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[en] Crack propagation rate by the process known as Delayed Hydride Cracking (DHC) is usually assessed in pressure tube materials. Curved Compact Toughness specimens are fatigue pre-cracked previous to DHC test. This work explores the validity of specimen pre-cracking by the same DHC process. It was demonstrated that crack velocity is not affected by the pre-cracking method. Results are compared with values obtained by using not pre-cracked specimens.
[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 report describes the experimental study performed to assess the efficiency of hydrogen water chemistry on the propagation rate of cracks generated by irradiation assisted stress corrosion cracking in high fluence material. The selection of the material and the test procedures followed for this study are presented. The test results obtained with 8.6 dpa specimen are discussed.
[en] The aim of this work is a second reply to Puls's comments on the author's first reply to the paper published in J. Nucl. Mater. 393 (2009) 350-367. The Dutton-Puls model indicates that the CGR is governed not by the stress gradient but by the ΔC that results from a decrease in the crack tip solubility due to the stress when compared to the bulk solubility, demonstrating that Puls's defense of the Dutton-Puls model is inconsistent and invalid. Given the fact that DHC involves three consecutive processes such as nucleation, growth and cracking of hydrides at the crack tip, Puls's claim that DHC is simply a diffusion-controlled process and the CGR is governed solely by the rate of hydride growth is incorrect, yielding many of the unsolved issues related to DHC. It is confirmed that Kim's criticism that the Dutton-Puls model for the crack growth rate (CGR) is established based on a faulty thermodynamic basis is correct.
[en] Acoustic Emission (AE) technique was applied to stress corrosion cracking of Inconel 600 to investigate the AE capability of detecting crack growth and to obtain the relation between AE characteristics and crack mechanism. The specimens were heat-treated in two conditions (600 .deg. C for 30 hrs or 700 .deg. C for 1 hr) and undergone CERT at two extension rates (2.5x10-5 or 1.25x10-4(mm/s)). It was found that the AE peak amplitude from plastic deformation was generally smaller than about 48dB (0.25mV), while Intergranular stress corrosion cracking (IGSCC) and ductile fracture produced higher values of 49 to 70dB (0.3mV to 3mV). The slopes of cumulative amplitude distribution (b-values) were linearly dependent on IGSCC susceptibility and the higher the susceptibility, the smaller the b-value. The monitoring of combined AE parameters such as event rate, amplitude, count and energy can provide effective means to clearly identify the transition from crack initiation and small crack growth to rapid growth of dominant cracks
[en] They conclude that the type-II boundary is a potential path for crack growth. While there are several theories for the mechanisms of the type-II boundary formation, they conclude that the type-II boundary forms from the allotropic σ-γ transformation at the base metal in the elevated austenitic temperature range. Moreover, many other crack growth experiments conclude that the type-II boundary and fusion boundary region of the weld metal are susceptible to SCC. Hou et al. investigate the microstructure and mechanical properties of the DMW of Alloy 182 and low-alloy steel A533 Gr. B. Using the tensile test, they found that type-II boundaries are high angle grain boundaries, which are more susceptible to SCC than low-angle grain boundaries. As the operation time of nuclear power plants using DMWs of Alloy 152 and A533 Gr. B increases, these DMWs must be evaluated for their resistance to SCC for long-term operations. However, only few studies have investigated the thermal aging effects induced by long-term operations at high temperature. So thermal aging effect by long-term operation, and existence of type-II boundary must be considered to evaluate the susceptibility to SCC of structural materials. Purpose of this study is to analyze the detailed microstructure of the type-II boundary region in the DMW of Alloy 152 and A533 Gr. B, after applying heat treatment simulating thermal aging effect of a nuclear power plant operation condition to evaluate the susceptibility of this region to SCC. Microstructural, grain boundary orientation, nanohardness analysis were conducted in the type-II boundary and fusion boundary region of the DMW between Alloy 152 and low-alloy steel A533 Gr. B in order to investigate the effect of thermal aging influence. Type-II boundaries are observed in the whole specimen, which seem to be arranged and then shifted away from fusion boundary as the heat treatment is applied. Increasing low-angle grain boundaries were observed as the heat treatment proceeded
[en] The thermodynamical strength acting on a crack tip, when the unilateral contact between the lips of the cracks is considered, is deduced in this note from energetic formulations. The formula obtained is particularly well suited to numerical computation
[fr]A partir de considerations energetiques, nous proposons dans cette Note une expression de la force thermodynamique qui s'exerce sur une fissure et ceci en prenant en consideration l'effet du contact unilateral entre le levres de la fissure. L'expression obtenue est particulierement adaptee au calcul numerique
[en] The paper presents the testing methodology on the CANDU pressure tube specimens (Zr-2.5% Nb alloy) to determinate the fracture mechanics parameters. The crack initiation and subsequent DHC propagation tests were performed on compact-tension (CT) specimens prepared from pressure tube off-cuts. Previously, the CT specimens were subjected to the specific method to increase the hydrogen concentration up to 50 ppm. This process consists of two stages: the first one is the electrolytic disposal of a uniform zirconium hydride layer and, the second one is to apply a homogenization thermal treatment causing the diffusion of hydrogen into the zirconium matrix from the hydride surface layer. The testing temperature was around 280oC, which is the temperature of interest for the pressure tubes operating in CANDU 6 reactor. (authors)
[en] Highlights: • The CEFM explains the theoretical aspects of the SCC of Alloy 22. • Effects of environmental variables on the SCC of Alloy 22 are explored. • SCC is possible in Yucca Mountain-type repositories, if the crack can nucleate. • Semi-elliptical surface cracks are attributed to the dependence of CGR on the ECL. • 2.5-cm outer shell may not be adequate for ensuring waste package integrity.
[en] Using Griffith's criterion for brittle fracture we analyze the effect of an enhanced crack resistance due to scaleinvariant fracture topology. To this end a relation between crack resistance, resp. fracture toughness, and fractal dimension of the fracture surface is derived on the basis of a scaling ansatz for the 'true' crack length. It turns out that this relation depends on the extension of the surface scaling range, the resistance of an ideal smooth crack in the same material, and remaining non-scaling features of the crack morphology. In general, there is no simple exponential dependency of toughness on fractal dimension for different materials. The theoretical predictions of the paper are discussed on the background of experimental results given in the literature. (orig.) With 2 figs
[de]Auf der Basis des Griffith'schen Kriteriums fuer sproeden Bruch wird der Einfluss selbstaehnlicher, skaleninvarianter Bruchmorphologien auf den Risswiderstand in sproeden Materialien untersucht. Mit Hilfe eines Skalenansatzes fuer die 'wahre' Risslaenge wird eine Beziehung zwischen Risswiderstand bzw. Bruchzaehigkeit und fraktaler Dimension der Bruchflaeche abgeleitet. Dabei zeigt sich, dass diese Beziehung ueber die fraktale Dimension hinaus weitere Parameter enthaelt, die von der Groesse des Skalenbereiches, dem Risswiderstand eines ideal glatten Risses im selben Material und anderen nicht skalierenden Eigenschaften der Bruchmorphologie abhaengen. So existiert im allgemeinen keine einfache exponentielle Abhaengigkeit der Bruchzaehigkeit von der fraktalen Dimension fuer verschiedene Materialien. Die erzielten theoretischen Aussagen werden im Zusammenhang mit experimentellen Ergebnissen anderer Autoren diskutiert. (orig.) With 2 figs