Browsing by Subject "Toughness"

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    Structure and mechanical properties of multiblock copolymers: toward the development of enhanced mechanical response materials
    (2014-01) Lee, Intaek
    Block polymers have attracted scientific interest for decades, and most studies have focused on the simplest molecular architectures: linear AB diblock and ABA triblock copolymers. Multiblock copolymers containing a large number of blocks are expected to have distinct microstructures and a mechanical response which is different from that of conventional diblock and triblock copolymers. This research addresses synthesis and characterization of poly(cyclohexylethylene)-polyethylene (CECECECEC) nonablock copolymers, poly(styrene-b-butadiene) (PS-PB) multiblock copolymers, and poly(lactide-b-butadiene) (PLA-PB) multiblock copolymers. CECECECEC nonablock copolymers having a large center C block were synthesized using sequential anionic polymerization followed by catalytic hydrogenation. The CECECECEC samples exhibited different morphologies with varying size of PE blocks. As the PE block size increased, the microstructure was transformed with the sequence of disordered homogeneous phase - lamellae with mixed phase of outer CECE blocks - layer-in-layer microstructure. Moreover, the secondary phase segregation of outer CECE blocks allowed tough mechanical behavior. PS-PB multiblock copolymers with alternating and random block sequences were synthesized using a combination of living anionic polymerization and polycondensation. Molecular characterization revealed the successful synthesis of the desired multiblock products through the proposed procedure. Structural analysis demonstrated a random bicontinuous-like morphology over a wide range of compositions, 0.69 ≤ fPS ≤ 0.85. Tensile tests showed yielding followed by necking and an overall ductility that translates into much greater toughness than that typically found in glassy continuous SBS triblock copolymers. PLA-PB multiblock copolymers (0.5 ≤ fPLA ≤ 0.9) were synthesized in a two-step procedure: PLA-PB-PLA triblock copolymers were prepared using ring-opening polymerization, followed by chain extension with the condensation reaction. Multiblock copolymer and homologous triblock materials exhibited nearly identical and well-ordered morphologies, in sharp contrast with the findings of PS-PB multiblock polymers. These results indicate a transition from classically ordered morphologies to a state of bicontinuous disorder for multiblocks containing <n> ≥ 10, where <n> is the average total number of blocks. In tensile tests, most PLA-PB multiblock copolymers exhibited dramatically enhanced mechanical properties compared to the corresponding LBL triblock copolymers. These results suggest that a multiblock copolymer strategy offers new possibilities to obtain unique microstructures and physical properties from many other combinations of polymers.
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    Variations in Power-Actuated Fastener Shear Capacity with Respect to Concrete Coarse Aggregate Properties
    (2019-12) Overacker, Scott
    Power-actuated fasteners (PAFs) securing the cold-formed steel track to concrete slab connection in non-structural partition walls are a critical failure point in buildings in low to high seismic regions. Concrete composition is hypothesized to have a major effect on the performance of PAFs; however, guidelines currently available in the United States for the evaluation of PAF performance in concrete do not include clear specifications on concrete mixes or aggregates. Testing and evaluation criteria for seismic applications is also not available. The focus of this thesis is on the dependency of PAF performance, in terms of capacity, embedment, stiffness, and shank bending, with respect to concrete coarse aggregate properties. A survey of concrete aggregates across a wide range of locations in the United States and Canada was conducted, and a classification system with recommended parameters for defining the toughness of concrete coarse aggregates was proposed. A loading device and methods for testing the out-of-plane behavior of groups of fasteners in cold-formed steel tracks under shear loading were developed, and several combinations of fasteners and concrete aggregates were tested. Results of the out-of-plane track shear tests were compared to shear tests of single fasteners installed into the same concrete mixes. The detriment of increased aggregate toughness on stick rate, embedment, and bending of the fastener shanks was noted. Additionally, an inverse relationship between coarse aggregate toughness and fastener capacity, as well as a strong group effect that reduced variability and increased capacities per fastener, was observed.