PSI - Issue 21

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

The # 1 and # 2 specimens were cooled to -60 °C for the experiment. Table 4 shows the respective absorbed energies and Fig. 11 shows the fracture surface photographs after the tests. Contrary to previous expectations that the absorbed energy of # 1 corresponding to scenario 1 is larger than that of # 2 corresponding to scenario 2, the opposite result was obtained with regard to absorbed energy. As a reason why contrary result was obtained about the absorbed energies, it can be raised that the trace of clear ductile fracture at the notch bottom of specimen # 2 is observed, that is, it is presumed that a large amount of energy was required for crack initiation. In addition, since the brittle running plate has a high strain energy just before crack initiation, it is considered that the brittle crack could not be arrested because the energy of the crack when entering the test part became too large. After the test, the fracture surfaces of the replacement part were observed, and the fracture surface roughness was measured. The result is shown in Table 5. From Fig. 12 as well as the result of the measurement (Table 5), it is confirmed that the fracture surface of #1 is rougher than that of #2 in both visual level and measurement index. The rate of increase in surface area is calculated by Eq. (4). This physical meaning is the surface area increase rate comparing completely flat plane. = 1 [∬ (√[1 + ( ( , ) ) 2 + ( ( , ) ) 2 ] − 1) ] (5) Where, Sdr is the rate of increase in the surface area, A is the area of the measuring surface, z(x,y) is the height of point (x, y) . The energy balance during crack propagation under the fixed boundary condition is expressed by Eq. (5) (K. B. Broberg, 1999). Since the dissipated energy term is consumed in the formation of the fracture surface, the larger the fracture surface roughness, the smaller the crack driving force. − = + (6) Where, is the potential energy, D is the dissipated energy, E k is the kinetic energy, A is the new projected crack area. Therefore, from the measurement of the fracture surface roughness, it was estimated that much more energy was consumed for the brittle crack propagation at the test part of specimen #1, and it was suggested that scenario 1 is easier to arrest brittle cracks than scenario 2.

Table 6 Absorbed energies of DWTT test at -60 ℃ Specimen mark Crack propagation direction Absorbed energy (J) Brittle fracture area (%) #1 thickness 1467 94 #2 width 3231 91