The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Mirco Daniel Chapetti - One of the best experts on this subject based on the ideXlab platform.
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crack closure and fatigue crack growth near Threshold of a metastable austenitic stainless steel
International Journal of Fatigue, 2015Co-Authors: D F Martelo, A Mateo, Mirco Daniel ChapettiAbstract:Abstract In this paper R-ratio effects on fatigue crack growth near Threshold Region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and Kmax = 23 MPa√m using a two-parameters approach (ΔK, Kmax and α in Kujawski’s model) and crack closure model (using Elber’s Kop and in Donald’s ACRn2 approaches). The Kop and ACRn2 were experimentally measured on a single edge tension specimens. The Kop measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris Region, they fail to correlate them in the Threshold Region. However, this correlation can be improved in the Threshold Region when a different α value from the Paris Region is used. The authors indicated that two different mechanisms operate; one in the Paris Region and another in the near Threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where ΔK is replaced by ΔKeff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify Kmax in the two-parameter approach.
Zdeněk Knesl - One of the best experts on this subject based on the ideXlab platform.
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effect of constraint on fatigue crack propagation near Threshold in medium carbon steel
Computational Materials Science, 2006Co-Authors: Pavel Hutař, Stanislav Seitl, Zdeněk KneslAbstract:Abstract The influence of constraint caused by different specimen geometry on fatigue crack growth in the Threshold Region is analyzed and a phenomenological model is proposed to correlate the fatigue crack propagation rate under different constraint levels. The approach is based on an application of two-parameter fracture mechanics and the corresponding calculations are performed according to the finite element method. The constraint level is quantified by the value of T -stress. The fatigue crack propagation rate is then expressed in terms of the stress intensity factor K and T -stress. The influence of K and T on the plastic zone size and crack opening near the fatigue crack tip is investigated. It is shown that assuming small scale yielding conditions, the plasticity induced crack opening and crack closure are not significantly influenced by the T -values, but the influence of T -stress on plastic zone size is significant and cannot be neglected. Plastic zone size is considered as a controlling variable for near Threshold fatigue crack behavior and a simple procedure making it possible to estimate the changes of the fatigue crack propagation rate in the Threshold Region due to different constraint level quantitatively is formulated. The numerically obtained results are compared with experimental data. It is concluded that experimental values correspond well with those numerically predicted. The result presented makes it possible to relate experimentally measured data obtained from specimens with different geometries and thus contribute to more reliable estimates of the residual fatigue life of structures.
Ruohe Yao - One of the best experts on this subject based on the ideXlab platform.
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a Threshold voltage definition for modeling asymmetric dual gate amorphous ingazno thin film transistors with parameter extraction technique
Journal of Applied Physics, 2019Co-Authors: Minxi Cai, Ruohe YaoAbstract:A physical definition of Threshold voltage has been proposed for asymmetric dual-gate amorphous InGaZnO (a-IGZO) thin-film transistors. Based on the definition, a compact model for drain current has been derived with complete parameter extraction methods. According to the distribution characteristics of density of states (DOS) in the a-IGZO bandgap, the Threshold voltage of bottom-gate (BG)-driven devices is defined by the ratio of the trapped-carrier density to the free-carrier density at the bottom surface of the channel layer. This definition has proven to give appropriate values of Threshold voltage for devices with DOS parameters varying in a wide range. The trapped-charge density in the above-Threshold Region can be expressed based on the defined Threshold voltage, with the form related to the BG voltage, the top-gate voltage, and the potential along the channel. The free-charge density and analytical drain current can thus be obtained in the above-Threshold Region. Finally, the continuous drain current expression is developed covering all operation Regions. In the modeling process, the determination and extraction strategies for parameters are also presented.A physical definition of Threshold voltage has been proposed for asymmetric dual-gate amorphous InGaZnO (a-IGZO) thin-film transistors. Based on the definition, a compact model for drain current has been derived with complete parameter extraction methods. According to the distribution characteristics of density of states (DOS) in the a-IGZO bandgap, the Threshold voltage of bottom-gate (BG)-driven devices is defined by the ratio of the trapped-carrier density to the free-carrier density at the bottom surface of the channel layer. This definition has proven to give appropriate values of Threshold voltage for devices with DOS parameters varying in a wide range. The trapped-charge density in the above-Threshold Region can be expressed based on the defined Threshold voltage, with the form related to the BG voltage, the top-gate voltage, and the potential along the channel. The free-charge density and analytical drain current can thus be obtained in the above-Threshold Region. Finally, the continuous drain cur...
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a physical model with the effects of self heating and variable resistance in above Threshold Region for hydrogenated amorphous silicon thin film transistor
Japanese Journal of Applied Physics, 2008Co-Authors: Yuan Liu, Ruohe YaoAbstract:A physical model that includes the effects of self-heating and variable resistance in the above-Threshold Region is presented for hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs). The self heating effect is due to the low thermal conductivity of the gate insulator and the glass substrate. In order to predict the temperature rise as a function of device dimensions and material parameters, a steady-state thermal model derived from a system of coupled one-dimensional (1-D) energy equations and boundary conditions is presented. Then the developed thermal model is applied to study the self-heating effect in the a-Si:H TFT. In addition, the bulk resistance is decreased due to the the mechanism of space charge limited conduction in a n+–i–n+ structure. By considering both the effects of self-heating and variable resistance, a physical model in the above-Threshold Region has been developed. Using this model, the non-saturating output current and kink effects can be accurately described for a typical a-Si:H TFT. The accuracy of the proposed model has been verified with the experimental data.
D F Martelo - One of the best experts on this subject based on the ideXlab platform.
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crack closure and fatigue crack growth near Threshold of a metastable austenitic stainless steel
International Journal of Fatigue, 2015Co-Authors: D F Martelo, A Mateo, Mirco Daniel ChapettiAbstract:Abstract In this paper R-ratio effects on fatigue crack growth near Threshold Region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and Kmax = 23 MPa√m using a two-parameters approach (ΔK, Kmax and α in Kujawski’s model) and crack closure model (using Elber’s Kop and in Donald’s ACRn2 approaches). The Kop and ACRn2 were experimentally measured on a single edge tension specimens. The Kop measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris Region, they fail to correlate them in the Threshold Region. However, this correlation can be improved in the Threshold Region when a different α value from the Paris Region is used. The authors indicated that two different mechanisms operate; one in the Paris Region and another in the near Threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where ΔK is replaced by ΔKeff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify Kmax in the two-parameter approach.
Genki Yagawa - One of the best experts on this subject based on the ideXlab platform.
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Near-Threshold fatigue crack propagation without oxide-induced crack closure.
Scientific reports, 2020Co-Authors: Koki Tazoe, Hiroto Tanaka, Masanori Oka, Genki YagawaAbstract:An accurate value for the Threshold stress intensity factor range, ΔKth, is a key parameter for studying crack-like defects. However, it is difficult to obtain accurate ΔKth values due to oxide-induced crack closure. In this study, we report conditions for minimizing the effects of oxide-induced crack closure near the Threshold Region, where a concave curve of the effect on the loading frequency on oxide-induced crack closure is achieved. The resulting conditions allow for an accurate determination of ΔKth, which is a key material parameter relating to the pertinent loading ratio.
