PSI - Issue 21

Taiko Aikawa et al. / Procedia Structural Integrity 21 (2019) 173–184 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Using the above calculation method, the twist angle was calculated for each connected grain boundary along crack propagation direction both in the plate thickness direction and in the plate width direction in the TMCP and NQT specimens, respectively. Arrows corresponding to the direction of crack propagation were drawn on the Grain map obtained by EBSD as shown in Fig. 8, and the Euler angle matrix was obtained for each Grain boundary on the three solid lines depicted in Fig. 8, then the twist angles were calculated. In order to equalize the number of grain boundaries to be investigated, the two Grain maps in Fig. 8 have different magnifications. The average calculated twist angles are summarized in Table 3. Fig. 9 shows the correlation of the absorbed energy at -90 °C (where the fracture had completely brittle character) and the average twist angle in the corresponding propagation direction. In the analysis of the absorbed energy by the average twist angle in the propagation direction, the correlation between the twist angle and the absorbed energy was obtained for each steel type. However, various factors affecting the investigation such as the number of grain boundaries and crack propagation distance have not been considered in this arrangement. Therefore, the authors introduced parameters considering the number of grain boundaries and the scanning distance corresponding to the crack propagation distance. This is expressed in Eq. (4). The average twist angle in the propagation direction is multiplied by the number of grain boundaries present per 1 μm = ̅ ∙ (4) Where ̅ is the average twist angle in the propagation direction shown in Table 3, N is the total number of grain boundaries examined, and l is the scanning distance. Table 4 shows the scanning distance and the number of grain boundaries per specimen and the numbe r of grain boundaries per 1 μm. Table 5 shows the absorbed energies at k and -90 ° C. Fig. 10 is a graph showing the relationship in Table 5. By this arrangement, it was suggested that the relationship between the twist angle and the absorbed energy could be unified by considering the number of grain boundaries for the crack propagation distance.

Fig. 9 EBSD Grain map and calculation line of twist angle at each Grain boundary

Table 3 Average twist angle for each direction of crack propagation in each steel Steel Surface Cross Surface-Cross TMCP 32.88° 24.58° 8.30° NQT 32.74° 29.05° 2.69°