PSI - Issue 2_B

2500 Yoshiki Nemoto et al. / Procedia Structural Integrity 2 (2016) 2495–2503 Author name / Structural Integrity Procedia 00 (2016) 000–000 steel at intervals 10°C within �1�0°C� � 100°C in Steel A and B, and �1�0°C� �1�0°C in Steel C. Quasi-CTOD is employed as a parameter of the fracture toughness. The quasi-CTOD  is calculated by simply applying the CTOD estimation formula of BS 7448 (1991), as 6

r W d a r W d V    ) ( ) ( p

2

2

(1

)

K











(1)

2

E



Y

where Y  is yield stress, r is the rotation factor (=0.4), W is specimen width (=10mm) and d is notch depth (=10mm). p V is a plastic component of the notch mouth opening displacement which is measured by a clip-gauge. K is a stress intensity factor.

1,0

1,0

0,8

0,8

0,6

0,6

0,4

0,4

A B C

Area fraction

Area fraction

0,2

0,2

0,0

0,0

0,00 0,02 0,04 0,06 0,08 0,10

0,00

0,02

0,04

0,06

Ferrite grain diameter [mm]

Perlite band thickness [mm]

Fig. 3. Distributions of grain sizes (a) Ferrite grain diameter (b) Pearlite band thickness

Table 3. Representative values of test steels

Ferrite grain diameter [  m]

Pearlite band thickness [  m]

Steel

Mean

Maximum

Mean

Maximum

A B C

38.7 56.0 45.8

83.1

14.0 17.8

85.4 68.5 31.3

120.6

99.0

9.2

The fracture initiation site were obtained by fractography using a SEM because fracture initiation sites were determined for the validation of the proposed model as well as the fracture toughness (Chen and Wang, 1998). The initiation sites were identified by tracing river pattern markings on the fracture surface. The distance from the notch root to the fracture initiation site was measured.

Rolling Direction

60

10

2

20

0.25

80

10

unit : mm

Notch Detail

Fig. 4. Schematic diagram of the fracture initiation process