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Effective treatment of carbon fiber components to improve delamination resistance is vital to the application of such materials since delamination is one of the biggest concerns regarding the use of composites in the aerospace sector. Due to the significant application benefit gained from increased stiffness to density ratio with composite materials, innovative developments resulting in improved through-thickness strength have been on the rise. The inherent anisotropy of composite materials results in an added difficulty in designing structural elements that make use of such materials. Proposed techniques to improve the through-thickness strength of laminar composites are many and varied; however all share the common goal of improving inter-laminar bond strength. This research makes use of novel materials in the field of wet flocking and Z-pinning. Cellulose nanofibers (CNFs) have already demonstrated excellent mechanical properties in terms of stiffness and strength, originating at the nano-scale. These materials were introduced into the laminate while in a sol-gel suspension in an effort to improve load transfer between laminate layers. The effect of CNFs as lightweight renewable reinforcement for CFRPs will be investigated. Carbon nanotube (CNT) additives were also considered for their beneficial structural properties. / Master of Science

The mechanical performance of composite materials depends not only on the matrix and the reinforcing fiber, but also to a great extent on the fiber/matrix interfacial adhesion. The focus of this work was to study carbon fiber surface chemical and physical properties and their effects on fiber/matrix adhesion. Untreated, commercially surface treated and oxygen plasma treated PAN based carbon fibers were used for study. XPS was used to determine fiber surface chemistry. A two-liquid tensiometric method was conducted to determine fiber surface energy and its dispersion and polar components. SEM was used to examine the fiber surface topography. Commercial surface treatment increased the carbon fiber surface oxygen content and fiber surface energy primarily in the polar component. An even higher level of fiber surface oxygen functionality and polar surface energy were achieved by oxygen plasma treatment. Oxygen plasma treatment also resulted in etching and pitting of AU-4 carbon fiber surface. Carbon fibers with varying surface properties were incorporated into epoxy matrices. Single fiber fragmentation tests were carried out to evaluate the strength as well as the temperature dependence and humidity durability of interfacial adhesion. Commercially treated carbon fibers which having a higher surface oxygen content and higher surface energy clearly produced superior interfacial adhesion, relative to untreated fibers. An even greater level of adhesion was achieved with oxygen plasma treated fibers. Fiber surface roughness improved durability under elevated temperature and relative humidity conditions. The presence of sodium on the fiber surface dramatically decreased durability at high relative humidity. / Master of Science

Composite durability can be influenced by varying the properties of the fiber/matrix interphase region. One method to modifY the properties of this interphase is through the application of a sizing to the carbon fiber. Recent work at Virginia Tech has shown that polymer-modified interphases can lead to increases by as much as two orders of magnitude in notched fatigue lifetime. In the present work, an apparatus was constructed to uniformly coat carbon fiber tow with water-soluble and dispersible polymers. Few such devices have been developed for use in academic settings because of the processing complexities presented by the aqueous coating system. Due to the high surface tensions of the aqueous solutions, fiber clumping and heterogeneous sizing deposition were major bottlenecks. Our novel process utilizes high tensions, high spreading, and low line speeds to accomplish the sizing step. Each processing independent variable can be continuously monitored and controlled which allowed for statistical correlation to the sizing level and uniformity. The sizing process was shown to satisfy three criteria for quality. 1. The average sizing level or weight percent on the final fiber can be readily controlled to achieve typical target values. 2. Filament clumping as a result of cohesion between corresponding filaments is kept to a minimum. 3. The sizing process must produce fiber with a consistent level of polymer sizing. In addition, characterization techniques for the sized fiber were developed. Pyrolysis in a high temperature nitrogen furnace was developed as a precise technique to ascertain the quantitative sizing level on the carbon fiber. SEM and ESCA were utilized to determine fiber clumping and sizing homogeneity. The sizing process along with the statistical process model and the characterization techniques allow for the precise development of optimal interphase materials that are tailored to meet the performance requirements of the composite consumer. / Master of Science

The Utah Department of Transportation (UDOT) is interested in the application of rehabilitation techniques to strengthen their AASTHO prestressed bridge girders for shear. Utah's bridges are exposed to deterioration from rain, snow, and the introduction of salt for ice removable. This requires innovative rehabilitation techniques to address the deteriorations of their highway bridges, especially the ends of bridge girders where water and salt are more common due to construction joints. Carbon Fiber Reinforced Polymers (CFRP) are becoming more prevalent as a tool in highway bridge rehabilitation. This research investigates the application of various CFRP systems that can be used as shear reinforcement for prestressed concrete girders. The experimental program involved full-scale destructive testing of six 40-year-old, AASHTO prestressed I-girders that were salvaged from the 45th South/I-215 bridge in Salt Lake City, Utah. The testing involved retrofitting five of the girders with various configurations of CFRP fabric. Based on the initial tests, the most effective configuration was then applied to another set of I-shaped concrete girders for verifications. After the experimental testing, two analytical models developed for predicting the additional shear contribution of the CFRP reinforcement were compared with the measured results from the experimental program. After testing and comparisons, a CFRP reinforcement configuration and theoretical model was selected as a reliable and effective method for application of external shear reinforcement of AASHTO prestressed I-shaped girders.

A study was undertaken to determine if light-weighting orbital launch vehicles (rockets) improves lifetime environmental impacts of the vehicle. Light-weighting is performed by a material substitution where metal structures in the rocket are replaced with carbon fiber reinforced polymers (CFRP’s). It is uncertain whether light-weighting the rocket in the same way as traditional vehicles are light-weighted would provide similar environmental benefits. Furthermore, the rocket system is significantly different from traditional vehicles and undergoes an atypical lifecycle, making analysis non-trivial. Seventy rocket configurations were sized using a Parametric Rocket Sizing Model (PRSM) which was developed for this research. Four different propellant options, three staging options, and eighteen different lift capacities were considered. Each of these seventy rockets did not include CFRP’s, thus establishing a baseline. The seventy rockets were then light-weighted with CFRP’s, making a total of seventy pairs of rockets. An environmental Life Cycle Assessment (LCA) was performed on each of the rockets to determine lifetime environmental impacts. During the Life Cycle Inventory (LCI), a Carbon Fiber Production Model was developed to determine the environmental burdens of carbon fiber production and to address issues identified with carbon fiber’s embodied burdens. The results of the LCA were compared across all rockets to determine what effects light-weighting had on environmental impact. The final conclusion is that light-weighting reduces lifetime environmental impacts of Liquid Oxygen-Rocket Propellant 1 and Nitrogen Tetroxide-Unsymmetrical Dimethylhydrazine rockets, while it likely benefits Liquid Oxygen-Liquid Hydrogen rockets. Light-weighting increases lifetime environmental impacts of Solid Propellant rockets.

Conserving energy in large structural buildings has become very important in today's economy. A number of buildings today are constructed with sandwich wall panels. Steel connections are most commonly used in these panels. The problem with steel is that it has a tendency to reduce the thermal resistance of the insulation. This project considers glass fiber reinforcing polymers (GFRP) and carbon fiber reinforcing polymers (CFRP) as an alternate material to steel. An experimental sandwich wall panel was constructed and subjected to freezing temperatures. The results of the experimental program were compared to a theoretical model using the ANSYS computer program. The model was verified using current analytical methods that determine the heat flux of a sandwich wall panel. The methods investigated include the parallel path, zone, parallel flow, and isothermal planes methods. The results suggest that the GFRP connectors perform slightly better than the steel and CFRP connectors. / ND EPSCoR

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<div>As the use of fiber-reinforced polymer composites grows in aerospace structures, there is an emerging need to implement damage tolerant approaches. The use of <i>in-situ</i> synchrotron X-ray tomography enables direct observations of progressive damage relative to the microstructural features, which is studied in a T650/5320 laminate composite with varying layup orientations (using 45^o and -45^o plies) in a compact tension specimen geometry. Specifically, the interactions of micromechanical damage mechanisms at the notch tip were analyzed through 3D image processing as the crack grew. First, monotonic tests were conducted where X-ray tomography was acquired incrementally between the unloaded state and maximum load. The analysis of the monotonic tension specimens showed intralaminar cracking was dominant during crack initiation, delamination became prevalent during the later stages of crack progression, and fiber breakage was, in general, largely related to intralaminar cracking. After the monotonic tension analysis, modifications were made to the specimen geometry and the loading assembly, and fatigue tests were conducted, also using <i>in-situ</i> synchrotron X-ray tomography. Specifically, tomography images were acquired after select intervals of cyclic loading to examine the crack growth behavior up to 5802 cycles. The analysis of the fatigue tests showed that intralaminar cracking was also dominant, while localized delamination allowed ply cross-over. A finite element analysis was conducted by comparing the crack profile at varying intervals of loading, and the change in stored energy per cycle, dU/dN, was calculated. The combined experimental and simulation analysis showed that when the per ply values of dU/dN were examined, the intralaminar cracking rate collapsed to one curve regardless of the ply orientation, where direct observations of fiber bridging were characterized and associated with a reduction in crack growth rate for the influenced ply. Overall, this work provides a physical understanding of the micromechanics facilitating intralaminar crack growth in composites, providing engineers the necessary assessments for slow crack growth approaches in structural composite materials.<br></div>