Theses and Dissertations - Department of Aerospace Engineering and Mechanics

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Browsing Theses and Dissertations - Department of Aerospace Engineering and Mechanics by Author "Allison, Paul Galon"

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    Carbon nanotube sheet scrolled fiber composite for enhanced interfacial mechanical properties
    (University of Alabama Libraries, 2017) Kokkada Ravindranath, Pruthul; Roy, Samit; University of Alabama Tuscaloosa
    The high tensile strength of Polymer Matrix Composites (PMC) is derived from the high tensile strength of the embedded carbon fibers. However, their compressive strength is significantly lower than their tensile strength, as they tend to fail through micro-buckling, under compressive loading. Fiber misalignment and the presence of voids created during the manufacturing processes, add to the further reduction in the compressive strength of the composites. Hence, there is more scope for improvement. Since, the matrix is primarily responsible for the shear load transfer and dictating the critical buckling load of the fibers by constraining the fibers from buckling, to improve the interfacial mechanical properties of the composite, it is important to modify the polymer matrix, fibers and/or the interface. In this dissertation, a novel approach to enhance the polymer matrix-fiber interface region has been discussed. This approach involves spiral wrapping carbon nanotube (CNT) sheet around individual carbon fiber or fiber tow, at room temperature at a prescribed wrapping angle (bias angle), and then embed the scrolled fiber in a resin matrix. The polymer infiltrates into the nanopores of the multilayer CNT sheet to form CNT/polymer nanocomposite surrounding fiber, and due to the mechanical interlocking, provides reinforcement to the interface region between fiber and polymer matrix. This method of nano-fabrication has the potential to improve the mechanical properties of the fiber-matrix interphase, without degrading the fiber properties. The effect of introducing Multi-Walled Carbon Nanotubes (MWNT) in the polymer matrix was studied by analyzing the atomistic model of the epoxy (EPON-862) and the embedded MWNTs. A multi-scale method was utilized by using molecular dynamics (MD) simulations on the nanoscale model of the epoxy with and without the MWNTs to calculate compressive strength of the composite and predict the enhancement in the composite material. The influence of the bias/over wrapping angle of the MWNT sheets on the carbon fiber was also studied. The predicted compressive strength from the MD results and the multiscale approach for baseline Epoxy case was shown to be in good relation with the experimental results for Epon-862. On adding MWNTs to the epoxy system, a significant improvement in the compressive strength of the PMC was observed. Further, the effect of bias angle of MWNT wrapped over carbon fiber was compared for 0, 45 and 90. This is further verified by making use of the Halpin-Tsai.
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    Modeling of nano-scale fracture mechanisms in a graphene sheet using the atomistic j-integral
    (University of Alabama Libraries, 2018) Roy, Anubhav; Roy, Samit; University of Alabama Tuscaloosa
    Researchers have performed studies with the addition and dispersion of a few weight percent of nanoscale particles in polymer matrices to mitigate the brittleness and microcracking of polymer matrices without incurring weight penalty and improve their strain to failure and fracture toughness. This thesis aims at studying these length scale effects in nano-fillers, identifying the existence of a lower bound on flaw-size that marks the transition from brittle fracture to strength-based failure in nanocomposites, resulting in a deviation from linear elastic fracture mechanics (LEFM) predictions. Crack-tip bond-order based prediction of critical value of stress intensity factor is also addressed in this work. The objective of this work also includes employment of an atomistic J-integral as a suitable metric for the evaluation of fracture behaviour in materials at nanoscale. Good agreement is observed between atomistic and LEFM predictions using far-field stress and J-integral computations. While the far-field stress based atomistic data enables global prediction of the system undergoing fracture, the J-integral around the crack tip sheds light on local near-crack-tip stress state. Both far-field and near tip predictions are seen to deviate from LEFM predictions below a certain length-scale. In addition, effects such as nonlocality in molecular dynamics (MD) computations and entropic effects at the atomistic scale add to the discrepancy with LEFM. The fracture study on crystalline (graphene) was performed to lay the foundation for atomistic predictions of fracture in amorphous (polymer) nanocomposite systems.