12. Fracture Toughness Evaluation of Low and Medium Strength Steels Based on the Stretched Zone

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概要

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• Fracture toughness evaluation based on the deformation behavior at the precrack tip is discussed. In part I it is shown that the stretched zone depth, as well as CTOD, is almost constant regardless of the test temperature when the crack initiates by fibrous crack, while, in cleavage initiation, it takes various values between the above-mentioned value and almost zero depending on the test temperature and plastic constraint. We call the value of CTOD at fibrous crack initiation δ_i (which is about the same value as 2(SZD) observed on the specimens in which the crack initiated by fibrous crack) and the fibrous-cleavage transition temperature in fracture initiation T_i. It is proposed to define the toughness of low and medium strength steels by two parametersδ_i and T_i. In part II the effect of strain aging induced by preloading on toughness of steels has been investigated. The embrittling effect of this kind will be important in the case of preloading for stress relief or proof test, for instance. According to the test results, SS41 and SM41A show considerable amounts of embrittlement due to this type of strain aging, while in SM50C, the effect of aging is hardly observed. Finally, in part III the effects of specimen thickness on T_i are investigated. T_i increases with increase in specimen size and takes upper limiting value (plane strain T_i) when plane strain state develops around the notch tip in the midthickness region. This situation is confirmed by the distribution of the stretched zone depth along the precrack front. Plane strain T_i may be assumed to be a material constant. As the specimen size requirement to give this plane strain T_i, the formula [numerical formula] is proposed, where a is crack length, B specimen thickness, W specimen depth,δ_i COD at fibrous crack initiation, E Young's Modulus, σ_Y yield strength and R the ratio of yield strength to ultimate tensile strength.

• 社団法人日本船舶海洋工学会の論文

著者

• Otsuka Akio

  Nagoya University

• Nishimura Seiji

  Nagoya University

• MIYATA Takashi

  Nagoya University

• KASAI Noboru

  Kobe Steel Corp.

関連論文

• 12. Fracture Toughness Evaluation of Low and Medium Strength Steels Based on the Stretched Zone
• 10. COD and Stretched Zone as the Criterion for Fracture Initiation from a Precrack at Various Temperatures

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