A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Modeling Viscoelastic Behavior Using Flexible Multibody Dynamics Formulations(2020) Nemani, Nishant; Bauchau, Olivier Prof.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Viscoelastic behavior is frequently observed in dynamical flexible multibody systems. In the simplest form it is manifested in one dimensional revolute and prismatic joints. Beyond which more complex force elements such as six degree of freedom flexible joints can also be found. Finally, beams, plates and shells are found to exhibit viscoelastic behavior too. In the past extensive work has been done on analyzing the dynamic response of three dimensional beams by performing cross-sectional analysis through finite element methods and subsequently solving the reduced beam problem. The approach is particularly relevant for the analysis of complex cross sections and helps improve computational efficiency significantly. A formulation which incorporates a viscoelastic model of the generalized Maxwell type with a solution of the three dimensional beam theory which gives an exact solution of static three dimensional elasticity problems is presented. Multiple examples incorporating the use of the aforementioned model in the context of viscoelastic beams and joints are presented. Shortcomings of the Kelvin-Voigt model, which is often used for flexible multibody systems, are underlined.Item STRUCTURE-PROPERTY RELATIONSHIPS IN CRYSTALLINE-AMORPHOUS BLOCK COPOLYMERS: SYNTHESIS, PHASE BEHAVIOR AND MECHANICAL PROPERTIES(2017) Hwang, Wonseok; Briber, Robert M; Sita, Lawrence R; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Block copolymers are composed of two or more polymer chains that are chemically distinctive and covalently bonded at the single point. Due to the incompatibility between blocks, the block copolymers undergo microphase separation generating long-range ordered microdomains. Considerable effort has been put on the understanding of the microdomain structure-mechanical properties relationships in block copolymers where both blocks are amorphous. Less research has been done on structure-property relationships in crystalline-amorphous block copolymers. In this thesis, the structure-property relationships in crystalline-amorphous block copolymers have been explored using two polymer systems, 1) well-defined isotactic-atactic-isotactic stereoblock (sbPP) and stereoirregular (irPP) polypropylene materials, 2) poly(1-hexene)-poly(methyl-1,3-cyclopentane) (PH-PMCP) block copolymers. The structure-property relationships in sbPP and irPP materials have been characterized using tensile testing, dynamic mechanical analysis (DMA) and rheology. The sbPP materials demonstrated higher tensile modulus exhibiting elongation ratios of more than 2,000 % while irPP materials showed lower tensile modulus with elongation of less than 100 %. Furthermore, sbPP materials exhibited more than 90 % of tensile recovery in contrast to 60-70 % tensile recovery for irPP materials. The discrete crystalline-amorphous-crystalline structure in sbPP materials provides both rigidity and flexibility. The morphologies of PH-PMCP materials have been investigated using in-situ small and wide angle X-ray scattering (SAXS/WAXS), rheology, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The melting of the crystalline PMCP followed by the microphase separation in PH-PMCP results in a modulus transition after the PMCP melting. In particular, sphere-forming PH-PMCP block copolymers exhibit a drop of 2 orders of magnitude in the storage modulus at the PMCP melting temperature. The recovery in storage modulus within 2 min results from the low molecular weight and the incompatibility between PH and PMCP blocks. By tuning PMCP content, this modulus transition has been demonstrated in sphere, cylinder, double gyroid, and lamellae forming block copolymers. The viscoelastic response in sphere-forming PH-PMCP block copolymers indicate that after the PMCP crystal melt the molten PMCP blocks participate the formation of block copolymer microdomains and improve the microdomain ordering.Item Prediction of Permanent Deformation in Asphalt Concrete(2012) Carvalho, Regis Luis; Schwartz, Charles W; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Permanent deformation is a major distress in flexible pavements that leads to the development of rutting along the wheel path of heavily trafficked roads. Early detection of rutting is very important for preventive maintenance programs and design of rehabilitation strategies. Rutting by definition is the accumulated permanent deformation that remains after removal of the load. Rigorous modeling of permanent deformations using nonlinear finite element analysis based on the correct physical mechanism of residual deformations after removal of the load provides important insights into the rutting problem. This dissertation documents the study of permanent deformation in asphalt concrete in pavement structures using a fully mechanistic model based on Schapery's viscoelasticity and Perzyna's viscoplasticity theories. The model is calibrated and implemented in a 3D finite element commercial software package. Two calibration procedures are performed and discussed. Two immediate practical applications are shown and a simulation of full scale accelerated pavement test is performed. This research demonstrates that the Perzyna-HiSS viscoplastic model can be successfully calibrated using either research-grade creep and recovery tests or the more simple and production-oriented Flow Number test. The importance of induced shear stress reversals under a moving wheel load is documented. The 3D finite element simulation is then used to identify the fundamental differences on how rutting develops in different pavement structures in terms of the differences in the transverse profile and distribution of rutting within the layer. The analysis results are used to develop new pavement-specific depth functions for potential future incorporation into the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG). Lastly, the 3D finite element model is used to predict rutting at one lane of the FHWA's full-scale Accelerated Load Facility experiment. After correction for some anomalies during the early loading cycles in the experiment, the predicted and measured rutting at the center of the wheel path were in good agreement.