PSI - Issue 2_A

Takuhiro Hemmi et al. / Procedia Structural Integrity 2 (2016) 2230–2237 Author name / Structural Integrity Procedia 00 (2016) 000–000

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1. Introduction Recently, steel plates of container carriers have become thicker because larger ships have been needed for carrying more baggage and reducing the transportation cost. This trend makes the risk of the brittle fracture higher. Arresting brittle crack propagation as well as crack initiation is essential as a “double integrity” for large steel structures. Although it is most effective for arresting brittle crack to prevent brittle crack initiation by controlling welding defects and fatigue cracks, it is nearly impossible to remove these faults completely. Therefore, it is necessary for the safety of a structures to arrest the brittle crack before serious accidents occurred. The application of steels with high arrestability to the structures is directly effective to ensure the integrity. There have been a lot of efforts on researches and developments of the steels with higher arrestability. In particular, it has been known conventionally that arrest toughness is a strongly related to microstructures of steels as an empirical knowledge supported by experiments such as (Ishikawa et al., (1995)). Although grain refining has been a promising approach of the development of steels with high arrestability (Ohmori et.al, (1976)), the detail theoretical mechanism has been scarcely revealed. As mentioned above, the reasonable theory to explain the quantitative and universal relationship between microstructures of steel and macroscopic brittle crack propagation and arrest behavior has never been established. Thus, to clarify these relationship in theory is much important to develop high-performance steel plate reasonably. In 2016, Shibanuma et al. and Yamamoto et al. proposed a multiscale model to simulate the complex behavior of brittle crack propagation and arrest, based on the model of Aihara and Tanaka (2011). The multiscale model is composed of two models; a microscopic model to simulate cleavage fracture in the grain scale and a macroscopic model to simulate brittle crack propagation and arrest behavior in the steel plate scale. This model was applied to temperature-gradient crack arrest tests of steel plates which have uneven texture distribution and those experiments proved that the model could simulate complicated crack propagation and arrest behavior. Although the model qualitatively showed a qualitative brittle crack propagation/arrest behavior, the microscopic energy absorbing mechanism in the model is too simple to simulate quantitative prediction of arrest toughness. Based on such a backup above, we conducted fundamental examination as follows for quantification of relationship between microstructure and arrest toughness in steel. (1) By carrying out microscopic calculation, evaluating the relationship between grain size and effective surface energy (2) By conducting a kind of crack arrest tests for some types of steel plates with different grain size, organizing the relationship between grainsize and arrest toughness (3) By identifying effective surface energy making consistent with experimental results, reviewing the validity of effective surface energy in the microscopic model and clarifying what is trouble to be improved the accuracy of multiscale model. Nomenclature s grain boundaries ߬ ଢ଼ shear strength ߪ ଢ଼ effective surface energy

yield stress ߝ ୤୫ shear strain ߛ A

an area of the entire domain c a ratio of width and height of uncracked ligament ݀ grain size ܭ ୡୟ arrest toughness P 0 crack initiation load T temperature a 0 initial crack length a a arrest length W specimen width S the distance between two jigs below the specimen