Fracture Toughness of Thin Plates by the Double-Torsion Test Method

Double torsion testing can produce fracture toughness values without crack length measurement that are comparable to those measured via standardized techniques such as the chevron-notch, surface-crack-in-flexure and precracked beam if the appropriate geometry is employed, and the material does not exhibit increasing crack growth resistance. Results to date indicate that 8 2 are required if crack length is not considered in stress intensity calculations. At L/W = 2, the normalized crack length should be 0.35 80) nonlinear effects were encountered

Evaluation of Alternative Sources of Supplementary Cementitious Materials (SCMs) for Concrete Materials in Transportation Infrastructure

Fly ash is the most utilized supplementary cementitious material (SCM) in the US. Nonetheless, rapid decline in coal-fired power generation threatens its supply. The objective of this study was to evaluate alternative SCMs for concrete transportation infrastructure in Region 6. SCMs investigated included reclaimed fly ash (RFA), reclaimed ground bottom ash (GBA), metakaolin (MK), and conventional Class F fly ash (FA) as a reference. SCMs were characterized and the fresh and hardened properties of concrete incorporating different dosages (i.e., 10, 20, and 30% cement replacement by mass) of the individual SCMs (i.e., binary systems) and blended SCM systems of RFA-MK and GBA-MK (i.e., ternary systems) were assessed. All the coal ashes met the requirements for pozzolanic component, CaO, SO3, moisture content, LOI, SAI, and water requirement to be classified as Class F pozzolan according to ASTM C618. MK met all the but the water requirement. Concrete using FA generally exhibited better workability than the control mixture (i.e., without SCMs), whereas concrete incorporating RFA, GBA, and MK presented decrements in workability. Mixtures implementing ternary systems also displayed decrements in workability. Air content of fresh concrete mixtures incorporating binary and ternary systems generally decreased. Relative to the control mixture, decrements in 28-day compressive strength (f’c) were reported when incorporating FA and RFA, yet this was generally not the case for the 90-day f’c. In the case of GBA mixtures, significant differences in f’c were not observed after 28 days nor 90 days. MK mixtures as well as RFA-MK and GBA-MK mixtures generally presented increments in 28-day and 90-day f’c. Concrete mixtures implementing coal ashes did not produce significant differences in 28-day surface resistivity (SR) at any cement replacement levels; yet after 90 days of curing, significant improvements in SR were reported. MK, RFA-MK, and GBA-MK mixtures exhibited significant increments in SR at all dosages after 28 and 90 days of curing. Notably, while the control mixture and mixtures incorporating coal ashes did not meet the 28-day SR requirement for class A1 concrete according to LaDOTD, mixtures implementing MK and ternary systems did in almost all cases. All SCMs were effective at reducing drying shrinkage. Binary systems reduced drying shrinkage by 24.2-69.1%, whereas ternary systems reduced drying shrinkage by 55.2-75.3%. With regards ASR, mixtures implementing SCMs presented significantly lower expansion and the increment in SCMs content further reduced the expansion; thus, signaling a positive effect in suppressing ASR related expansion, specially at high dosages

Evaluation of Sustainable and Environmentally Friendly Stabilization of Cohesionless Sandy Soil for Transportation Infrastructure

Ordinary Portland cement (OPC) is generally used to stabilize cohesionless sandy soils that are often found in coastal areas. Due to its high carbon footprint, many studies are being conducted to identify a suitable green alternative for stabilizing cohesionless soils. Previous studies have shown that partially replacing OPC with waste materials such as nano-silica and coal waste reduces the overall carbon footprint without significantly impacting the performance. Geopolymer (GP) received a lot of attention in the past few decades owing to its similar properties to that of OPC yet with a lower carbon footprint. This study investigated the feasibility of stabilizing cohesionless sandy soils with metakaolin-based GP. Engineering and characterization tests such as shrinkage, strength, pH, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) were performed to evaluate various characteristics of the stabilized mixes with different dosages of geopolymer and relate them to microstructural changes. Notably, GP-treated soils did not deteriorate during the durability tests, whereas the OPC-treated soil only retained about 75% of its strength. This is an indication that GP could be a better choice than OPC in coastal areas where cohesionless soils often experience heavy rainfall and flooding. Overall, an optimum dosage of GP improved both the mechanical properties and durability of cohesionless soils

Phase-Field Model of Silicon Carbide Growth During Isothermal Condition

Silicon carbide (SiC) emerges as a promising ceramic material for high-temperature structural applications, especially within the aerospace sector. The utilization of SiC-based ceramic matrix composites (CMCs) instead of superalloys in components like engine shrouds, combustors, and nozzles offers notable advantages, including a 25% improvement in fuel efficiency, over 10% enhanced thrust, and the capability to withstand up to 500$^{\circ}$C higher operating temperatures. Employing a CALPHAD-reinforced multi-phase-field model, our study delves into the evolution of the SiC layer under isothermal solidification conditions. By modeling the growth of SiC between liquid Si and solid C at 1450$^{\circ}$C, we compared results with experimental microstructures and quantitatively examined the evolution of SiC thickness over time. Efficient sampling across the entire model space mitigated uncertainty in high-temperature kinetic parameters, allowing us to predict a range of growth rates and morphologies for the SiC layer. The model accounts for parameter uncertainty stemming from limited experimental knowledge and successfully predicts relevant morphologies for the system. Experimental results validated the kinetic parameters of the simulations, offering valuable insights and potential constraints on the reaction kinetics. We further explored the significance of multi-phase-field model parameters on two key outputs, and found that the diffusion coefficient of liquid Si emerges as the most crucial parameter significantly impacting the SiC average layer thickness and grain count over time. This study provides valuable insights into the microstructure evolution of the Si-C binary system, offering pertinent information for the engineering of CMCs in industrial applications

Development of Alternative Stabilization Methods for Transportation Infrastructure Based on Geopolymers

Current soil stabilization methods are often limited by durability and leaching issues and do not always offer sustainable treatments. This research explores the use of geopolymers to stabilize clays in the North Texas area. In recent years, geopolymer has received much attention as an eco-friendly and sustainable alternative to conventional chemical additives, since it can be processed at room temperature from aqueous solutions by utilizing waste materials and/or abounded natural sources. Two subgrade soils from North Texas were treated with GP mix at a ratio of 8 wt% dry GP to dry soil. GP is shown to reduce swelling and shrinkage potential of soil considerably while an increase in unconfined compressive strength is observed as well. Therefore, further studies are recommended to understand the mechanism of GP and soil bonding resulting in said changes

High-Performance Metal/Carbide Composites with Far-From-Equilibrium Compositions and Controlled Microstructures

The prospect of extending existing metal-ceramic composites to those with the compositions that are far from thermodynamic equilibrium is examined. A current and pressure-assisted, rapid infiltration is proposed to fabricate composites, consisting of reactive metallic and ceramic phases with controlled microstructure and tunable properties. An aluminum (Al) alloy/Ti(2)AlC composite is selected as an example of the far-from-equilibrium systems to fabricate, because Ti(2)AlC exists only in a narrow region of the Ti-Al-C phase diagram and readily reacts with Al. This kind of reactive systems challenges conventional methods for successfully processing corresponding metal-ceramic composites. Al alloy/Ti(2)AlC composites with controlled microstructures, various volume ratios of constituents (40/60 and 27/73) and metallic phase sizes (42–83 μm, 77–276 μm, and 167–545 μm), are obtained using the Ti(2)AlC foams with different pore structures as preforms for molten metal (Al alloy) infiltration. The resulting composites are lightweight and display exceptional mechanical properties at both ambient and elevated temperatures. These structures achieve a compressive strength that is 10 times higher than the yield strength of the corresponding peak-aged Al alloy at ambient temperature and 14 times higher at 400 °C. Possible strengthening mechanisms are described, and further strategies for improving properties of those composites are proposed