PSI - Issue 13

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

1222

2

In the first place, the relation between average grain size and crack propagation resistance was revealed by performing systematic crack arrest toughness tests on steels whose average grain size was only different. Secondly, we experimentally measured the fracture condition at the grain level and the absorbed energy at the failure, and verified the validity of the estimation of the energy absorption amount. Finally, by applying the formula of the absorbed energy derived from experiments to the developed model, we compared the experimental fact with the result by model calculation. 2. Evaluation of macroscopic resistance against cleavage crack propagation 2.1. Material In this study, cleavage crack propagation tests were carried out using two types of ferrite-pearlite steels with different grain size. The chemical compositions are shown in Table 1. Mechanical properties of these steels are shown in Table 2. Microstructures of these steels by optic microscope observation are shown in Fig. 1.

Table 1 Chemical compositions of steels employed (mass%)

C

Si

Mn

P

S

Al

N

S1 S2

0.1 0.1

0.19

1.5

0.01

0.003 0.003

0.028

0.0029 0.0028

0.2

1.48

0.011

0.03

Table 2 Mechanical properties of steels employed

FATT[ ℃ ] -48.1

Average grain size[ μm ]

Yield stress at room temperature [MPa]

Tensile strength at room temperature [MPa]

㻌

S1 S2

319 231

459 405

31.3 53.1

-32.0

S1 S2 Fig. 1 Microstructures of steels employed by optic microscope observation (magnitude: 100)

2.2. Experimental procedure Using the two types of steels shown in 2.1, cleavage crack propagation resistance was evaluated by DCB tests at multiple temperatures from -90 ℃ to -50 ℃ . In these tests, as the crack arrested due to the decrease of the stress intensity factor with crack growth, the temperature in these specimens was set to a constant value. The specimen of DCB tests is shown in Fig. 2.