PSI - Issue 13

Yuta Suzuki et al. / Procedia Structural Integrity 13 (2018) 1221–1225 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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propagation model based on XFEM (Moës et al., 2002; Gonzalez et al., 2013; Shibanuma et al., 2018). This model made it possible to reproduce fracture surface with very complicated cleavage crack propagation with high precision and easily by defining finite elements independently of cracks and grains. In cleavage crack propagation in steels, it is pointed out that the absorbed energy due to cleavage plane formation is just a little and energy dissipation by ductile fracture of ligaments between cleavage planes along grain boundaries amounts for a large proportion (Shibanuma et al., 2016; Yamamoto et al.,2016). The broken ligament is called as tear ridge. Therefore, in accordance with previous studies (Aihara and Tanaka, 2011; Shibanuma et al., 2016; Yamamoto et al.,2016), the energy absorption calculation considered only tear ridge formation in this study. Plastic work in forming tear-ridge per unit volume has been expressed by the shear yield stress and the critical strain . Based on Tresca’s yielding condition assuming perfectly plastic solids, t he energy absorption amount γ due to tear ridge formation was calculated by γ = 2 1 ∫ ℎ 2 (1) where is target area, and is the ratio of the thickness of the unbroken ligament to the height. Here, was set to 0.1 and was set to 0.7 based on SEM observation conducted in previous studies (Shibanuma et al., 2016; Yamamoto et al., 2016).

S1 S2 Fig. 4 Simulation result of tear-ridge formation

Using the grain size distribution of S1, and S2 shown in 2.1, we evaluated the fracture surface formation energy of each steel. The obtained results are shown in Fig. 5. From this figure, similarly to the experimental result of Section 2, it could be said that the larger grain size is, the larger the fracture surface formation energy, that is, cleavage crack propagation resistance is.

0 500 Absorbed energy [J/mm 2 ] 1.000 1.500 2.000 2.500 S1 S2

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Average Grain size [μm]

Fig. 5 Influence of grain size on absorbed stress (Model simulation)