PSI - Issue 13

Yoshiki Mikami et al. / Procedia Structural Integrity 13 (2018) 1804–1810 Author name / Structural Integrity Procedia 00 (2018) 000–000

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bending and tensile loading during unloading. Therefore, the region where the residual stress changed was limited to the notch root as shown in Fig. 8(c). The required length of the precrack extension is 2.5 mm for a 50-mm-thick specimen; therefore, the residual stress modified region is enough to introduce the precrack.

a

b

c

1500

AW

RB

t/4 t/2 3t/4 t/2 t/2 3t/4

RB

Residual stress, σ y (MPa) −1000 −500 0 500 1000

Reverse bent

AW

t /4

t /4

t /2

t /2

3 t /4

3 t /4

As welded

Ligament

Ligament

Machined notch

Machined notch

0

1

2

3

4

5

Distance from notch root, x (mm)

Fig. 8. Residual stress distribution in front of the notch root; (a) as-welded specimen, AW; (b) reverse bent specimen, RB; (c) distribution of the residual stress in front of the notch root at locations t /4, t /2, and 3 t /4, where t is the thickness (50 mm).

4.2. Effect of weld residual stress on the P–V g curve and crack opening profile The relationship between the bending load P and the notch mouth opening displacement V g is presented in Fig. 9(a). The P – V g curves for the AW and RB cases are different from that for the BM case owing to the weld residual stress. The difference in the residual stress distribution between the AW and RB cases is limited in the vicinity of the notch root (Figs. 8(a) and (b)); therefore, the P – V g curves—an overall response of the specimens against bending—of AW and RB cases are similar. In contrast, the crack opening profile is clearly affected by the residual stress. The crack opening profiles at the same V g level, 0.6 mm, are presented in Fig. 9(b) for cases BM, AW, and RB. The contour shows the stress distribution in the crack opening direction ( σ y ). Figure 9(b) presents the crack opening profile by cutting the model by the cross-section at mid-thickness. Two lines that correspond to the blunting crack-tip shape at mid-thickness and the bottom surface can be seen. The CTOD values for these cases were calculated using a 45°- intersection definition as shown in Fig. 9(b). As seen from the figure, the calculated CTOD value for the AW case is quite different from that for the BM case, which is the effect of the weld residual stress. When the residual stress distribution was modified by reverse bending, the crack-tip opening profile became comparable with that for the BM case. This means that the influence of the residual stress on the crack-tip opening behavior was reduced by the reverse bending treatment. Such a difference in the crack-tip opening behavior could not be evaluated by the apparent P – V g curves and could be clarified only by the proposed fracture mechanics simulations considering complex residual stress distributions that exist in actual welds.

a

b

200

BM AW RB

RB

BM

AW

(MPa) y

150

0.033 mm

0.055 mm

0.057 mm

Load, P (kN)

100

0.095 mm

0.065 mm

0.12 mm

50

0

bottom mid-thickness

bottom mid-thickness

mid-thickness bottom

Notch mouth opening displacement, V g (MPa) 0 0.5

Fig. 9. (a) P – V g curves; (b) crack opening profile at V g = 0.6 mm.