Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16510号 / 工博第3503号 / 新制||工||1530(附属図書館) / 29167 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 落合 庄治郎, 教授 田中 功 / 学位規則第4条第1項該当

The addition of hexagonal boron nitride (h-BN) has introduced a comprehensive reinforcing effect to the mechanical and electrochemical properties of commercial concrete, including fiber reinforced concrete (FRC) and steel fiber reinforced concrete (SFRC). Although this has been proven effective and applicable, further investigation and study is still required to optimize the strengthen result which will involve the exfoliation of h-BN into single-layered nano sheet, improving the degree of dispersion and dispersion uniformity of h-BN into concrete matrix. There is currently no direct method to test the degree of dispersion of non-conductive particles, including h-BN, in concrete matrix, therefore it is necessary to obtain an analogous quantification method like SEM, etc. The reinforcing mechanism on concrete, including FRC and SFRC is now attracting a great number of interest thanks to the huge potential of application and vast demand across the world. This study briefly describes the reinforcing mechanism brought by h-BN. In this study, different samples under varied conditions were prepared according to the addition of h-BN and dispersant to build a parallel comparison. Characterization is mainly focused on their mechanical properties, corrosive performance and SEM analysis of the cross-section of post-failure samples.

This research investigated effect of submicron glass fiber modification on mechanical performance of short carbon fiber reinforced vinyl ester resin. Firstly, the mixture of resin and glass fiber was made by mixing submicron fiber into resin in a homogenizer at the speed of 5000 rpm in 30 minutes. Then, this modified resin was reinforced by short carbon fiber at the length of 1 mm, 3mm and 25 mm. The modifying effects were accessed by evaluating mechanical properties such as: bending, tensile, impact test as well as dynamic mechanical analysis. To explain some manners of material caused by adding glass fiber into resin, some techniques were also used (IFSS, SEM, laser microscope scanner, ultrasonic S-scan, X-ray ...). / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

Carbon nanofibers (CNFs) used as nano-scale reinforcement have been extensively studied since they are capable of improving the physical and mechanical properties of conventional fiber reinforced polymer composites. However, the properties of CNFs are far away from being fully utilized in the composites due to processing challenges including the dispersion of CNFs and the viscosity increase of polymer matrix. To overcome these issues, a unique approach was developed by making carbon nanopaper sheet through the filtration of well-dispersed carbon nanofibers under controlled processing conditions, and integrating carbon nanopaper sheets into composite laminates using autoclave process and resin transfer molding (RTM). This research aims to fundamentally study the processing-structure-property-performance relationship of carbon nanopaper-based nanocomposites multifunctional applications: a) Vibrational damping. Carbon nanofibers with extremely high aspect ratios and low density present an ideal candidate as vibrational damping material; specifically, the large specific area and aspect ratio of carbon nanofibers promote significant interfacial friction between carbon nanofiber and polymer matrix, causing higher energy dissipation in the matrix. Polymer composites with the reinforcement of carbon nanofibers in the form of a paper sheet have shown significant vibration damping improvement with a damping ratio increase of 300% in the nanocomposites. b) Wear resistance. In response to the iv observed increase in toughness of the nanocomposites, tribological properties of the nanocomposite coated with carbon nanofiber/ceramic particles hybrid paper have been studied. Due to high strength and toughness, carbon nanofibers can act as microcrack reducer; additionally, the composites coated with such hybrid nanopaper of carbon nanofiber and ceramic particles shown an improvement of reducing coefficient of friction (COF) and wear rate. c) High electrical conductivity. A highly conductive coating material was developed and applied on the surface of the composites for the electromagnetic interference shielding and lightning strike protection. To increase the conductivity of the carbon nanofiber paper, carbon nanofibers were modified with nickel nanostrands. d) Electrical actuation of SMP composites. Compared with other methods of SMP actuation, the use of electricity to induce the shape-memory effect of SMP is desirable due to the controllability and effectiveness. The electrical conductivity of carbon fiber reinforced SMP composites can be significantly improved by incorporating CNFs and CNF paper into them. A vision-based system was designed to control the deflection angle of SMP composites to desired values. The funding support from National Science Foundation and FAA Center of Excellence for Commercial Space Transportation (FAA COE CST) is acknowledged.

<p>The properties and microstructure of Ti-5Al-5V-5Mo-3Cr were characterized under various stress states after the following heat treatments: 1) annealing above the β transus, followed by cooling at various rates and ageing for different times; 2) solution heat treatment in the α-β range, fan-cooling and ageing for various temperatures and times.</p> <p>Heat treatment above the ptransus temperature causes complete recrystallization of the as-forged microstructure. The as-cooled microstructure consists of equiaxed β grains with an average grain size of 200μm. Water quenching from above the β transus results in precipitation of a dispersion of nano-sized ω phase; while the fan-cooled microstructure contains nano-sized ω and α precipitates. Ageing of the fan-cooled microstructure at 790°C or 600°C precipitates sub micron acicular α throughout the β grains. The tensile properties of this condition could not be determined using standard tensile specimens due to brittle failure at the grips.</p> <p>Controlled cooling from above the β transus to the ageing temperature at slower rates produces a coarser α+β microstructure. Acicular α laths are produced with cooling rates of 1°C/min, while lamellar α develops at cooling rates of 3.4°C/min. The β annealed and fan-cooled condition is characterized by relatively low strength (~850MPa) and low ductility (~6% elong.).The fracture mode is by intensely localized slip and the creation of transgranular cracks. Localization of slip is attributed to shearing of the nano-scale ω precipitates by dislocations. A linear relationship between the grain size, d⁻¹/² , and the yield and fracture stresses was established, as described by the Hall-Petch relation. With controlled cooling, the strength and ductility improve by precipitation of lamellar α within the β matrix. Improvements in ductility and strength are achieved by reducing the slip length.</p> <p>Solution heat treatment below the β transus and fan-cooling results in complete dissolution of the as-forged acicular α phase. Solutionizing at 50°C below the ptransus yields a volume fraction of 16.5% primary α in a matrix of retained β. Low angle grain boundaries and globular primary α, each measuring 2-4μm average in diameter, are uniformly distributed throughout the retained β matrix. The tensile strength in this condition is relatively low, i.e. (900MPa) and the ductility relatively high (~16% elong.). With ageing in the 500°C to 600°C temperature range, precipitation of α within the retained β begins within 5 minutes of the start of ageing. Precipitating is heterogeneously nucleated at dislocations and grain boundaries. The yield and ultimate tensile strengths reach values of roughly 1200 and 1300MPa, respectively, and remain relatively constant for up 48 hours ageing</p> <p> The fracture stresses for the solution treated condition and for material subsequently aged at 500°C and 600°C are quite similar in magnitude. This similarity is due to the fact that the fracture mechanism, which controls the fracture stress, is the same for all these conditions. The fracture mechanism for all the solution treated conditions begins with shear decohesion of the primary α/β interfaces.</p> <p>For each condition, the damage mechanisms and final fracture modes were evaluated and rationalized on the basis of microstructural features. The yield and fracture stresses for the various conditions were calculated and plotted on a two-principal stress axis coordinate system, thus creating the failure envelope for Ti-5553. For the β annealed and fan cooled and for the α-β solution heat treated and aged conditions the yield and fracture envelopes are two concentric ellipses in good agreement with the shear strain energy (van Mises) model for failure.</p> <p>The fracture toughness and stress corrosion cracking behaviour for the STA condition were evaluated and compared against other β titanium alloys.</p> / Thesis / Doctor of Philosophy (PhD)

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Dry High speed and wet milling strategies have both been used to machine hardened die and mold H13 tool steel (HRC 45-58). The TiAlCrSiYN-based family of PVD coatings prepared with various architectures (mono-, multi- and multilayer with an TiAlCrN interlayer) were studied to evaluate the coating micro-mechanical properties that affect tool life during dry high-speed milling of H13 tool steel. A systematic design of varying TiAlCrN interlayer thickness within a multilayer coating structure was developed and its influence on coating properties and cutting performance was investigated. A comprehensive characterization of the coatings was performed using a transmission electron microscope (TEM), focused ion beam (FIB), scanning electron microscope (SEM), X-ray powder diffraction (XRD), room-temperature nanoindentation, a nano-impact, ramped load scratch and a repetitive load wear test. The incorporation of an interlayer into the multilayer coating structure was found to increase the crack propagation resistance (CPRs) to 5.8 compared to 1.9 for the multilayer and 1.6 for the monolayer coatings, which resulted in a 60% tool life increase. The wear test at a load of 1.5 N showed that although the 500nm interlayer exhibited the best coating adhesion, a decline in the H3/E2 ratio was observed to worsen the machining performance. An approximate 40% increase in the tool life was achieved with the 300 nm interlayer by obtaining a balance between mechanical and adhesion properties. To investigate the tool performance during the wet milling of hardened tool steels, the (AlCrN-TiAlN) bi-layer PVD coating was post-treated by WPC (Wide Peening Cleaning) at various pressures and times. Fatigue resistance of the coating following the application of post treatment was observed to improve as the micro-mechanical characteristics (such as H3/E2 ratio, yield stress) were increased. A deterioration in the coating’s adhesion with increasing WPC pressure was also observed as measured by wear test applying a load of 1 N. Through experimentation a balance between fatigue resistance and adhesion was found with tool life being improved by 35% at a WPC applied pressure of 0.2 MPa. / Dissertation / Doctor of Philosophy (PhD) / Over the last 50 years, PVD (physical vapor deposition) coatings have played an increasingly important role in manufacturing where tool cost takes up 3% of the total expenses of the production process. Optimization of these coatings can expedite production wherever machining is conducted under extreme cutting conditions and consequent high material removal rates. These considerations assert significant importance on conducting research on PVD coating development specifically for milling of H13 tool steel, the material widely used in the mold and die industry. This research work seeks to enhance the micro-mechanical and adhesion properties of PVD coatings through architectural design and careful process development while relating desired properties to the high-performance milling of H13 tool steel.

Yes / Ultra-high performance concrete (UHPC) is an innovative cementitious composite, that has been widely applied in numerous structural projects because of its superior mechanical properties and durability. However, ensuring the safety of UHPC structures necessitates an urgent need for technology to continuously monitor and evaluate their condition during their extended periods of service. Self-sensing ultra-high performance concrete (SSUHPC) extends the functionality of UHPC system by integrating conductive fillers into the UHPC matrix, allowing it to address above demands with great potential and superiority. By measuring and analyzing the relationship between fraction change in resistivity (FCR) and external stimulates (force, stress, strain), SSUHPC can effectively monitor the crack initiation and propagation as well as damage events in UHPC structures, thus offering a promising pathway for structural health monitoring (SHM). Research on SSUHPC has attracted substantial interests from both academic and engineering practitioners in recent years, this paper aims to provide a comprehensive review on the state of the art of SSUHPC. It offers a detailed overview of material composition, mechanical properties and self-sensing capabilities, and the underlying mechanisms involved of SSUHPC with various functional fillers. Furthermore, based on the recent advancements in SSUHPC technology, the paper concludes that SSUHPC has superior self-sensing performance under tensile load but poor self-sensing performance under compressive load. The mechanical and self-sensing properties of UHPC are substantially dependent on the type and dosage of functional fillers. In addition, the practical engineering SHM application of SSUHPC, particularly in the context of large-scale structure, is met with certain challenges, such as environment effects on the response of SSUHPC. Therefore, it still requires further extensive investigation and empirical validation to bridge the gap between laboratory research and real engineering application of SSUHPC.