When bulk helium-4 is cooled below T = 2.18 K, it undergoes a phase transition to a superfluid, characterized by a complex wave function with a macroscopic phase and exhibits inviscid, quantized flow. The macroscopic phase coherence can be probed in a container filled with helium-4, by reducing one or more of its dimensions until they are smaller than the coherence length, the spatial distance over which order propagates. As this dimensional reduction occurs, enhanced thermal and quantum fluctuations push the transition to the superfluid state to lower temperatures. However, this trend can be countered via the proximity effect, where a bulk 3-dimensional (3d) superfluid is coupled to a low (2d) dimensional superfluid via a weak link producing superfluid correlations in the film at temperatures above the Kosterlitz-Thouless temperature. Recent experiments probing the coupling between 3d and 2d superfluid helium-4 have uncovered an anomalously large proximity effect, leading to an enhanced superfluid density that cannot be explained using the correlation length alone. In this work, we have determined the origin of this enhanced proximity effect via large scale quantum Monte Carlo simulations of helium-4 in a topologically non-trivial geometry that incorporates the important aspects of the experiments. We find that due to the bosonic symmetry of helium-4, identical particle permutations lead to correlations between contiguous spatial regions at a length scale greater than the coherence length. We show that quantum exchange plays a large role in explaining the anomalous experimental results while simultaneously showing how classical arguments fall short of this task.

In this study, ABS Publication 115, “Guidance on Fatigue Assessment of Offshore Structures” is briefly reviewed. Emphasis is on the S-N curves based fatigue assessment approach of non-tubular joints, and both size and environment effects are also considered. Further, fatigue tests are performed to study the fatigue strength of load-carrying and non-load-carrying steel cruciform joints that represent typical joint types in marine structures. The experimental results are then compared against ABS fatigue assessment methods, based on nominal stress approach, which demonstrates a need for better fatigue evaluation parameter. A good fatigue parameter by definition should be consistent and should correlate the S-N data well. The equivalent structural stress parameter is introduced to investigate the fatigue behavior of welded joints using the traction based structural stress approach on finite element models of specimens, and representing the data as a single Master S-N curve.

In this study, the effect of ultrasonic treatment (UST) parameters such as amplitude, sonication time, and melt temperature on microstructure and microhardness of Al 6061 alloy is evaluated. The effect of UST on the dispersion of tungsten disulfide (WS2) and carbon nanotubes (CNT) as reinforcement particles in Al 6061 during casting is also studied. The cast Al 6061 with UST demonstrated 32% grain size reduction and 8% increase in the microhardness for optimum processing conditions. The cavitation process induced by UST is responsible for the refinement in microstructure and increase of hardness by enhancing the degassing and nucleation process. UST treated 6061 Al alloy demonstrated Hall-Petch relationship for all processing conditions. The UST process also aids in excellent dispersion of WS2 and CNT as reinforcement particles. UST treated WS2 and CNT reinforced Al 6061 composites displayed improved wear resistance as compared to samples without cavitation.

Fusion energy has been said to be the solution to all the world’s energy problems. The International Thermonuclear Experimental Reactor (ITER) is the flagship project to demonstrate the feasibility of fusion energy. The Central Solenoid (CS), an important component of the reactor, is needed to induce plasma current, initiate, ramp-up, ramp-down, and sustain plasma in a very controlled manner. In order to achieve this, the CS coil packs must be manufactured under controlled conditions. The CS conductor is an advanced cable-in-conduit Nb3Sn superconductor. The CS cable will be made in long continuous sections but with thousands of meter of cable needed, splices will have to be made in the field during construction of the ITER reactor. With the ends of the CS cable being exposed to the environment for an unspecified amount of time, concern has been expressed about the effect of the cable exposure on the quality of the splice. As a result an experimental program was devised to replicate and expedite the environmental damage the cable may see while in the field. The CS cable samples were exposed to 100% humidity at 60, 80, and 100oC for periods ranging from one week to four weeks. Once the samples were soaked for a period of time they were then sintered as would be done in the field. After sintering the mechanical tests were done to determine the load required to push the sintered strands out of the copper sleeve. Initial results obtained with samples having the sleeve thickness of 1.25mm (0.05in) were inconclusive due to the presence of a fold in the copper sleeve formed during the compaction of the sleeve around the cable. To prevent the fold formation, another set of samples were prepared with thicker copper sleeve of 5mm (0.20in). Results from these samples yielded data that was more conclusive and showed a possible correlation between aging temperature and sintering strength. The experimental data suggests that the thin oxide layer formed during the elevated temperature soak at 100% humidity may even be beneficial to the sinter quality.

Alkali Silica Reaction in concrete is a chemical deterioration process occurring between alkalis in cement paste and reactive aggregates. ASR increases expansion and cracking as well as other durability problems such as freezing and thawing. It is most probable that concrete structure will collapse unless mechanical, mineral, or chemical preventive measures are taken against ASR or this problem is realized and solved in the design stage of the concrete structure or later on. Rather than ordinary preventive measures in which mineral admixtures are used, mechanical ones were investigated in this study. In the experiment done by using the accelerated mortar bar method, reinforced concrete specimens on which pre-stressing force was applied were examined. The effects of reinforcement ratio and pre-stressing force on ASR based expansion and cracking were studied. Expansion and cracking developments in time were followed, and the connection between these phenomena and the energy produced by ASR was made. By applying the same mechanical preventive measures on the specimens prepared by using different reactive aggregates, the effectiveness of these methods with respect to the degree of v ASR was investigated. Also, the methods in question were compared with traditional preventive measures (fly ash). The investigation results show that reinforcement and pre-stressing force play a significant role in diminishing the effects of ASR.Keywords:Alkali-Silica Reaction, Reinforced Concrete Specimen, Pre-stressed Concrete, Mechanical Preventive Measures, Energy of ASR

In calcium aluminate cement (CAC) systems the hydration process is different than portland cement (PC) systems. The hydration products of CAC are subjected to conversion depending on temperature, moisture, water-cement ratio, cement content, etc. Consequently, strength of CAC system can be seriously reduced. However, presence of other inorganic binders or additives may alter the hydration process and improve various properties of CAC based composites. The objective of this study is to investigate the temperature effect on the behaviour of CAC based composite binders. Throughout this research, several combinations of CAC-PC, CAC-gypsum, CAC-lime, CAC-ground granulated blast furnace slag (CAC-GGBFS) were studied. These CAC based composite binders were subjected to seven different curing regimes and their strength developments were investigated up to 210 days. In addition, the mechanism of strength development was examined by XRD analyses performed at 28 and 210 days. Finally, some empirical relationships between strength-time-curing temperatures were formulated. Experimental results revealed that the increase in ambient temperature resulted in an increase in the rate of conversion, thereby causing drastic strength reduction, particularly in pure CAC mix. It has been observed that inclusion of small amount of PC, lime, and gypsum in CAC did not induce conversion-free CAC binary systems, rather they resulted in faster conversion by enabling rapid formation of stable C3AH6 instead of metastable, high strength inducing CAH10 and C2AH8. On the other hand, in CAC-GGBFS mixes, the formation of stable straetlingite (C2ASH8) instead of calcium aluminate hydrates hindered the conversion reactions. Therefore, CAC-GGBFS mixes, where GGBFS ratio was over 40%, did not exhibit strength loss due to conversion reactions that occurred in pure CAC systems.

Slurry infiltrated fiber concrete (SIFCON) was first produced in 1979 in the USA, by incorporating large amounts of steel fiber in molds to form very dense network of fibers. The network is then infiltrated by a fine liquid cement-based slurry or mortar. The steel fiber content can be as high as 30 % by volume. This percentage usually does not exceed 2 % in normal fiber reinforced concrete (FRC) for reasons related to mixing and workability. Due to its high fiber content, SIFCON demonstrates unique and superior mechanical properties in the areas of both strength and ductility. Most of previous research work on SIFCON has focused mainly on investigating the mechanical properties of this material. On the other hand, the studies carried out in the field of durability of SIFCON are quite limited. v Therefore, it seemed that it would be worth to study the various durability aspects of SIFCON. In view of the above, the objectives of this study are to investigate and provide information about durability of SIFCON, mainly permeability, resistance to chloride penetration, freezing and thawing and drying shrinkage. This information will help in providing the necessary database and knowledge about the ability of SIFCON to withstand the conditions for which it has been designed without deterioration, especially when it is intended to be used in aggressive environments The investigations included studying the effects of the following on durability of SIFCON: (i) matrix type (slurry or mortar), (ii) fiber contents (7%, 9.5%, and 12% by volume), and (iii) steel fiber geometry (hooked or crimped). The results obtained indicated that SIFCON, especially when prepared using mortar not slurry, has shown good durability characteristics in spite of its apparent high water absorption. The SIFCON made with the highest possible fiber volume fractions showed the best results. However, it was concluded that SIFCON needs to be protected with suitable low permeability overlays to ensure ideal improved performance by protecting the steel fibers exposed on the surfaces especially against chloride attack.

A combinatorial study was carried out for hydrogen storage alloys which involve processes similar to those normally used in their fabrication. The study utilized a single sample of combined elemental (or compound) powders which were milled and consolidated into a bulk form and subsequently deformed to heavy strains. Material library was obtained in a post annealing treatment carried out at elevated temperatures which brings about solid state reactions between the powders yielding equilibrium phases in the respective alloy system. A sample comprising the material library was then pulverized and screened for hydrogen storage composition. X-ray diffraction was used as a screening tool, the sample having been examined both in as-processed and hydrogenated state. The method was successfully applied to Mg-Ni, and Mg-Ni-Ti yielding the well known Mg2Ni as the storage composition. It is concluded that partitioning of the alloy system into regions of similar solidus temperature would be required to enrich the material library.

Self consolidating concrete (SCC) is defined as an innovative concrete that does not require vibration for placing and compaction and it is able to flow under its own weight, completely filling formwork and achieving full compaction. Although significant amount of research has been carried out regarding the fresh properties, mix design, placing methods and strength of various SCC mixes, only a very limited amount of work has been done to assess the durability performance of SCC. Concretes in cold climates are subjected to freeze-thaw cycles which are one of the major durability problems, and if the concrete is in a saturated or nearly saturated condition, those cycles lead to expansion of the water in the capillary pores of concrete causing great internal stresses. For a durable concrete subjected to freeze-thaw cycles, an adequate air void system is obtained by using air-entraining admixtures. The performance of the air void system is characterized by air void parameters that are determined using microscopical examination of the concrete microstructure. In this thesis a software tool, based on image analysis of concrete surface, is developed to evaluate the air void parameters of concrete using both American and European standards. Later on, an experimental program is conducted to evaluate the effect of freezing-thawing on self consolidating concrete that contain different percentages of fly ash (FA) and air entraining agents. For this purpose, a total of ten self consolidating concrete mixtures that contain four different contents of fly ash, and three different levels of air entrainment were prepared. During the casting operation, the workability properties of SCCs were observed through slump flow time and diameter, air content, V-funnel flow time, L-box height ratio, and segregation ratio. Hardened properties were evaluated by compressive strength, permeability tests (water absorption, sorptivity and rapid chloride permeability test), freezing-thawing test, resonant frequency test, ultrasonic pulse velocity test. The developed tool was used to characterize and evaluate the effects of air void parameters of SCC on its resistance to freeze-thaw cycles. At the end of this experimental investigation, it was concluded that the addition of air entraining agent increased the flowability and an increase in the fly ash content decreased the effect of air entraining agent. On the other hand, during image processing, it was observed that the surface preparation procedures have a crucial effect on processing quality. Moreover, spacing factor -which is the most important air void characteristic that is utilized for determination of the resistance to freezing-thawing- should not be restricted to 0.2 mm for SCC, since SCCs with spacing factors smaller than 0.4 mm could still exhibit good freezethaw resistance.

The measurement and monitoring of the elastic properties of cement-based materials is very important for assessing their quality, integrity and performance. Due to the nonhomogeneous and time-dependent characteristics of these materials, it is difficult to observe the developments in elastic properties with traditional destructive methods. The aim of this thesis is to determine and monitor elastic properties of mortar specimens made with different cements by using resonant frequency and ultrasonic pulse velocity test methods, and to obtain relationships between these elastic properties. For this purpose, eight different cement mortar mixtures were prepared with different constituent CEM I cements. Dynamic elastic moduli, static elastic moduli, dynamic Poisson&rsquo / s ratio and strength of these mixtures were observed for different ages. The relationships between these elastic properties are determined and the results obtained from two different nondestructive test methods are compared. Although nondestructive tests made it possible to obtain elastic properties of mortar mixtures, the results revealed that it is very difficult to develop a single relationship between different elastic properties of mortars with varying mixture proportions. This situation is mainly due to the anisotropy and nonlinear behavior of the mortar and the difficulty of describing the actual behavior of mortar by formulations defined for perfectly elastic materials.