velocity impact (LVI) which may occur due to tool drop and other similar scenarios. The objectives of this work were (a) to create a finite element (FE) model of a component of a spacesuit, (b) validation of the FE model through experiments, (c) …

Design, Analysis And Experimental Evaluation Of 3d Printed Variable Stiffness Structures, Rossana R. Fernandes

Doctoral Dissertations and Master's Theses

The rapid progress of additive manufacturing (AM) introduces new opportunities but also new challenges for design and optimization to ensure manufacturability, testability and accurate representation/prediction of the models. The present dissertation builds a bridge between design, optimization, AM, testing and simulation of advanced optimized variable-stiffness structures. The first part offers an insight on the mechanical, viscoelastic and failure characteristics of AM continuous fiber composites. This understanding was used in the second part to investigate the feasibility of different topology and fiber-orientation optimization methods and the manufacturability of the resulting models. The study also assesses the effects of the manufacturing constraints …

Additively Manufactured Dielectric Elastomer Actuators: Development And Performance Enhancement, Stanislav Sikulskyi

Doctoral Dissertations and Master's Theses

The recently emerging and actively growing areas of soft robotics and morphing structures promise endless opportunities in a wide range of engineering fields, including biomedical, industrial, and aerospace. Soft actuators and sensors are essential components of any soft robot or morphing structure. Among the utilized materials, dielectric elastomers (DEs) are intrinsically compliant, high energy density polymers with fast and reversible electromechanical response. Additionally, the electrically driven work principle allows DEs to be distributed in a desired fashion and function locally with minimum interference. Thus, a great effort is being made towards utilizing additive manufacturing (AM) technologies to fully realize the …

Effect Of Galvanic Corrosion On Crack Propagation Of Cold Expanded Holes In Aa2024-T3, Ken Shishino

Doctoral Dissertations and Master's Theses

A search for the balance between weight reduction and durability of a structure has always been a popular topic within the aerospace industry. Among numerous researches to improve a structure’s fatigue life, processes on mechanically fastened joints have been a topic of focus as it is naturally a discontinuity in the material leading to a local stress concentration under loading. Cold expansion is a well-known technique with a wide range of applications in both new and repaired structures. The benefits include the extension of fatigue life and a delay in crack growth without addition of weight. However, localized galvanic corrosion …

Investigation Of Bio-Inspired Pin Geometries For Heat Transfer Applications, Anish Prasad

Doctoral Dissertations and Master's Theses

Array of circular cylindrical pins or tubes are one of the most widely used type of convection cooling systems, profoundly used in the internal cooling of gas turbine blades. They promote heat transfer due to flow acceleration, secondary flows and wake shedding, at the expense of large pressure loss and unsteadiness in the flow. The need to reduce pressure loss and maintain the heat transfer rates are a much needed requirement for a variety of industries to improve the cooling efficiency. One such prominent line of research is conducted on optimizing the design of the circular cylindrical pins to increase …

Analysis Of After Impact Characteristics And Structural Optimization Of Cfrp Composite Plate, Zhennan He

Doctoral Dissertations and Master's Theses

Low-velocity impact (LVI) on composites may cause Barely Visible Impact Damage (BVID), which is one of the most common damage types and may result in a decrease in strength of the composite. The objectives of this research were (a) to identify the characteristics of a carbon/epoxy composite plate after low-velocity impact, (b) to create a validated model to simulate the impact process and progressive failure, and (c) to perform sizing and shape optimization of the laminate for improving the damage resistance of the plate. As a part of this thesis, composite plates were fabricated using carbon fiber reinforced polymer (CFRP) …

Parametric Analysis Of Particle Spreading With Discrete Element Method, Yuxuan Wu

Doctoral Dissertations and Master's Theses

The spreading of metallic powder on the printing platform is vital in most additive manufacturing methods, including direct laser sintering. Several processing parameters such as particle size, inter-particle friction, blade speed, and blade gap size affect the spreading process and, therefore, the final product quality. The objective of this study is to parametrically analyze the particle flow behavior and the effect of the aforementioned parameters on the spreading process using the discrete element method (DEM). To effectively address the vast parameter space within computational constraints, novel parameter sweep algorithms based on low discrepancy sequence (LDS) are utilized in conjunction with …

Experimental Analysis Of Heat Generation In Carbon, Mounisha Ganesan

Doctoral Dissertations and Master's Theses

The purpose of this research is to study a novel non-covalent functionalization technique of single-walled carbon nanotubes (SWCNTs) and to examine the feasibility of using carbon nanotube films as thin film heaters. We developed a unique methodology to synthesize a well dispersed, environmentally friendly solution of CNTs that could be easily spin coated as well as spray coated on different substrates based on the application. Using the produced solutions, thin films of SWCNTs were fabricated on a glass substrate. Two sets of experiments were conducted to study the influence of polyaromatic moiety and the pH on the electro thermal characteristics …

Effect Of Cold Work Expansion On The Fatigue Life Of Pre-Cracked Al 2024-T3, Christopher Leirer

Doctoral Dissertations and Master's Theses

No abstract provided.

Piezoresistive Hybrid Nanocomposites For Strain And Damage Sensing: Experimental And Numerical Analysis, Audrey Jean-Miche Gbaguidi

Doctoral Dissertations and Master's Theses

Carbon nanomaterials such as carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) demonstrate remarkable electrical and mechanical properties, which suggest promising structural and functional applications as fillers for polymer nanocomposites.

The piezoresistive behavior of these nanocomposites makes them ideal for sensing applications. Besides, hybrid nanocomposites with multiple fillers like carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) are known to exhibit improved electrical and mechanical performance when compared to mono-filler composites.

To comprehensively understand the mechanisms of electrical percolation, conductivity, and piezoresistivity in hybrid nanocomposites, the author develops a two-dimensional (2D) and a three-dimensional (3D) computational Monte Carlo percolation network models for …

Topology Optimization And Analysis Of Thermal And Mechanical Metamaterials, Lee Alacoque

Doctoral Dissertations and Master's Theses

To take advantage of multi-material additive manufacturing technology using mixtures of metal alloys, a topology optimization framework is developed to synthesize high-strength spatially periodic metamaterials possessing unique thermoelastic properties. A thermal and mechanical stress analysis formulation based on homogenization theory is developed and is used in a regional scaled aggregation stress constraint method, and a method of worst-case stress minimization is also included to efficiently address load uncertainty. It is shown that the two stress-based techniques lead to thermal expansion properties that are highly sensitive to small changes in material distribution and composition. To resolve this issue, a uniform manufacturing …

Analysis Of Local Convection Heat Transfer Rates Of Supercritical Carbon Dioxide In Tubes, Yang Chao

Doctoral Dissertations and Master's Theses

Numerical investigation on thermodynamic characteristics of pure supercritical carbon dioxide (𝑆𝐶𝑂2) and mixtures of Helium and 𝑆𝐶𝑂2 in horizontal straight tubes with diameter of 6mm under cooling and heating conditions is carried out by using SST 𝑘 − 𝜔 turbulence model. The computations are conducted at operating pressure of 8MPa with various operating conditions. Simulations are conducted by using the latest poly-hexcore mesh method in ANSYS FLUENT 19.2 and tetrahedral mesh method in CFX 19.2, results and solving time are compared. The effect of heat flux, mass flux, inlet temperature and buoyancy on heat transfer performance are studied, pressure drop …

Viscoplastic Study Of 3d Printed Carbon Fiber Composites, Alexander M. Seifans

Doctoral Dissertations and Master's Theses

The research presented here involves an attempt to model the viscoplastic response of 3D printed carbon fiber composites made using the Mark Two 3D printer by Markforged. The ability to manufacture composites through the use of 3D printers is relatively new and contains gaps in the various mechanical properties of the material, of interest is the viscoplastic response.

Due to the fabrication method, the properties of the composites cannot be assumed to be similar to that of conventionally made composites as the fused deposition in printing alters the material properties. In order to understand the effects of the printed fiber …

Shape Optimization Of Microfiber Composite Energy Harvesters, Suyash Lahoti

Doctoral Dissertations and Master's Theses

A model of energy harvesting beam with a piezoelectric material, microfiber composite (MFC), in a unimorph configuration was setup in Matlab using the governing equations of motions of a coupled electromechanical system. The equations of motion were derived using Hamilton’s variational principles and constitutive relations of a piezoelectric material. The mathematical model developed in Matlab was validated with an experiment and frequency response functions. The validated model was used to perform shape optimization so as to obtain the shape of the beam and the patch that harvests the largest voltage. The shape variables were length of the beam (LB), length …

In-Situ Measurement Of Resin Shrinkage For Aerospace Components, Samarth Motagi

Doctoral Dissertations and Master's Theses

Resin shrinkage due to chemical changes and thermal gradients plays an important role in the residual stress evolution during composite manufacturing. In this thesis, we develop a new in-situ experimental approach for measuring resin shrinkage during curing with the help of digital image correlation (DIC) instrumented on a specially designed autoclave with borosilicate viewports. The images of the resin sample are captured at regular intervals with the help of DIC to measure the displacement and strains during the curing process. We utilize this method for a comparative analysis of resin shrinkage in two common aerospace epoxies, Epoxy 105 and EPON …

Hybrid Structural Composites With Energy Harvesting Capabilities, Simon Furnes

Doctoral Dissertations and Master's Theses

Hybrid materials have received significant interest due to the potential enhancements they provide over traditional materials such as sensing, actuating, energy scavenging, thermal management, and vibration damping. While traditional materials can be utilized for either one of these functions or loadbearing, the hybrid materials are superior as they allow combination of a wide array of functionalities whilst being suitable for load-bearing purposes.

The goal of this thesis is to elucidate the synergistic effects of hybridization of two piezoelectric materials; zinc oxide nanowires (ZnO NWs) and thin film of lead zirconium titanate (PZT) on the mechanical and energy harvesting of beams …

Thermal Bending, Buckling And Post-Buckling Analysis Of Unsymmetrically Laminated Composite Beams With The Effects Of Moisture And Geometric Imperfections, Claudia Barreno

Doctoral Dissertations and Master's Theses

This thesis is concerned with the analytical study of the thermal bending, buckling, and post-buckling of unsymmetrically laminated composite beams with imperfection under hygrothermal effects. Three different boundary conditions will be considered on this study. The non-linear governing partial differential equations are derived by taking into account the von-Karman geometrical nonlinearity for an imperfect unsymmetrical laminated composite beam. Classical beam theory (CBT) as well as first order shear deformation theory (FSDT) will be used. The effects of temperature, angle of orientation, moisture variations, imperfection, and geometrical parameters, will be evaluated and discussed. Two different laminated composite laminates will be considered: …

Hypervelocity Impact Analysis Of Hybrid Nanocomposite Sensors For Inflatable Space Structures, Yachna Gola

Doctoral Dissertations and Master's Theses

Future space exploration requires easy-to-transport, and easy-to-build and deploy space habitats. NASA and Bigelow Aerospace have collaborated so that human habitation can be made safe and easy with inflatable space habitats (Litteken, 2017). One of the biggest threats faced by these structures in outer space is impact damage by micrometeoroid orbital debris (MMOD) traveling at velocities as high as 15 km/s (Lemmens, Krag, Rosebrock, & Carnelli, 2013). This work presents fabrication and testing of hybrid nanocomposites with carbon nanotubes (CNT) and coarse graphene nanoplatelets (GNP) as fillers and flexible epoxy matrix, that are proposed to be used for sensing the …

Non-Linear Bending Analysis Of Functionally Graded Beams With Spring Constraints And Thermal Effects, Swetha Suresh

Doctoral Dissertations and Master's Theses

No abstract provided.

Shape-Stabilization Of Pcm/Hdpe Composites With 3d-Printing Applications For Battery Thermal Management, Thomas Benjamin Freeman

Doctoral Dissertations and Master's Theses

Phase-change materials (PCMs) are a useful alternative to more traditional methods of thermal management of Li-ion batteries in electric or hybrid-electric vehicles. PCMs are materials which absorb large amounts of latent heat and undergo solid-to-liquid phase change at near-constant temperature. The goal of this thesis is to experimentally investigate the thermal properties of a novel shape-stabilized PCM/HDPE composite extruded filament. The extruded filament can then be used in a 3D printer for custom PCM/HDPE shapes. These custom PCM/HDPE shapes can be used to help reduce weight, energy consumption, and complexity of the thermal management of the ESS. The PCM used …

A Novel Magnetorheological Fluid Damper For A Small Spacecraft With Flexible Appendages, Robert Waelchli

Doctoral Dissertations and Master's Theses

Small satellites have become increasingly popular over the past thirty years, particularly since the adoption of the common CubeSat architecture early this century. Because of their restricted volume and electrical budgets however, there are practical limits to the missions that small satellites may adopt. One potential near-term solution to the problem of limited electrical power may be the adoption of larger, flexible solar arrays. However, spacecraft with flexible appendages have historically presented attitude control challenges relating to platform stability given the dynamic response of the flexible components to applied torques. These challenges may be particularly disruptive to a small spacecraft …