Friction stir processing (FSP) has been developed based on the basic principles of friction stir welding (FSW), a solid-state joining process originally developed for aluminum alloys. What is attractive about FSP is that it can be incorporated in the overall manufacturing cycle as a post-processing step during the machining operation to provide localized modification and control of microstructures in near-surface layers of metallic components. FSP has emerged as an important post-processing technique, and has been identified as a process that may have a high impact, and perhaps is a disruptive manufacturing process. In this study, FSP has been applied to Al cast alloy A206, which is a high strength, widely used cast alloy in the manufacturing industry. Motivations behind this work are to (1) investigate the feasibility of FSP on manipulating the cast microstructure and strengthening the material, and (2) to explore the viability of FSP to produce a localized particle reinforced zone in cast A206 aluminum components. The thesis will show that we have optimized FSP for processing of Al alloys to locally manipulate the cast microstructure, eliminate casting defects, and attain grain refinement and second phase homogenization. We have established the mechanism leading to the microstructure evolution and have evaluated the resultant mechanical properties, i.e. hardness, tensile property and fatigue properties. We have also synthesized a localized composite material in the A206 work piece with three different reinforcement materials via FSP. These results will be presented and discussed.

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Individual carbon nanotubes have been theoretically and experimentally proven to be the strongest and stiffest materials discovered to date with tensile strengths ranging from 1-5 TPa and elastic modulus values as high as 150 GPa. In this work, the recent development of continuous sheets of CNTs, produced by Nanocomp Technologies Inc ., are investigated for their potential as reinforcement in polymer matrix composite (PMC) materials. The potential of these nanotube-based PMC materials have been reported by researchers at Florida State University (FSU). Through the use of mechanical stretching procedures to increase the alignment of the nanotubes within the CNT sheets, the tensile strength and Young's modulus of the composites in the FSU study averaged 3081 MPa and 350 GPa, respectively. These values are for composites fabricated from 40% stretched CNT sheets and are 48% and 107% improvements over composites fabricated from the pristine, unstretched CNT sheets. However, the test specimens used in the FSU study consisted of a single CNT ply and each coupon was individually stretched and cured for testing. Therefore, the process used to generate the coupons which exhibited these high mechanical properties would be difficult to scale to a usable size for aerospace structural components. In the current study, a scalable process has been developed in which 2-ply, 3" x 3" panels of CNT and resin composites are fabricated. An apparatus and methodology for mechanically stretching the CNT sheets used in these composite panels has also been developed. After initial testing was conducted with the CNT composites and the coupons exhibited significant elongation at failure, along with the absence of a linear elastic region, conventional test standards for material testing were deemed impractical. For this reason, new mechanical testing methodologies have been developed to determine the mechanical properties of specific strength and specific modulus of CNT-polymer composites. In order to obtain the maximum benefits of a fiber in any matrix in terms of stiffness and strength, it is preferable to align the high strength and stiffness fibers in the direction of loading. Given that these CNT sheets essentially consist of billions of short, discontinuous CNTs of 2-3mmin length, the process of mechanical stretching is used in an attempt to align these tubes in the direction of the applied tensile load. Here we have explored methodologies for stretching, fabricating, and mechanical testing. Having identified a process which seems viable, an examination into the effect of the mechanical stretching to increase the alignment of the nanotubes within the CNT sheets, and thus to increase the material properties of the 2-ply composites constructed from them, is conducted. In order to correlate the enhancements in the mechanical properties with the increased alignment of the CNTs, polarized Raman spectroscopy techniques have been used. Lastly, Scanning Electron Microscopy (SEM) is used to examine the effect of stretching on the pristine CNT sheet, as well as examine the fracture surfaces of failed test coupons to better characterize the failure modes. In this report, polarized Raman spectroscopy has been used to confirm the enhancedalignment of nanotubes within the CNT sheets through the used of a nematic order parameter. Unstretched sheets exhibit an order parameter of 0.07 and 0.09 for untreated and Acetone treated sheets, respectively. Upon stretching the untreated sheets to 45%, the order parameter increases to 0.1409 and, when stretched to 30%, Acetone treated sheets have an order parameter of 0.1518. During the mechanical testing of 2-ply composites fabricated from stretched CNT sheets, the effect of this increased alignment is made apparent. Untreated CNT sheets are used to fabricate 2-ply composites after being stretched and are compared to baseline values of panels fabricated using sheets which are not stretched. In the panels fabricated with PEI resin and 43% stretched, untreated CNT sheets, a 137% increase in average specific strength and a 44% increase in average specific modulus over the baseline panel is observed. For panels fabricated with BMI and 33% stretched, untreated CNT sheets, a 169% increase in average specific strength and 105% increase in average specific modulus is observed when compared to the baseline panel. These increases are evidence for the potential of mechanical stretching to align the nanotubes within the CNT sheets and bolster the mechanical properties of resulting CNT-polymer composites. / Master of Science

This study examines the use of embedded shape memory alloy (SMA)actuators for adaptive control of the themomechanical response of composite structures. Control of static and dynamic responses are demonstrated including thermal buckling, thermal post-buckling, vibration, sonic fatigue, and acoustic transmission. A thermomechanical model is presented for analyzing such shape memory alloy hybrid composite (SMAHC) structures exposed to thermal and mechanical loads. Also presented are (1) fabrication procedures for SMAHC specimens, (2) characterization of the constituent materials for model quantification, (3) development of the test apparatus for conducting static and dynamic experiments on specimens with and without SMA, (4) discussion of the experimental results, and (5) validation of the analytical and numerical tools developed in the study. The constitutive model developed to describe the mechanics of a SMAHC lamina captures the material nonlinearity with temperature of the SMA and matrix material if necessary. It is in a form that is amenable to commercial finite element (FE) code implementation. The model is valid for constrained, restrained, or free recovery configurations with appropriate measurements of fundamental engineering properties. This constitutive model is used along with classical lamination theory and the FE method to formulate the equations of motion for panel-type structures subjected to steady-state thermal and dynamic mechanical loads. Mechanical loads that are considered include acoustic pressure, inertial (base acceleration), and concentrated forces. Four solution types are developed from the governing equations including thermal buckling, thermal post-buckling, dynamic response, and acoustic transmission/radiation. These solution procedures are compared with closed-form and/or other known solutions to benchmark the numerical tools developed in this study. Practical solutions for overcoming fabrication issues and obtaining repeatable specimens are demonstrated. Results from characterization of the SMA constituent are highlighted with regard to their impact on thermomechanical modeling. Results from static and dynamic tests on a SMAHC beam specimen are presented, which demonstrate the enormous control authority of the SMA actuators. Excellent agreement is achieved between the predicted and measured responses including thermal buckling, thermal post-buckling, and dynamic response due to inertial loading. The validated model and thermomechanical analysis tools are used to demonstrate a variety of static and dynamic response behaviors associated with SMAHC structures. Topics of discussion include the fundamental mechanics of SMAHC structures, control of static (thermal buckling and post-buckling) and dynamic responses (vibration, sonic fatigue, and acoustic transmission), and SMAHC design considerations for these applications. The dynamic response performance of a SMAHC panel specimen is compared to conventional response abatement approaches. SMAHCs are shown to have significant advantages for vibration, sonic fatigue, and noise control. / Ph. D.

Thermal degradation during composite fabrication is the main impediment for the wide use of agro-based fibers as filler and reinforcement in engineering thermoplastic composites. Different thermal, chemical and physical techniques (e.g., alkali, steam explosion and retting) aiming to increase the fiber-matrix adhesion or reduce the plant fibers water absorption have been presented in the literature. However, there have been very few attempts to solve the difficulties associated with processing engineering thermoplastics with plant fibers. Most of these attempts involved the use of additives (such as plasticizers and salts) to lower the polymers processing temperature and plant fibers with inherent higher thermal stability (such as Curaua and cellulose). Despite all these efforts, no important progress has been achieved. Therefore, to explore the full potential of wheat straw and expand its use in commercial applications, an experimental study was carried out to develop different methodologies to improve the thermal stability of wheat straw fiber. In this thesis, most attention is given to wheat straw because of the relevance and potential of entering the market as commercial filler today. It is reported here that the thermal stability and chemical composition of wheat straw do not seem to significantly vary with wheat straw type and cultivation region. For example, the main thermal degradation of wheat straw samples starts in a narrow window of temperature which goes from 220.8 to 237.8 °C and from 224.8 to 238.1 °C for air and nitrogen atmospheres, respectively. On the other hand, lignin and inorganic materials are the wheat straw components with the highest relative variation. In addition, it is showed here that silane modification is an efficient method to increase the temperature of degradation of wheat straw. The highest improvements were achieved with chlorosilane modifiers and combinations of alkoxysilane and chlorosilane modifiers. In fact, the silane treated samples have lower thermal degradation during the fabrication of composites with polyamide-6. It is observed here that the extruded and injection molded composites containing silane treated wheat straw samples have significant smaller thermal degradation than those utilizing untreated wheat straw samples. Equally important, it seems that the mechanical properties of the composites are not affected by the addition of silane treated samples in comparison with untreated wheat straw. In addition, another efficient treatment method is presented in this thesis. This method employs ultraviolet light to modify and improve the thermal stability of wheat straw. This method offers important economical and environmental benefits. Significant improvements (e.g., 40 ºC increase on the temperature at 2% of weight loss) were achieved after treatment for short periods of time (up to 15 minutes) and without the use of any pre-treatment or production of toxic by-products. This treatment method represents a novel application for ultraviolet light with potential for industrial use.

Thermal degradation during composite fabrication is the main impediment for the wide use of agro-based fibers as filler and reinforcement in engineering thermoplastic composites. Different thermal, chemical and physical techniques (e.g., alkali, steam explosion and retting) aiming to increase the fiber-matrix adhesion or reduce the plant fibers water absorption have been presented in the literature. However, there have been very few attempts to solve the difficulties associated with processing engineering thermoplastics with plant fibers. Most of these attempts involved the use of additives (such as plasticizers and salts) to lower the polymers processing temperature and plant fibers with inherent higher thermal stability (such as Curaua and cellulose). Despite all these efforts, no important progress has been achieved. Therefore, to explore the full potential of wheat straw and expand its use in commercial applications, an experimental study was carried out to develop different methodologies to improve the thermal stability of wheat straw fiber. In this thesis, most attention is given to wheat straw because of the relevance and potential of entering the market as commercial filler today. It is reported here that the thermal stability and chemical composition of wheat straw do not seem to significantly vary with wheat straw type and cultivation region. For example, the main thermal degradation of wheat straw samples starts in a narrow window of temperature which goes from 220.8 to 237.8 °C and from 224.8 to 238.1 °C for air and nitrogen atmospheres, respectively. On the other hand, lignin and inorganic materials are the wheat straw components with the highest relative variation. In addition, it is showed here that silane modification is an efficient method to increase the temperature of degradation of wheat straw. The highest improvements were achieved with chlorosilane modifiers and combinations of alkoxysilane and chlorosilane modifiers. In fact, the silane treated samples have lower thermal degradation during the fabrication of composites with polyamide-6. It is observed here that the extruded and injection molded composites containing silane treated wheat straw samples have significant smaller thermal degradation than those utilizing untreated wheat straw samples. Equally important, it seems that the mechanical properties of the composites are not affected by the addition of silane treated samples in comparison with untreated wheat straw. In addition, another efficient treatment method is presented in this thesis. This method employs ultraviolet light to modify and improve the thermal stability of wheat straw. This method offers important economical and environmental benefits. Significant improvements (e.g., 40 ºC increase on the temperature at 2% of weight loss) were achieved after treatment for short periods of time (up to 15 minutes) and without the use of any pre-treatment or production of toxic by-products. This treatment method represents a novel application for ultraviolet light with potential for industrial use.