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[en] This study presents a series of 4-point bending tests performed to describe the delayed behavior of unreinforced pre-cracked beams under low, moderate and high sustained loading levels. The deflection creep rate, the failure time and the load level were assessed. A linear relation, in a semi-log scale, was found for the deflection creep rate at high load levels. In addition, a linear relation, in a log–log scale, between the secondary deflection creep rate and failure time was observed. Besides, it was shown that the secondary creep deflection rate increases with the sustained loading level and the macrocrack propagation rate when macrocrack propagation occurs during the sustained loading. Physical mechanisms are proposed to explain these results and may be summarized as follows: the delayed behavior of an unreinforced cracked concrete specimen under sustained loading is mainly due to the cracking evolution, thus the creation of microcracks and/or the propagation of a macrocrack
[en] This article describes the key design principles and application of a mini-bellows loaded irradiation creep frame technology developed for use in the high flux isotope reactor (HFIR). For this irradiation vehicle, the bellows, frame, sample, and temperature monitor are contained within a hydraulic or fixed “rabbit” capsule of a few inches in length. Of critical importance and key to this technology is the viability and stability of the metallic bellows under the elevated temperature irradiation environment. Conceptual design and supporting analysis have been performed for tension and compression specimens. Benchtop verification has substantiated the modeling regarding the ability of the bellows to produce sufficient stress to induce irradiation creep in subsize specimens. Discussion focuses on the possible stress ranges in specimens induced by the miniature gas-pressurized bellows and the limitations imposed by the size and structure of thin-walled bellows. A brief discussion of pre- and post-irradiation measurement of the integrity of load frames irradiated to 4.4 × 1025 n/m2 (E > 0.1 MeV) is presented. Following this protocol, the pre-irradiation loading to a sample is determined and post-irradiation loading inferred
[en] There is a trend towards the progressive use of higher operating temperatures and stresses to achieve improved efficiencies in power-generation equipment. It is important to perform the crack assessment under high temperature and high-pressure conditions. The C(t)-integral is a key parameter in crack assessment for transient creep states. The estimation of the C(t)-integral is complex when considering the mechanical and thermal loads simultaneously. In this paper, we study estimation of C(t)-integral under combined mechanical and thermal load depending on loading conditions
[en] The article Statistical filtering of useful concrete creep data from imperfect laboratory tests, written by Mohammad Rasoolinejad, Saeed Rahimi-Aghdam, Zdenek P. Bazant, was originally published online without Open Access. After publication in volume 51, article ID 153 RILEM decided to grant the author to opt for open choice and to make the article an open-access publication.
[en] Creep rupture life and microstructural degradation have been studied in two heats of Gr.91 steels. The coarsening of subgrains and precipitates, mainly M23C6 and MX, has been evaluated during static aging and creep. Hardness of head (static aging) and gauge (creep) portions of crept samples were measured to know their relation with microstructural degradation during long-term exposure. The correlation between subgrain size and spacing of precipitates and hardness has been equated. As an example, there is a close correlation between hardness value and inverse subgrains size in Gr.91 steels, regardless of aging or creep conditions. The appearance of three recovery mechanisms was found during long-term creep, namely: strain-induced recovery, pure static recovery and strain-assisted static recovery. By equated correlations between subgrain size, precipitates and hardness, the contribution of three recovery mechanisms to subgrain coarsening and hardness drop were calculated for two creep conditions at 700 °C in long-term creep region, where breakdown of creep strength has happen. The calculated data accord well with experimental data obtained from aged and crept samples. The contribution of static recovery to the subgrain coarsening and consequent hardness drop during long-term creep increases with increasing creep time. The significant contribution of both static recovery mechanisms can result in the breakdown of creep strength in long-term creep region
[en] This objective of this work was to develop an experimental facility that can perform in situ high temperature proton irradiation-induced creep experiments on a range of materials. This was achieved by designing an irradiation chamber and stage that allows for load application and removal, provides a method for controlling and monitoring temperature and proton flux, and a means to make in situ measurement of dimensional change of the samples during the experiment. Initial experiments on POCO Graphite Inc. ZXF-5Q grade ultra-fine grain samples irradiated at 1000 °C at a damage rate of 1.15 × 10−6 dpa/s exhibited a linear dependence of measured creep rate on applied stress over a range of stresses from 10 MPa to 40 MPa
[en] We have studied the dynamics of propagation in one dimension of a magnetic domain wall (DW) in the presence of an exchange bias field in nanowires patterned in two-dimensional ultra-thin perpendicularly magnetized ferromagnetic–antiferromagnetic (F–AF) films. The DW velocity v(H) is strongly reduced when the DW propagates against the exchange bias field, and its field dependence is consistent with a creep process with a critical exponent μ = 1/4. The increase in energy barrier responsible for the reduction of v(H) cannot be explained by the introduction of a macroscopic exchange bias field. Instead it arises from the AF–F interface that provides an extra-source of random pinning. (paper)
[en] The concept of a dual-cooled fuel was developed to achieve low temperature operation along with high power density. Dual-cooled fuel consists of two claddings and annular pellets, and is designed to provide double coolant channels. Korea Atomic Energy Research Institute (KAERI) is doing the R and D project for the development of dual-cooled fuel technology. There are two kinds of tubes, i.e. an inner and outer cladding tube, which would deform in reverse directions by the creep process. The creep rates are far different between the opposed creep modes. Also, differences in gap closure for the two claddings can be expected, which affect the heat split performance directly. Therefore, the creep should be analyzed for the safe design of dual-cooled fuel. In this paper, creep deformations were estimated based on the experimental and analytical methods for the dual-cooled cladding tubes
[en] It is desired to accurately predict creep strains in zirconium claddings, subjected to cases such as sudden load drops or reversals, using the BISON fuel performance code. It has been experimentally proven that primary creep reinitializes at sudden load drops and reversals, which is different than normal operating conditions, where stresses increase gradually. A new visco-elastic primary creep model is added to BISON, which is capable of handling situations of load drops or reversals. The model is coupled with secondary creep models namely Hayes-Hoppe and Limback-Anderson, available in BISON to make predictions on cladding creep strains. The results are compared to Halden in-pile experiments IFA-585 and IFA-699. Based on the comparison, it is recommended to use the visco-elastic primary creep model coupled with Hayes-Hoppe secondary model for consistency and performance.
[en] Creep through nanoindentations has attracted increasing research attention in recent years. Many studies related to indentation creep tests, however, have simply focused on the characteristics of steady-state creep, and there exist wide discrepancies between the uniaxial test and the indentation test. In this study, we performed a computational simulation of spherical indentations, and we proposed a method for evaluating the creep properties onsidering transient creep. We investigated the material behavior with variation of creep properties and expressed it using regression equations for normalized variables. We finally developed a program to evaluate the creep properties considering transient creep. By using the proposed method, we successfully obtained creep exponents with an average error less than 1.1 and creep coefficients with an average error less than 2.3 from the load-depth curve