Papers by Keyword: Brittle Fracture

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Authors: Yoichi Kayamori, Takehiro Inoue, Tetsuya Tagawa
Abstract: The ratio of J-conversion CTOD to CMOD-based CTOD was experimentally evaluated and analytically estimated in shallow crack specimens. It was demonstrated that the low strain hardening exponent in the Ramberg-Osgood relation reduced the CTOD ratio. A CTOD transformation equation, which was proposed by the authors, can transform CMOD-based CTOD into J-conversion CTOD with reasonable accuracy for 0.15≤a/W≤0.5.
Authors: Fabio Sorbello, Peter E.J. Flewitt, A.G. Crocker, Gillian E. Smith
Abstract: It is well established that within the lower-shelf temperature range of Fe2-3Si polycrystalline steels, the brittle fracture occurs predominately by transgranular cleavage, unless subject to embrittling heat-treatments. The cleavage fracture develops on the well established {001} planes of the bcc structure. In this paper we revisit the growth, of these cleavage cracks by considering crack propagation in single crystals of Fe2Si steel. Three point bend specimens manufactured from oriented crystals have been tested by impact loading at a temperature of -196°C. High spatial resolution focused ion beam imaging combined with ion milling is used to examine in detail the crack propagation path and has provided a new insight into the mechanisms involved. In particular it has been established that within the process zone of the propagating cracks local strain is accommodated by the formation of {112} twins. The results are discussed with respect to the overall crack propagation mechanism.
Authors: Dominique Leguillon, Sébastien Murer
Abstract: Cotterell and Rice theory (1980) on the kinking of a crack submitted to a biaxial loading in a homogeneous material has been recently revisited (Leguillon and Murer 2008). The mixed criterion for fracture which involves both an energetic and a stress condition (Leguillon 2002) allows defining a positive threshold of the T-stress below which no branching can occur (Selvarathinam and Goree 1998). This analysis enters within a more general mixed-mode analysis (I+II+T-stress). Despite the complex terms and the oscillations, results extend to interfacial cracks. The assumption of a crack jump as a consequence of the energy balance allows getting rid of the problem brought by the oscillations due to these complex terms. This approach brings a new insight on the prediction of crack kinking out of a bimaterial interface.
Authors: Cheng Long Zhang, Ping Fa Feng, Zhi Jun Wu, Ding Wen Yu
Abstract: Rotary ultrasonic machining is a hybrid machining process that combines diamond grinding and ultrasonic machining. The mathematical predictive material removal rate models have been developed in rotary ultrasonic machining with a constant pressure. However, there is no report on mathematical predictive cutting force model in rotary ultrasonic drilling at a constant feedrate presently. Since cutting force can not only reflect the processing state, but also affect the machined surface quality, it is necessary to develop a mathematical model for predicting cutting force which can forecast the machining results. This paper presents a mathematical model to predict the cutting force in rotary ultrasonic machining. On the basis of this model, the relations between cutting force and controllable machining parameters are researched by numerical computation method. This paper also researches the influences of spindle speed and feedrate on cutting force by experiments. The results observed through the experiments agree well with the relations generated from the mathematical model, which verify the developed model.
Authors: Yoichi Kayamori, Takehiro Inoue, T. Tagawa
Abstract: There have been two different CTOD calculations, geometric CTOD calculation in BS7448 and J-based conversion in ASTM E1290 since the ASTM International revised E1290 in 2002, and it is useful to transform one CTOD into the other CTOD. In this study, analytical and experimental investigations into the ratio of ASTM E1290-CTOD to BS7448-CTOD were carried out, and two ratios of ASTM-CTOD to BS-CTOD, R1 and R2, were characterised in terms of cleavage cracking. These two characteristic CTOD ratios are helpful in simply transforming one critical CTOD to the other.
Authors: J. Zhou, Winston O. Soboyejo
Authors: Chun Zhi Du, Wen Yan Qiu
Abstract: To reduce the coal mining risk, drainage of methane with drills and hydraulic fracture has been put in practice. In the paper, an advanced damage propagation model of brittle fracture is studied based on mode I crack. The localization band of damage is inducted, and the modal of localization of damage around crack-tip is established using fracture and damage mechanics. Besides, the stress distribution in localization of damage is assumed as obeying a quadratic function, thus the stress field around the crack is obtained, and the calculation formula of crack length is presented, which can provide reference for mine production.
Authors: Volker Hardenacke, Jörg Hohe, Valérie Friedmann, Dieter Siegele
Abstract: The objective of the present study is the development of a micromechanically based probabilistic model for the assessment of the cleavage fracture probability of ferritic steels. Brittle fracture of ferritic steels is a probabilistic process, triggered by the failure of randomly distributed brittle particles. These particles fracture due to plastic deformation of the surrounding matrix, resulting in the nucleation of micro-cracks. Once nucleated, the local stress state controls the possible instability of the defects. In this context, the local stress-triaxiality is assumed to govern the blunting of freshly nucleated micro-defects. The local approach models available in literature account for the above-mentioned correlations only in a simplified manner. Based on Representative Volume Elements (RVE) of the microstructure, accounting for the grain-structure as well as for the brittle particles, the cleavage initiation process was modelled in order to investigate the relevant parameters and their interactions. The RVE’s were loaded according to the local mechanical field quantities determined numerically for a variety of specimen types at the cleavage-origins. Thus, the behaviour of the particles against the micromechanical conditions could be specified, resulting in a better understanding of the processes at cleavage fracture initiation. Based on the results, an enhanced probabilistic cleavage model is proposed.
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: Ya Jie Li, Li Jun Xin, Zhi Yong Wang
Abstract: MS1470 steel is mainly used in the automotive industry, which has high strength, light weight and low cost. However, there always appear grains coarsening, brittle increasing, lower intensity and other issues. To solve this problem, we have analyzed the welding process, the fracture morphology and microstructure etc. The results shown that the microstructure were mainly ferrites and lath martensites. However, grains coarsening in the HAZ were serious, and fracture failure also occurred in the heat affected zone. Meanwhile, MS1470 steel had formed a small amount of pores during rolling process, which also is an important reason for cracks formation.
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