PSI - Issue 13

Rui Guo et al. / Procedia Structural Integrity 13 (2018) 2202–2209 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

2205

4

the Y direction is defined and the fully constrained boundary conditions are applied to both ends of the Y direction, thus axial welding simulated residual stress distribution was obtained. Another identical finite element model was established and the material was defined in accordance with the actual conditions of the safety end welded joints of the primary circuit in PWR, as is shown in Table 1. Keep the boundary constraints on both sides of the y direction unchanged, and introduce the residual stress field in the homogeneous linear elastic material model by using the predefined field. By comparing with the original data by Hayashi et al. (2009) , Figure 4 is obtained. Obviously the residual stress distribution obtained by the stress field introduction method is more reasonable.

400

400

Standard Residual Stress Predefined Residual Stress

Standard Residual Stress Predefined Residual Stress

200

200

-400 Residual Stress  r (MPa) -200 0

-400 Residual stress  r (MPa) -200 0

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

Distance from pipe inside to outside(mm)

Distance from pipe inside to outside(mm)

Figure 3 Simulation of residual stress by predefined temperature field method

Figure 4 Simulation of residual stress by stress import method

3. Numerical Experiments In order to study the influence of crack length on the stress and strain around crack tips under the effects of the residual stress field in the safety end of the primary circuit pressure vessel in nuclear power plants, a crack model was established for every 5 mm of the Alloy 182 welding zone in the geometric model, as shown in Figure 1(c), crack position 1. The effect of the residual stress field on the stress and strain field around crack tips when the crack exists or extends to this point. In order to study the variation of the stress and strain field around crack tips adjacent to the welding interface, as shown in Figure 1(c), crack position 2, cracks of 4, 5, and 6 mm were taken near the welding interface of A533B-182, to make it approach, reach, and pass over the welding interface, and observe the changes in stress and strain fields around the crack tips. The same crack tip mesh refinement was adopted for all the models during this study, as shown in Figure 1(c), to ensure the consistency of the accuracy of the simulation results. The plane strain CPE8R type unit was adopted, and the amount of model elements is around 30,000. 4. Results and Analysis 4.1 The analysis of fracture parameters around Crack tips of different length midst the seam By calculation, crack tip stress and strain fields with lengths of 5mm, 10mm, 15mm, 20mm, 25mm, and 30mm were obtained, respectively. Von Mises stress distribution within 1 mm radius around 6 crack tips is shown in Fig. 5. As shown in Fig. 5, when a the crack length is less than 10 mm, the Von Mises Stress of the crack tips  e become larger as the crack length increases. When a is longer than 10 mm and shorter than 25 mm, the  e gradually decreases, and when a reached 25 mm, the  e reached its minimum. Then the  e distribution is independent of the crack when a reaches 30 mm. Note that in Fig. 5(c)(d)(e)(f), the stress distribution on two sides of the seam are asymmetric. This is due to that the axial residual stress is tensile stress within about 17 mm from the inner wall of the pipe in the safe end welding structures of the primary circuit of nuclear power plant. In the range of 17mm to 50mm, the stress is compressive, as shown in Fig. 2. When the crack length reaches 15mm, the stress on both sides of the crack is released, breaking the original stress balance. Besides, because the crack is in the welding area, the material mechanics of the two base metals and the welding material on both sides are different, and the weld shape is “double V”, which leads to the occurrence of as ymmetry of axial stress field around crack tips near the welding interface in the welding area. It can be seen from the previous study that the equivalent stress value and the Y