Papers by Author: Mitsuru Ohata

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Authors: Yasuhito Takashima, Mitsuru Ohata, Fumiyoshi Minami
Abstract: Charpy impact toughness values show large statistical scatter, particularly in the ductile-to-brittle transition temperature (DBTT) range. Although the statistical distribution of Charpy absorbed energy has not been clarified, critical values of the stress intensity factor, J-integral and crack-tip opening displacement (CTOD) at brittle fracture generally show the Weibull distribution with two or three parameters. This study proposes a brittle fracture model, based on the weakest link theory, for evaluating the scatter in Charpy absorbed energy KV. The numerical results show that the amplitude of the opening stress fields ahead of the V-notch at varying amounts of KV are uniquely characterized as the square of the applied load. With these numerical results, the Weibull shape parameter of the statistical distribution of KV is almost equal to 2. The proposed statistical model is verified through experimental results. It is found that the statistical distribution of KV is characterized by a two-parameter Weibull distribution with the shape parameter of 2 under the condition of pure brittle fracture.
Authors: Gyu Baek An, Mitsuru Ohata, Masahito Mochizuki, Han Sur Bang, Masao Toyoda
Abstract: It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.
Authors: Yasuhito Takashima, Mitsuru Ohata, Fumiyoshi Minami
Authors: Hiroshi Shimanuki, Fumiyoshi Minami, Mitsuru Ohata
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.
Authors: Mitsuru Ohata, Anna Ui, Fumiyoshi Minami
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