Browsing by Subject "Fracture toughness"

Now showing 1 - 4 of 4
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    Determination of Optimum Time for the Application of Surface Treatments to Asphalt Concrete Pavements - Phase II
    (Minnesota Department of Transportation, 2008-06) Marasteanu, Mihai; Velasquez, Raul; Herb, William; Tweet, John; Turos, Mugur; Watson, Mark; Stefan, Heinz G.
    Significant resources can be saved if reactive type of maintenance activities are replaced by proactive activities that could significantly extend the pavements service lives. Due to the complexity and the multitude of factors affecting the pavement deterioration process, the current guidelines for applying various maintenance treatments are based on empirical observations of the pavement surface condition with time. This report presents the results of a comprehensive research effort to identify the optimum timing of surface treatment applications by providing a better understanding of the fundamental mechanisms that control the deterioration process of asphalt pavements. Both traditional and nontraditional pavement material characterization methods were carried out. The nontraditional methods consisted of X-Ray Photoelectron Spectroscopy (XPS) for quantifying aging, while for microcracks detection, electron microprobe imaging test (SEM) and fluorescent dyes for inspection of cracking were investigated. A new promising area, the spectral analysis of asphalt pavements to determine aging, was also presented. Traditional methods, such as Bending Beam Rheometer (BBR), Direct Tension (DTT), Dynamic Shear Rheometer (DSR) and Fourier Transform Infrared Spectroscopy (FTIR) for asphalt binders and BBR and Semi-Circular Bending (SCB) for mixtures were used to determine the properties of the field samples studied in this effort. In addition, a substantial analysis of measured pavement temperature data from MnROAD and simulations of pavement temperature using a one-dimensional finite difference heat transfer model were performed.
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    Investigation of Low Temperature Cracking in Asphalt Pavements National Pooled Fund Study 776
    (Minnesota Department of Transportation, 2007-10) Marasteanu, Mihai; Zofka, Adam; Turos, Mugur; Li, Xinjun; Velasquez, Raul; Li, Xue; Buttlar, William; Paulino, Glaucio; Braham, Andrew; Dave, Eshan; Ojo, Joshua; Bahia, Hussain; Williams, Christopher; Bausano, Jason; Gallistel, Allen; McGraw, Jim
    Good fracture properties are an essential requirement for asphalt pavements built in the northern part of the US and in Canada for which the predominant failure mode is cracking due to high thermal stresses that develop at low temperatures. Currently, there is no agreement with respect to what experimental methods and analyses approaches to use to investigate the fracture resistance of asphalt materials and the fracture performance of asphalt pavements. This report presents a comprehensive research effort in which both traditional and new experimental protocols and analyses were applied to a statistically designed set of laboratory prepared specimens and to field samples from pavements with well documented performance to determine the best combination of experimental work and analyses to improve the low temperature fracture resistance of asphalt pavements. The two sets of materials were evaluated using current testing protocols, such as creep and strength for asphalt binders and mixtures as well as newly developed testing protocols, such as the disk compact tension test, single edge notched beam test, and semi circular bend test. Dilatometric measurements were performed on both asphalt binders and mixtures to determine the coefficient of thermal contraction. Discrete fracture and damage tools were utilized to model crack initiation and propagation in pavement systems using the finite element method and TCMODEL was used with the experimental data from the field samples to predict performance and compare it to the field performance data.
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    Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics
    (2016-08) Li, Tuoqi
    The great commercial importance of several brittle plastics continuously drives research efforts to be devoted to fabricating well defined structures in these materials for effectively toughening them. Amphiphilic block copolymers can be appropriately designed to generate nanometer scaled structures in a brittle plastic matrix at relatively low loadings (< 5% by weight). The resultant nanostructured plastics exhibit significant toughness enhancement without sacrificing other desirable properties such as transparency, stiffness and use temperature. The goal of this dissertation is to understand the nanostructure formation of block copolymers and the consequent toughening effect under various conditions. In this work we designed different types of block copolymer modifiers in concert with several commercially important brittle plastics, including epoxy thermosets and poly(lactide) (PLA) thermoplastics. The block copolymer toughening strategy was first established in bulk epoxies as well as in epoxy coatings through a model system study with the Jeffamine resin. Two distinct types of diblock copolymers formed spherical micelles in cured bulk epoxies and 15 micrometer thick coatings, but the process of solvent-casting affected the micelle size and distribution in the coating. The toughness enhancement observed in bulk epoxies (up to 5-fold increase in the critical strain energy release rate GIc) successfully translated to coatings, as evidenced by the over 40% increase in the coating abrasive wear resistance with only 5 wt.% of modifiers. Transmission electron microscopy (TEM) revealed that similar toughening mechanisms as those in bulk epoxies (micelle cavitation and matrix shear yielding) still held in thin coatings. Moreover, the hardness, modulus, transparency and glass transition temperature (Tg) of modified coatings were not appreciably affected compared to unmodified ones. Based on this model system study, we proceeded to investigate the commercially viable Cardolite resin system that is more complex thermodynamically but industrially relevant. A series of poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) diblock copolymers were synthesized at fixed composition (31% PEO by volume) and varying molecular weight expanding on a commercial product under the tradename Fortegra™ 100. Direct application of this product resulted in little improvement of the poor fracture toughness of the cured material. Modification of the resin formulation and curing protocol led to the development of well-defined spherical and branched wormlike micelles in cured resins. Thermodynamic interactions and the curing reaction together controlled the micelle formation as evidenced by small angle x-ray scattering (SAXS) measurements. A 9-fold increase in GIc over the neat bulk epoxy, and an over 30% improvement in the coating abrasive wear resistance over the unmodified coating were achieved at 5 wt.% loading of wormlike micelles. We then took one step further to explore the toughening efficacy of block copolymer micelles in hybrid composite systems in the presence of a second type of modifier, rigid graphene fillers with amine-functionalization. Both types of modifiers were well dispersed in cured epoxies with no observable interactions under TEM. The crosslink density of the epoxy network strongly affected the toughening effect. In the matrix with the lowest crosslink density, the combination of micelles and graphene drastically enhanced the GIc value to 19 times that of the neat material with no reduction in the elastic modulus and Tg. Additionally, hybrid ternary composites exhibited a synergistic toughening effect, revealing some positive mutual interference to the toughening mechanisms noted for micelles and graphene particles. Lastly, we extended the block copolymer toughening strategy to the PLA thermoplastic matrix. A low molar mass PEO-PBO diblock copolymer was uniformly dispersed as short cylindrical micelles in a commercial high molecular weight glassy PLLA plastic. This structure formation resulted from the negative Flory-Huggins interaction parameter (X) between PEO and PLLA. Those micelles could effectively toughen the matrix through concurrent cavitation, crazing and shear yielding. At only 5 wt.% of loading, micelles led to a greater than 10-fold increase in the tensile toughness and notched Izod impact strength over the neat PLLA in the glassy state. This toughening effect was retained in plastic films prepared with modified blends via a film blowing process.
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    Stress localization and size dependent toughening effects in SiC composites.
    (2010-08) Beaber, Aaron Ross
    Coatings with high wear resistance have generated a great deal of interest due to a diverse range of applications, including cutting tools, turbine blades, and biomedical joint replacements. Ceramic nanocomposites offer a potential combination of high strength and toughness that is ideal for such environments. In the current dissertation research, silicon and silicon carbide based films and nanostructures were deposited using a hybrid of chemical vapor deposition and nanoparticle ballistic impaction known as hypersonic plasma particle deposition (HPPD). This included SiC/Ti-based multilayers and Si-SiC core-shell composite nanotowers. Using a combination of nanoindentation and confocal Raman microscopy, the role of plasticity and phase transformations was studied during fracture events at small length scales. In a parallel study, HPPD synthesized Si nanospheres and vapor-liquid-solid (VLS) Si nanotowers were compressed uniaxially inside the TEM. These experiments confirmed inverse length scale dependent relationships for strength and toughness in Si based on dislocation pile-up and crack tip shielding mechanisms, respectively. A transition was also identified in the deformation of Si under anisotropic loading below a critical size and used as the basis for a new toughening mechanism in Si-SiC composites. Overall, these results demonstrate the importance of nanoscale confinement and localized stress in the design of mechanically robust nanocomposites.