This paper describes the analytical work done by modelling and evaluating a thermal shock in a WWER-440 reactor pressure vessel due to an emergency case. An axial oriented semielliptical underclad/surface crack is assumed to be located in the core weld line. Threedimensional finite element models are used to compute the global transient temperature and stress-strain fields. By using a three-dimensional submodel, which includes the crack, the local crack stress-strain field is obtained. With a subsequent postprocessing using the j-integral technique the stress intensity factors KI along the crack front are obtained. The results for the underclad and surface crack are provided and compared, together with a critical discussion of the VERLIFE code.

The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfil ASTM 1820 accuracy requirements. Therefore some investigations have been performed to resolve this problem. In those studies it was considered that measured compliance should be corrected for various factors, but satisfying results were not attained. In the presented work the problem was attacked from the other side, the measured specimen compliance was taken as a correct value and what had to be adjusted was the calculation procedure. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths the investigation was carried out. Finally, a calculation procedure enabling accurate crack length calculation up to 5mm of plastic deflection was developed. Applying the new procedure, out of investigated 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirements, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for the specimens of different than Charpy size.

Asphalt mixtures or asphalt concrete are used to pave about 93% of about 2.6 million miles paved roads and highways in the US. Asphalt concrete is a composite of aggregates and asphalt binder; asphalt binder works as a glue to bind the aggregate particles. The mechanical response of the asphalt binder is dependent on the time/rate of loading, temperature and age. An asphalt concrete mixture inherits most of these characteristics from the asphalt binder. Also the asphalt binder plays a critical role in providing the asphalt concrete the ability to resist tensile stresses and relaxing thermally induced stresses that can lead to fatigue and low temperature cracking, respectively. Hence, it is very important (but not sufficient) to ensure that asphalt binders used in the production of asphalt concrete are inherently resistant to cracking, rutting and other distresses that a pavement may undergo. Current binder specification (AASHTO M-320) to evaluate its fatigue cracking is based on the stiffness of the binder and not on its tensile strength. Also, measurements following current specifications are made on test specimens subjected to a uniaxial mode of loading that does not produce the same stress state in the binder as in the case of asphalt concrete. Another challenge in being able to produce binders with inherently superior performing characteristics is the fact that the asphalt binders produced in a refinery do not have a consistent chemical composition. The chemical composition of asphalt binder depends on the source and refining process of crude oil. There is a need to better quantify the tensile strength of asphalt binder and understand the relationship between the chemical composition of asphalt binders and its mechanical properties. The knowledge from this study can be used to engineer asphalt binders that have superior performance characteristics. The objective of this research was to quantify the tensile strength of asphalt binder, develop a metric for the tensile strength and identify the relationship between chemical composition and mechanical properties of asphalt binder. Laboratory tests were performed on binders of different grades using a poker chip geometry to simulate confined state by varying the film thickness, rate of loading and modes of loading. The chemical properties of asphalt binder were studied based on SARA fractionation. The findings from this research showed that the modified correspondence principles can unify and explain the rate and mode dependency of asphalt binder. This study also quantified the relationship between chemical composition, and rheological and mechanical properties of asphalt binder. Finally, a composite model was developed based on the individual properties of chemical fractions which could predict the dynamic modulus of the asphaltenes doped and resins doped binder. / text

Material property heterogeneity is present ubiquitously in various natural and man-made materials, such as bones, seashells, rocks, concrete, composites, and functionally graded materials. A fundamental understanding of the structure-property relationships in these material systems is crucial for the development of advanced materials with extreme properties. Well-developed homogenization schemes exist to establish such relationships in elasticity, electrostatics, magnetism, and other time- or history-independent material properties. Nevertheless, one’s understanding of the effective fracture properties of heterogeneous media is remarkably limited. The challenge here is that heterogeneous fracture, as a history-dependent process, involves complex interaction and negotiation of a discontinuity front with local heterogeneities. The determination of effective fracture properties necessitates a critical interrogation of this evolutionary process in detail. In this work, a combined experimental and modeling effort is made to examine and control fracture mechanisms in heterogeneous elastic solids. A two-phase laminated composite, which mimics the key microstructural features of many tough biological materials, is selected as a model material. In the computational part of this work, finite element analysis with cohesive zone modeling is used to model crack propagation and arrest in the laminated direction. A crack-tip-opening controlled algorithm is implemented to overcome the instability problems associated with inherently unstable crack growth. Computational results indicate that the mismatch of elastic modulus is an important factor in determining the fracture behaviors of the heterogeneous model material. Significant enhancement in the material’s effective fracture toughness can be achieved with appropriate modulus mismatch. Systematic parametric studies are also performed to investigate the effects of various material and geometrical parameters, including modulus mismatch ratio, phase volume fractions, T-stress, and cohesive zone size. Concurrently, a novel stereolithography-based additive manufacturing system is developed and used for fabricating heterogeneous test specimens with well-controlled structural and material properties. Fracture testing of each specimen is performed using the tapered double-cantilever beam (TDCB) test method. With optimized material and geometrical parameters, heterogeneous TDCB specimens are found to exhibit higher fracture toughness than their homogenous counterparts, which is in good agreement with the computational predictions. The integrative computational and experimental study presented here provides a fundamental mechanistic understanding of the fracture mechanisms in brittle heterogeneous materials and sheds light on the rational design of ultra-tough materials through patterned heterogeneities.

Titanium based alloys reinforced uniaxially with silicon carbide fibres (Ti/SiC) are advanced and innovative materials for aerospace vehicles. To avoid potential problems, these new materials should be extensively tested and analyzed before application. This research focuses on experimental fracture toughness study on 0.5 mm thick Ti/SiC composite materials for aerospace applications. The fracture toughness tests are mainly based on BS 7448 with some modifications for transversely isotropic behaviour of the composite materials. By loading on specimens in the direction perpendicular to the fibre axis, three critical values of fracture toughness parameters characterizing fracture resistance of material, plane strain fracture toughness [Plane strain fracture toughness }, critical crack tip opening displacement [Critical crack tip opening displacement ] and critical J-integral [Critical at the onset of brittle crack extension or pop-in when Δa is less than 0.2 mm. ]are measured for two kinds of titanium alloy specimens and three kinds of Ti/SiC composites specimens. The values of [Provisional value of Plane strain fracture toughness ] obtained from the fracture toughness tests are not valid [Plane strain fracture toughness ] for these materials, since the thickness of specimens is insufficient to satisfy the minimum thickness criterion; however, the results could be used as particular critical fracture toughness parameter for 0.5 mm thick structures of the materials. The valid values of [Critical J at the onset of brittle crack extension or pop-in when Δa is less than 0.2 mm. ] and [Critical crack tip opening displacement ] could be used as fracture toughness parameters for all thickness of structures of the materials. The results also show that: fracture toughness of the titanium alloys decreases dramatically after being unidirectional reinforced with SiC fibre, which is mainly triggered by poor fibre/matrix bonding condition. Moreover, Ti-Al3-V2.5 reinforced with 25% volume fraction SiC fibre performs better than the other two composites in fracture resistance.

The primary focus of this research is to evaluate the time-dependent crack growth in rocks using lab tests and numerical modeling and its application to geologic hazard problems. This research utilized Coconino sandstone and Columbia granite as the study materials and produced the subcritical crack growth parameters in both mode I and II loadings using the rock materials. The mode I loading test employs three different types of fracture mechanics tests: the Double Torsion (DT), the Wedge Splitting (WS), and the Double Cantilever Beam (DCB) test. Each test measured the mode I crack velocity. The DT test indirectly measured the crack velocity using the load relaxation method. The WS and DCB tests directly measured the crack velocity by monitoring using a video recording. The different mode I subcritical crack growth parameters obtained from the three tests are discussed. For the mode II loading test, this study developed a new shear fracture toughness test called the modified Punch-Through Shear (MPTS). The MPTS test conducted at different loading rates produced the mode II subcritical crack growth parameters. These fracture mechanics tests were calibrated and simulated using the distinct element method (DEM) and the finite element method (FEM). DEM analysis employed the particle flow code (PFC) to simulate the mixed mode crack growth and to match with the failure strength envelop of the triaxial compressive tests. FEM analysis employed the Phase2 program to analyze the crack tip stress distribution and the FRANC2D program to calculate the modes I and II stress intensity factors. The fracture mechanics tests and numerical modeling showed well the dependency of the mode II subcritical crack growth parameters according to confining pressure, loading rate, and the mode II fracture toughness. Finally, the UDEC modeling based on DEM is utilized in this study to forecast the long-term stability of the Coconino rock slope, as one of geologic hazards. The fracture mechanics approach is implemented in the program using the modes I and II subcritical crack growth parameters obtained from the lab tests and numerical modeling. Considering the progressive failure of rock bridges due to subcritical crack growth, the UDEC results predicted the stable condition of the Coconino rock cliff over 10,000 years. This result was validated by comparing it with the previous planar failure case.

The structure-property relationships of epoxy nanocomposites with inorganic layer-structure nanofillers have been studied to obtain the fundamental understanding of the role of nanofillers and the physics of polymer nanocomposites in this dissertation. Several polymer nanocomposite systems with modified montmorillonite (MMT) or α-zirconium phosphate (ZrP) nanofillers were prepared with epoxy matrices of different ductility and properties. The successful nanofiller's exfoliations were confirmed with X-ray diffraction and transmision electronic microscopy (TEM). Dynamic mechanical analysis (DMA) on the prepared epoxy nanocomposites revealed the significant increase in rubbery plateau moduli of the epoxy nanocomposite systems above Tg, as high as 4.5 times, and tensile test results showed improved modulus by the nanofiller addition, while the fracture toughenss was not affected or slightly decreased by nanofillers. The brittle epoxy nanocomposite systems were toughened with core shell rubber (CSR) particles and showed remarkable increase in fracture toughness (KIC) value up to 270%. The CSR toughening is more effective at ductile matrices, and TEM observation indicates that major toughening mechanisms induced by the CSR addition involve a large scale CSR cavitation, followed by massive shear deformation of the matrix.

Carbon Capture and Storage (CCS) is recognised as having a significant role to play in tackling climate change and reducing carbon dioxide (CO2) emissions. In CCS schemes, CO2 is captured from anthropogenic sources, and transported to suitable sites either for EOR (Enhanced Oil Recovery) or storage. The transport of such huge amount of CO2 causes new challenges. The main concern is in the difference between natural gas and CO2 transportation pipelines. CO2 phase behaviour during decompression, existence of different impurities and very high operating pressure are some of the new challenges for pipeline designer and operators. This PhD study has taken a systematic approach to understand the mechanics involved in the fracture of pipes containing high pressure flue-gas CO2. The work involved the development of a novel weight function stress intensity factor solution that can be used with complex stress fields induced by residual and/or thermal stresses in addition to applied pressure. In addition, the thesis reports a substantial experimented test programme which involved low temperature fracture toughness tests linked to a detailed finite element based stress analysis. Overall, the thesis presents an integrated engineering criticality means to assess the suitability or otherwise of a pipeline system to transport high pressure flue-gas CO2.

ABSTRACT DETERMINATION OF MECHANICAL PROPERTIES OF HYBRID FIBER REINFORCED CONCRETE Yurtseven, Alp Eren M.Sc. Department of Civil Engineering Supervisor: Prof. Dr. Mustafa Tokyay Co-Supervisor: Asst. Prof. Dr. . &Ouml / zg&uuml / r Yaman August 2004, 82 pages Fiber reinforcement is commonly used to provide toughness and ductility to brittle cementitious matrices. Reinforcement of concrete with a single type of fiber may improve the desired properties to a limited level. A composite is termed as hybrid, if two or more types of fibers are rationally combined to produce a composite that derives benefits from each of the individual fibers and exhibits a synergetic response. This study aims to characterize and quantify the mechanical properties of hybrid fiber reinforced concrete. For this purpose nine mixes, one plain control mix and eight fiber reinforced mixes were prepared. Six of the mixes were reinforced in a hybrid form. Four different types of fibers were used in combination, two of which were macro steel fibers, and the other two were micro fibers. Volume percentage of fiber inclusion was kept constant at 1.5%. In hybrid reinforced mixes volume percentage of macro fibers was 1.0% whereas the remaining fiber inclusion was v composed of micro fibers. Slump test was carried out for each mix in the fresh state. 28-day compressive strength, flexural tensile strength, flexural toughness, and impact resistance tests were performed in the hardened state. Various numerical analyses were carried out to quantify the determined mechanical properties and to describe the effects of fiber inclusion on these mechanical properties. Keywords: Fiber Reinforcement, Hybrid Composite, Toughness, Impact Resistance

ABSTRACT PRODUCTION AND CHARACTERIZATION OF RESOL TYPE PHENOLIC RESIN / LAYERED SILICATE NANOCOMPOSITES TaSan, Cemal Cem M.S., Department of Metallurgical and Materials Engineering Supervisor: Assoc.Prof. Cevdet Kaynak April 2005 133 Pages Polymer / layered silicate (P/LS) nanocomposites belong to one of the most promising group of materials of the past few decades and most probably for the near future. Combining two of the most widely studied topics of material science: composite materials and nanotechnology / P/LS research have drawn great attention starting with the pioneering works of Toyota Research Group in 1980&rsquo / s. The research is now being carried out world wide / since the excellent properties of these new materials, which is achieved by using very low amounts of a cheap reinforcement material (clay), increases the interest on these materials everyday after. In this present study, the object was to investigate the production parameters of phenol formaldehyde based layered silicate nanocomposites. For this purpose, 14 different specimen groups were produced / using two different resol type phenolic resins (PF76 and PF76TD) as the matrix / and 9 different montmorillonite clays (Rheospan, Resadiye, Cloisite Na+, 10A, 15A, 20A, 25A, 30B, 94A) as the reinforcement phase. Initially the curing schedules for the available resins were experimentally determined. Then, a short and effective mixing procedure for the thermosetting resin and the montmorillonite clay was developed. The effects of several processing parameters / such as clay type, clay source, clay content, clay modification, resin type, resin cure type, cure cycle and mixing cycle were determined by X-ray Diffraction, Scanning Electron Microscopy and Mechanical Tests. Then, Transmission Electron Microscopy was used to investigate the level of intercalation and/or exfoliation of the layered silicates. Finally, Differential Scanning Calorimetry was also carried out to analyse thermal properties of the specimens. It was concluded that, a partially intercalated and/or exfoliated structure could be obtained in resol type phenolic resin based systems at very low clay contents (such as 0,5%) leading to remarkable increases in mechanical properties (e.g. 66% increase in fracture toughness).