Influence of Hydrogen Content on Axial Fracture Toughness Parameters of Zr-2.5Nb Pressure Tube Alloy in the Temperature Range of 306-573 K

Tubes fabricated from dilute Zr-alloys serve as miniature pressure vessels in Pressurized Heavy Water Reactors and are subjected to stress, aqueous corrosion and intense irradiation during service. Hydrogen evolved during the corrosion reaction may enter into the material and precipitate as hydride phase, which acquire platelet shaped morphology in Zr-alloys and are known to embrittle the host matrix. Since hydride embrittlement is a major life limiting factor for the components made from these alloys, several theoretical and experimental studies have been carried out to understand the influence of hydrogen/hydride on the mechanical properties in general and micromechanisms assisting crack nucleation and its propagation in the presence of hydride, in particular. For ductile materials like Zr-alloys, crack initiation follows void nucleation and its growth in the plastic zone. Nucleation of voids is associated with fracture of second phase particle or separation of matrix-precipitate interface. Hydrides are suspected to be fracture initiating sites in Zr-alloys and the presence of hydride platelets normal to tensile load significantly influences crack propagation. However, the role of hydrides in crack nucleation and its propagation and influence of temperature on the same has not been delineated clearly. In this work, influence of hydrogen and temperature on the axial fracture toughness parameters of Zr-2.5Nb pressure tube alloys is reported. The fracture toughness tests were carried out using 17 mm width curved compact tension specimens machined from gaseously hydrogen charged tube-sections and tested in the temperature range of 306 to 573 K. Metallography of the samples revealed that hydrides were predominantly oriented along axial-circumferential plane of the tube. The fracture toughness parameters like JQ, J0.15, JMax, J1.5, dJ/da, KJC and KMax were determined as per the ASTM standard E-813, with the crack length measured using direct current potential drop technique. The plane strain K values were computed from the corresponding J values. The critical crack length for catastrophic failure was determined using a numerical method, which is widely used in literature. It is observed that for a given test temperature both the fracture toughness parameters representing crack initiation, such as JQ, J0.15 and KJC and crack propagation, such as JMax, J1.5, and KMax, decrease mildly with increase in hydrogen content whereas mean dJ/da is practically unaffected by hydrogen content. Also, for a given hydrogen content crack initiation fracture toughness parameters showed large scatter with a tendency to decrease with increase in test temperature whereas the crack propagation fracture toughness parameters increased with temperature to a saturation value

Experimental evaluation of mode II fracture properties of Eucalyptus globulus L.

research projects BIA 2015-64491-P. UIDB/00667/2020 (UNIDEMI).Eucalyptus globulus Labill is a hardwood species of broad growth in temperate climates, which is receiving increasing interest for structural applications due to its high mechanical properties. Knowing the fracture behaviour is crucial to predict, through finite element models, the load carrying capacity of engineering designs with possibility of brittle failures such as elements with holes, notches, or certain types of joints. This behaviour can be adequately modelled on a macroscopic scale by the constitutive cohesive law. A direct identification of the cohesive law of Eucalyptus globulus L. in Mode II was performed by combining end-notched flexure (ENF) tests with digital image correlation (DIC) for radial-longitudinal crack propagation system. The critical strain energy release for this fracture mode, which represents the material toughness to crack-growth, was determined by applying the Compliance Based Beam Method (CBBM) as data reduction scheme and resulted in a mean value of 1.54 N/mm.publishersversionpublishe

Preparation and characterization of cogon grass natural fiber as a concrete filler for gamma radiation shielding

Concrete is a combination of cement, fine aggregate, coarse aggregate, and water. Concrete has a high shielding potential against gamma radiation from radioactive source. Concretes having a higher attenuation coefficient may be produced by varying the additives of various specific densities to increase the shielding performance. This study uses a cogon grass fiber, an invasive and unwanted grass due to their ability to grow, and disrupt desirable vegetation efficiently as a reinforcement material mixed into the concrete composite to observe its performance in radiation shielding. Natural fiber is known to have a tolerance to post splitting, high energy absorption and increased fatigue resistance of cement-based composites. Cogon grass fibers are use with different percentages of 0%, 0.5%, 1.0% and 1.5% fiber content with four different thicknesses. In this study, the chemical and physical properties of cogon grass were characterized by the hydrolysis process (Chesson Method) and Atomic Force Microscopy (AFM), respectively. The effect of different percentage of cogon grass fiber in concrete with different thickness to linear attenuation coefficient, the difference of linear attenuation coefficient value between lead concrete and cogon grass fiber concrete and half value layer (HVL) were also investigated. The possibility of using cogon grass natural fiber as a block of concrete for gamma radiation shielding was also evaluated based on the results obtained in this study. The results show that the hemicellulose, cellulose and lignin content of raw cogon grass fiber were 27.60%, 36.62% and 6.11%, respectively. AFM micrograph of cogon grass showed a blocky and rough surface. The calculated linear attenuation coefficient of cogon grass fiber showed an increase with the increase of fiber content and thickness. However, lead concrete showed a higher attenuation coefficient and lower HVL value compared to cogon grass fiber. Despite that, cogon grass fiber concrete may have the potential to be used as shielding material with some improvement

Inspection scheduling based onreliability updating of gas turbinewelded structures

This article presents a novel methodology for the inspection scheduling of gas turbine welded structures, based on reliability calculations and overhaul findings. The model was based on a probabilistic crack propagation analysis for welds in a plate and considered the uncertainty in material properties, defect inspection capabilities, weld geometry, and loads. It developed a specific stress intensity factor and an improved first-order reliability method. The proposed routine alleviated the computational cost of stochastic crack propagation analysis, with accuracy. It is useful to achieve an effective design for manufacturing, to develop structural health monitoring applications, and to adapt inspection schedules to airplane fleet experience.We are grateful to the Mechanical Technology Department of ITPAero (R) for supporting and helping us with this study. The invaluable guidance and feedback from Jose Ramon Andujar is recognized with great appreciation