This paper presents the methods used to fabricate carbon fiber tubes with different geometries that impact their critical failure modes. Two types of carbon fiber were used in the manufacturing process: seamless sleeve carbon fiber and stitched bonded sheet carbon fiber (PRI 2000-1-C). A manufacturing process for the tubes was developed for both geometries. Different epoxy systems were used for each fiber type. After curing, the surfaces of the tubes were inspected using flash thermography to characterize surface defects. The tube samples were placed in a three-point bending setup with an induced crack. The crack propagation was documented using a digital image correlation system. The process for finding the shape factors and energy release rate are presented. The fracture behavior of the tubes is compared to the data from the compact tension samples to develop damage tolerant design guidelines for tube type structures. Plate samples were prepared to compare the capacity to the demand of the circular hollow section samples. With the results of this study, design guidelines for damage tolerant structures are developed, which can be applied to many industries such as aviation, alternative energy production, and construction. This is crucial to the longevity and safety of structures and systems that are used daily in society.

Filament used in 3D printers can vary by size, color, and material. Most commonly thermoplastics are used for rapid prototyping by industry. Recycled filament has the potential to reduce cost and provide a more sustainable and energy efficient approach to 3D printing. This can be a viable option if recycled parts show comparable mechanical characteristics to non-recycled material. This report focuses on the development of a methodology to efficiently characterize recycled filament for application in industry. A crush sample in the shape of a hollow cube and a dog-bone shaped specimen will be created using a filament extruder and 3D printer. The crush sample will be broken and extruded to produce a recycled filament. The crush sample will undergo a varying number of recycles (i.e. breakings) per sample group to simulate mechanical degradation; 0, 1, 2, and 5 recycling loops. The samples will undergo micro mechanical (microscopy analysis) and macro mechanical (tensile) characterization.

This thesis document outlines the construction of a device for preparation of cylindrical ice-aluminum specimens. These specimens are for testing in a uniaxial load cell with the goal of determining properties of the ice-metal interface, as part of research into spray ice material properties and how such ice might be better removed from maritime vessels operating in sub-freezing temperatures. The design of the sample preparation device is outlined, justifications for design and component choices given and discussion of the design process and how problems which arose were tackled are included. Water is piped into the device through the freezers lid and sprayed by a full cone misting nozzle onto an aluminum sample rod. The sample rod is supported with Ultra High Molecular Weight Polyethylene pillars which allow for free rotation. A motor, timing belt and pulley assembly is used to rotate this metal rod at 1.25 RPM. The final device produces samples though intermittent flow in a 5 minutes on, 20 minutes off cycle. This intermittent flow is controlled through the use of a solenoid valve which is wired into the compressor. When the thermostat detects that the freezer is too warm, the compressor kicks on and the flow of water is stopped. Additional modifications to the freezer unit include the addition of a fan to cool the compressor during device operation. Recommendations are provided towards the end of the thesis, including suggestions to change the device to allow for constant flow and that deionized water be used instead of tap water due to hard water concerns.

Carbon nanotube (CNT) membranes (buckypaper) are manufactured with multiple procedures, vacuum filtration, surfactant-free, and 3D printing. A post-manufacturing process for resin impregnation is subjected to the membranes. The effects of manufacturing processes on the microstructure and material properties are investigated for both pristine and resin saturated samples manufactured using all procedures. Microstructural characteristics that are studied include specific surface area, porosity, pore size distribution, density, and permeability. Scanning electron microscopy is used to characterize the morphology of the membrane. Brunauer-Emmett-Teller analysis is conducted on membrane samples to determine the specific surface area. Barrett-Joyner-Halenda analysis is conducted on membrane samples to determine pore characteristics. Once the microstructure is characterized for each manufacturing process for both pristine and resin saturated samples, material properties of the membrane and nanocomposite structures are explored and compared on a manufacturing basis as well as a microstructural basis. Membranes samples are interleaved in the overlap of carbon fiber polymer matrix composite tubes, which are subjected to fracture testing. The effects of carbon nanotube membrane manufacturing technology on the fracture properties of nanocomposite structures with tubular geometries are explored. In parallel, the influences of manufacturing technology on the electromechanical properties of the membrane that effect a piezoresistive response are investigated for both pristine and resin saturated membranes manufactured using both methods. The result of this study is a better understanding of the relationships between manufacturing technology and the effected microstructure, and the resulting influences on material properties for both CNT membranes and derivative nanocomposite structures. Developing an understanding of these multiscale relationships leads to an increased capacity in designing manufacturing processes specific to optimizing the expression of desired characteristics for any given application.

This paper presents the methods and materials used to investigate the fatigue fracture properties of i) seamless twill weave carbon fiber and ii) stitch bonded biaxial carbon fiber polymer matrix composite. Additionally, the effect of notch tip placement relative to longitudinal fiber toes is investigated. The process for observing and characterizing fatigue crack damage propagation is presented. The fatigue fracture behavior is compared with data acquired from compact tension samples subjected to static tension tests in order to develop damage tolerant design guidelines for tube structures under fatigue loading.

There is a concerted effort in developing robust systems health monitoring/management (SHM) technology as a means to reduce the life cycle costs, improve availability, extend life and minimize downtime of various platforms including aerospace and civil infrastructure. The implementation of a robust SHM system requires a collaborative effort in a variety of areas such as sensor development, damage detection and localization, physics based models, and prognosis models for residual useful life (RUL) estimation. Damage localization and prediction is further complicated by geometric, material, loading, and environmental variabilities. Therefore, it is essential to develop robust SHM methodologies by taking into account such uncertainties. In this research, damage localization and RUL estimation of two different physical systems are addressed: (i) fatigue crack propagation in metallic materials under complex multiaxial loading and (ii) temporal scour prediction near bridge piers. With little modifications, the methodologies developed can be applied to other systems.

Current practice in fatigue life prediction is based on either physics based modeling or data-driven methods, and is limited to predicting RUL for simple geometries under uniaxial loading conditions. In this research, crack initiation and propagation behavior under uniaxial and complex biaxial fatigue loading is addressed. The crack propagation behavior is studied by performing extensive material characterization and fatigue testing under in-plane biaxial loading, both in-phase and out-of-phase, with different biaxiality ratios. A hybrid prognosis model, which combines machine learning with physics based modeling, is developed to account for the uncertainties in crack propagation and fatigue life prediction due to variabilities in material microstructural characteristics, crack localization information and environmental changes. The methodology iteratively combines localization information with hybrid prognosis models using sequential Bayesian techniques. The results show significant improvements in the localization and prediction accuracy under varying temperature.

For civil infrastructure, especially bridges, pier scour is a major failure mechanism. Currently available techniques are developed from a design perspective and provide highly conservative scour estimates. In this research, a fully probabilistic scour prediction methodology is developed using machine learning to accurately predict scour in real-time under varying flow conditions.

The flexural behavior of epoxies was investigated by performing mechanical tests and applying statistical Weibull theory and analytical methods to the results. The effects of loading systems and environmental conditions were also considered. Three kinds of epoxies were studied: Epon E863, PRI 2002, and PR520. In total, 53 three-point-bending (3PB) Epon E863 samples and 26 3PB PR520 were tested immediately after curing, together with 26 four-point-bending (4PB) PRI2002 samples stored at 60°C and 90% Rh for 48 weeks. The Weibull parameters were estimated using both linear regression and the moments method. The statistical character of the Weibull model leads to uncertainty in the evaluated parameters, even for a large number of experiments. This study analyzed the ratio of flexural strength to tensile strength in bulk epoxy resin polymers. An analytical method previously developed by the authors to study the relationship between uniaxial tension/compression stress-strain curves and flexural load-deflection response was used to obtain the ratio. The results show that the Weibull model overpredicted the aforementioned ratio in different load arrangements.

Although high performance, light-weight composites are increasingly being used in applications ranging from aircraft, rotorcraft, weapon systems and ground vehicles, the assurance of structural reliability remains a critical issue. In composites, damage is absorbed through various fracture processes, including fiber failure, matrix cracking and delamination. An important element in achieving reliable composite systems is a strong capability of assessing and inspecting physical damage of critical structural components. Installation of a robust Structural Health Monitoring (SHM) system would be very valuable in detecting the onset of composite failure. A number of major issues still require serious attention in connection with the research and development aspects of sensor-integrated reliable SHM systems for composite structures. In particular, the sensitivity of currently available sensor systems does not allow detection of micro level damage; this limits the capability of data driven SHM systems. As a fundamental layer in SHM, modeling can provide in-depth information on material and structural behavior for sensing and detection, as well as data for learning algorithms. This dissertation focusses on the development of a multiscale analysis framework, which is used to detect various forms of damage in complex composite structures. A generalized method of cells based micromechanics analysis, as implemented in NASA's MAC/GMC code, is used for the micro-level analysis. First, a baseline study of MAC/GMC is performed to determine the governing failure theories that best capture the damage progression. The deficiencies associated with various layups and loading conditions are addressed. In most micromechanics analysis, a representative unit cell (RUC) with a common fiber packing arrangement is used. The effect of variation in this arrangement within the RUC has been studied and results indicate this variation influences the macro-scale effective material properties and failure stresses. The developed model has been used to simulate impact damage in a composite beam and an airfoil structure. The model data was verified through active interrogation using piezoelectric sensors. The multiscale model was further extended to develop a coupled damage and wave attenuation model, which was used to study different damage states such as fiber-matrix debonding in composite structures with surface bonded piezoelectric sensors.

Polymer and polymer matrix composites (PMCs) materials are being used extensively in different civil and mechanical engineering applications. The behavior of the epoxy resin polymers under different types of loading conditions has to be understood before the mechanical behavior of Polymer Matrix Composites (PMCs) can be accurately predicted. In many structural applications, PMC structures are subjected to large flexural loadings, examples include repair of structures against earthquake and engine fan cases. Therefore it is important to characterize and model the flexural mechanical behavior of epoxy resin materials. In this thesis, a comprehensive research effort was undertaken combining experiments and theoretical modeling to investigate the mechanical behavior of epoxy resins subject to different loading conditions. Epoxy resin E 863 was tested at different strain rates. Samples with dog-bone geometry were used in the tension tests. Small sized cubic, prismatic, and cylindrical samples were used in compression tests. Flexural tests were conducted on samples with different sizes and loading conditions. Strains were measured using the digital image correlation (DIC) technique, extensometers, strain gauges, and actuators. Effects of triaxiality state of stress were studied. Cubic, prismatic, and cylindrical compression samples undergo stress drop at yield, but it was found that only cubic samples experience strain hardening before failure. Characteristic points of tensile and compressive stress strain relation and load deflection curve in flexure were measured and their variations with strain rate studied. Two different stress strain models were used to investigate the effect of out-of-plane loading on the uniaxial stress strain response of the epoxy resin material. The first model is a strain softening with plastic flow for tension and compression. The influence of softening localization on material behavior was investigated using the DIC system. It was found that compression plastic flow has negligible influence on flexural behavior in epoxy resins, which are stronger in pre-peak and post-peak softening in compression than in tension. The second model was a piecewise-linear stress strain curve simplified in the post-peak response. Beams and plates with different boundary conditions were tested and analytically studied. The flexural over-strength factor for epoxy resin polymeric materials were also evaluated.