PSI - Issue 2_A

Junichiro Yamabe et al. / Procedia Structural Integrity 2 (2016) 525–532 J Yamabe et al/ Structural Integrity Procedia 00 (2016) 000–000

527

3

In addition to the FCG test under a constant load range, Δ P , the FCG test was carried out under a constant stress intensity factor range, Δ K , to clarify the effect of f on the FCG rate. These tests are referred to as Δ P -constant and K constant tests, respectively. The crack size was obtained by means of the compliance method with crack-opening displacements (CODs) as follows: 5 4 3 2 2143.6 1214.9 236.82 18.46 / 1.0010 4.6695 x x x x x u u u u u a W − + − + − = = α (1)

1 g 1/ 2 ] 1) ([ − + = P EV B

(2)

u x

where a is the crack length, W is the specimen width, B is the specimen thickness, E is Young’s modulus, V g is the COD. The Δ K was calculated as follows: ( ) ( ) 5.6 ) 14.72 (0.886 4.64 13.32 1 2 4 3 2 3/ 2 α α α α α α − + − + + ∆ + ∆ = B W P K (3).

The FCG tests were performed under the following small-scale yielding (SSY) condition:

K

K

∆

2

2

YS max σ π

W a

(4)

]

) ( 4 )[ =

( 4 )(

− ≥

(1 ) R −

YS σ π

where σ YS is the yield stress. After the FCG test, slip deformations were observed by laser microscopy (LM). 3. Results and discussion

3.1. Hydrogen diffusivity of cold-rolled steels

Fig. 1(a) shows the Arrhenius plot of hydrogen diffusivity, D , of the cold-rolled JIS-SM490B. Literature data (Kiuchi et al. (1983); Asano et al. (1974)) are also shown in Fig. 1(a). The D of the cold-rolled steel was lower than that of α -iron and annealed low-carbon steel. This is because of lattice defect produced by the cold rolling. The activation energy of D for the steel with the rolling ratio of 40 % was nearly equal to that of the cold-rolled low-carbon steel reported by Asano et al. (1974). Fig. 1(b) shows the D of the cold-rolled steel at 303, 363 and 423 K. Irrespective of the measured temperatures, the values of D decreased with an increase in the rolling ratio; however, became constant for rolling ratios higher than 20 %. Since severe plastic deformation is produced at crack tip, the hydrogen diffusivity of which is represented by that of the steel with higher rolling ratios obtained here. Based on the Oriani’s equilibrium theory (Oriani (1970)), the experimental data were fitted by the following equation as N X / N L and E B were unknown parameters:

a

b

10 -7

10 -6

Rolling ratio 〇 5 % △ 10 % ▽ 40 % Literature data

10 -7

10 -8

10 -11 Hydrogen diffusivity, D [m 2 /s] Temperature 〇 303 K △ 363 K □ 423 K 10 -10 10 -9

10 -11 Hydrogen diffusivity, D [m 2 /s] 10 -10 10 -9 10 -8

b

a

a, b: α -iron (Kiuchi et al.) c: annealed low-carbon steel (Asano et al.) d: low-carbon steel with 40% rolling (Asano et al.)

c

d

10 -12

10 -12

0

10

20

30

40

50

3.

2.5

3

2

Rolling ratio [%]

1000 / T [1/K]

Fig. 1. (a) Temperature dependence of hydrogen diffusivity; (b) Hydrogen diffusivity vs rolling ratio at 303, 363 and 423 K.