Results 1 - 10 of 341
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[en] The recent demand for high-performance vehicles requires the development of vehicle engines with improved output. As the engine becomes more advanced, the exhaust gas temperature increases due to an increase in the rate of heat generation. Thus, to ensure reliability in engine development, an analysis of changes in the mechanical characteristics of the engine exhaust components due to high temperature must be conducted. Especially, for an engine exhaust valve that is operated for long hours at high temperatures, the creep characteristics of its material must be determined. Ni-Cr NCF3015 steel with excellent high-temperature characteristics is under development as an engine valve material. Therefore, it is necessary to study the evaluation of the high-temperature creep characteristics of the material, considering a realistic operating environment. In this study, high-temperature creep tests were performed and the variations in creep rupture life with aging hours were analyzed by observing microstructural changes.
[en] In the nuclear reactor environment with the simultaneous presence of pressure, temperature and irradiation, creep resistant properties of materials is the second most important property after the nuclear properties. In this regard ordered phases are expected to perform better because long-range order structure is partially maintained in the radiation environment. The Zr3Al based materials have, therefore, been proposed for structural applications in the pressurized nuclear reactors. This paper reports the high temperature creep properties of the Zr3Al-Nb alloys
[en] One of today’s most commonly used test on a Dynamic Shear Rheometer (DSR) is the Multiple Stress Creep Recovery (MSCR) test. The test is described in the standard EN 16659, which is valid in the Czech Republic since October 2016. The principle of the test is based on repeated loading and recovering of a bitumen sample, according to which it is possible to determine the percentage of elastic recovery (R) and non-recoverable creep compliance (Jnr) of the bituminous binder. This method has been recently promoted as the most suitable test for assessing the resistance of bituminous binders to permanent deformation. The test is performed at higher temperatures and is particularly suitable for modified bituminous binders. The paper deals with the comparison of the different input parameters set on the DSR device - different levels of stress, temperature of test, the geometry of the measuring device and also a comparison of the results for a different number of loading cycles. The research study was focused mainly on modified bituminous binders, but to compare the MSCR test it is performed even with conventional paving grade binders. (paper)
[en] First experimental results on fluid injection influence on a mode of interblock contact deformation are reported. It is found that deformation energy consumed to radiate seismic waves largely depends on parameters of a fluid injected into a fracture. Dilatant fluid appears to be the most efficient to decrease seismic energy as viscosity of such fluid grows with increasing deformation rate. In laboratory tests the slip-stick motion mode transforms into a quasi-stable creep in a single deformation cycle after fluid injection is ceased.
[en] The high-temperature deformation process of the recrystallized 16CrODS ferritic steel was investigated at 1000 °C for the stress loading perpendicular to the elongated grain structure. The strain rate was varied in the range from 1.0 × 10−2 to 1.0 × 10−5 s−1. At the strain rate over 1.0 × 10−4 s−1, deformation is dominated by the conventional dislocation creep. Decreasing strain rate from 1.0 × 10−4 s−1, grain boundary sliding becomes prominent. Accommodation process for the localized stress induced by grain boundary sliding could be dislocation creep at 1.0 × 10−4 s−1, and by diffusional creep at 1.0 × 10−5 s−1 or less. These were verified through the observation of void formation and localized strain accumulation by KAM map.
[en] We propose an investigation combining numerical and analytical tools to estimate the ageing viscoelastic properties of cement-based materials within a multi-scale framework.With analytical homogenization the properties at the cement paste and mortar scales are estimated by a combination of Generalized Self-Consistent (GSC) and Mori-Tanaka (MT) schemes. With numerical homogenization the effective properties at the concrete scale are estimated. Numerical homogenization has the advantage of allowing assessing local information and to study more complex geometries. This combined strategy constitutes a promising tool to investigate how different mechanisms leading to ageing at the hydrated products scale, as well as other features of cement-based materials such as the Interfacial Transition Zone (ITZ), affect the viscoelastic behaviour at superior scales. In this context, we study the solidification of non-ageing constituents as the mechanisms leading to the ageing behaviour combined or not with a space-filling process in C-S-H. Relaxation and creep results are presented. (authors)
[en] Steam generators of the small modular Very High Temperature Reactor (VHTR) will require use of Alloy 800 and 2.25Cr-1Mo steel in different regions depending on temperature and corrosion resistance requirements. As a result, Dissimilar Metal Welds (DMWs) between these two materials are critical for design, development, and manufacturing of the VHTR. DMWs are also used in many other applications in the power generation and petrochemical industries. Experience has demonstrated that failures of such DMWs can occur prematurely well below the expected creep life of either base metal, and these failures have significant consequences in terms of plant reliability, safety, and economic impact. These DMWs failures are caused by the abrupt change in composition that occurs at the weld interface between the two alloys. In this research, the concept of developing and optimizing Graded Transition Joints (GTJ) was developed through an integrated modeling and experimental approach. With GTJs, the composition is gradually graded between the two alloys, thus eliminating the sharp change in composition that causes failure. Microstructural modeling was first conducted to identify the optimum grade length required for improved resistance to carbon diffusion that is largely responsible for premature failure. Next, GTJs were fabricated and characterized in detail to understand how the microstructure evolves during fabrication and long term aging. Finally, the creep behavior of the GTJs was investigated and compared to that of conventional DMWs to demonstrate the superior creep strength of the GTJs. The results of this research provide significant insight into design, fabrication, microstructural evolution, and resultant performance of GTJs that can be used for reducing or eliminating DMW failures in a number of high temperature applications. The results also demonstrate that the GTJs can be readily fabricated with either conventional or additive manufacturing processes, and detailed guidelines are provided on the length of grades needed for optimal performance. The results of this research benefits the general public by improving the reliability of power generation and petrochemical plants that rely on DMWs for safe and reliable operation to deliver power and other products.
[en] Highlights: • Experimental and theoretical studies on grid to rod fretting are reviewed. • The effects of geometry, wear, creep, turbulent flow and vibration were studied. • Priorities and recommendations for future studies are presented. - Abstract: This paper briefly summaries the recent developments of grid-to-rod fretting (GTRF) wear analyses, as well as the remaining challenges. Attention is given to the aspects of spring/dimple geometry design, fretting wear, turbulent flow, vibrations and material behaviors such as creep, swelling and corrosion. These factors are closely related to the loss of spring force and the wear damage of fuel claddings. Due to enormous number of publications for each factor, this brief review is not intended to be completely comprehensive. The goal is to understand how these factors affect the GTRF problem and give a big picture of it. Significant progress has been made in understanding each individual factor, which provided guidelines to the design of next generation fuel plant. However, state of art uncertainty analyses are needed to evaluate their contributions under the in-reactor conditions. Analyses to couple those factors together are challenging yet necessary to completely understand and eliminate the GTRF induced fuel failures.