Papers by Keyword: Microvoid

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Authors: Bum Joon Kim, Byeong Soo Lim, Sung Jin Song
Abstract: Generally, as the hold time of static load increases, the degradation of material becomes more rapid and the creep-fatigue life decreases. Under the creep-fatigue interaction, the cause of life decrease was mainly the initiation and growth of microvoid with increasing hold time. In this study, using the newly developed alloy of P92, the artificial creep-fatigue degradation tests and its ultrasonic inspection were carried out to evaluate creep-fatigue life. From the relations between the creep-fatigue life and the nondestructive degradation assessment by backscattered ultrasound inspection, the new nondestructive life evaluation parameter, SDA (Slope of Decaying Amplitude) was proposed. Also, to verify the nondestructive life evaluation ability, the life evaluation results by SDA were compared with those of the destructive life evaluation obtained through the fraction of cavity area. From the test result of backscattering Rayleigh surface wave, the linear relationship between SDA and experimental life could be obtained. The good agreement between the experimental life and the predicted life by SDA was also obtained. Finally, it can be stated that the new life evaluation/degradation parameter, SDA (Slope of Decaying Amplitude) could be utilized for the evaluation of the material degradation under creep-fatigue interaction.
Authors: Yuan Dong Liu, Yi Hui Yin, Yun Tan
Abstract: Hydrogen in the steel wall can cause hydrogen embrittlement of the wall material and thereby change the carrying capacity of the vessel. A theoretical model of hydrogen diffusion in the steel wall of a high pressure vessel was established and the formula of hydrogen content in the steel wall was deduced. Based on the hydrogen content formula, the formula of hydrogen pressure within microvoids which naturally exist in the steel wall of a spherical pressure vessel was deduced. At last, as an example to demonstrate the meaning of solving hydrogen pressure in microvoids, by using a representative volume element (RVE) model to carry out FE numerical simulation, the effects of hydrogen pressure on equivalent mechanical properties of the wall material were studied. It is found that the higher the gas pressures are, the lower the ultimate strength, specific elongation and percentage contraction of area are, which is in good accordance with the phenomena of metal hydrogen embrittlement.
Authors: Byeong Soo Lim, Bum Joon Kim
Abstract: This paper investigates the influence of various hold times on creep-fatigue life at 600oC. The relationship between the crack growth behavior and hold time was studied, and a metallurigical investigation to examine the effect of creep was performed. To examine the relationship between creep-fatigue life and microvoids, the fraction of micro-voids/cavity area was analyzed at the crack tip. The crack growth rate of the HAZ was found to be faster than that of base metal while creep-fatigue life was found to be shorter. Finally, it can be stated that the fraction of cavity area, Fca could be utilized for the life prediction under creep-fatigue interaction. As the hold time increased, the creep damage was observed along the prior austenite grain boundaries and inside and boundaries of delta-ferrite.
Authors: H. Mizubayashi, I. Sakata, H. Tanimoto
Abstract: For hydrogenated amorphous silicon (a-Si:H) films deposited at temperatures between 423 K and 623 K (a-Si:H423K and so on), the light-induced changes in the internal friction between 80 K and 400 K were studied. The internal friction is associated with H2 motion in microvoid networks, and shows the mild temperature dependence between about 80 K and 300 K (Q-180-300K) and the almost linear increase above 300 K (Q-1>300K). Both Q-180-300K and Q-1>300K decrease with increasing the deposition temperature, and show the mild temperature dependence in a-Si:H623K. The white light soaking with 100 mW/cm2 (WLS100 and so on) below 300 K caused a change in Q-180-300K and no changes in Q-1>300K, respectively, and the light-induced changes in Q-180-300K recovered after annealing at 423 K. The wide distribution of activation energies for H2 motions between microvoids indicate that most of neighboring microvoids are connected through windows, i.e., the microvoid networks are existing in a-Si:H, and the spatially loose or solid structures are responsible for the low or high activation energies for the H2 motion between microvoids, respectively. Furthermore, the light-induced hydrogen evolution (LIHE) was observed for WLS200 to WLS400 in a vacuum between 400 and 500 K, resulting in the disappearance of the internal friction due to the H2 motion in the microvoid network.
Authors: F. Hori, A. Yunjia, Yasuhisa Aono, M. Takenaka, Eiichi Kuramoto
Authors: T. Riewcharoon, Z. Tang, Yasuyoshi Nagai, Mitsuru Hasegawa
Authors: Y.Y. Shan, C.C. Ling, H.L. Au, S. Fung, C.D. Beling, Y.Y. Wang
Authors: Mitsuru Ohata, Takuya Fukahori, Fumiyoshi Minami
Abstract: This study pays attention to reveal the material properties that control resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. The crack growth tests using 3-point bend specimens with fatigue pre-crack were conducted for two steels that have different ductile crack growth resistance with almost the same CTOD level for crack initiation, whereas both steels have the same “Mechanical properties” in terms of strength and work hardenability. The observation of crack growth behaviors provided that different mechanisms between ductile crack initiations from fatigue pre-crack and subsequent growth process could be applied. It was found that two “Mechanical properties” associated with ductile damage of steel could mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from the surface of notched specimen, and the other one is a dependence of stress triaxiality on ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve was proposed taking the two “ductile properties” into account, where ductile crack initiation from crack-tip was in accordance with critical local strain based criterion, and subsequent crack growth GTN (Gurson-Tvergaard-Needleman) based triaxiality dependent damage criterion. The proposed model accurately predicted the measured R-curve for the two steels used with the same “strength properties” through ductile crack initiation to growth.
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