PSI - Issue 5

Etienne Bonnaud et al. / Procedia Structural Integrity 5 (2017) 310–317 Bonnaud / Structural Integrity Procedia 00 (2017) 000 – 000

316

7

Here again the optimization algorithm finds a sequence giving almost zero displacement but simulation of this computed sequence gives a much higher displacement value, which indicates that this optimization procedure, as it is, does not work properly either. A closer look at the kinematic of the problem again gives the explanation. During pipe butt welding, two competing phenomena influence the inwards/outwards deformation, namely hoop shrinkage and axial shrinkage. Hoop shrinkage always leads to an inward deformation whereas axial shrinkage can lead to either inward or outward deformation, depending on the location of the pass being laid and on the momentary stiffness of the weld. This is illustrated in Fig. 9 showing deformations when all beads in the inside are welded before all beads in the outside (Sequence Inside-Outside) and when all beads in the outside are welded before all beads in the inside (Sequence Outside-Inside).

1

3

6

2

5

5

6

2

4

1

7

3

8

4

7

8

Fig. 9. Illustration runs. Beads 1, 2, 5 and 6 can give very different displacement values depending on their position in the sequence, see black arrows.

When welding 3-4-7-8 (first half of the red curve and second half of the blue curve), both hoop and axial shrinkages contribute to an inward displacement. Each displacement value is therefore negative and relatively high in both sequences. When welding 1 2-5-6 (first half of the blue curve and second half of the red curve), hoop and axial shrinkages act against each other. When welding 1-2-5-6 first, hoop shrinkage dominates over axial shrinkage and displacements are negative. On the contrary, when welding 1-2 5-6 last, axial shrinkage dominates over hoop shrinkage and displacements are positive. This is what misleads the surrogate algorithm: beads 1, 2, 5 and 6 can either cause a displacement inwards or outwards depending on their position in the sequence. A way to correct this difficulty is to store two different displacement values for pairs involving these particular beads, depending on whether beads 3 and 4 have already been deposited or not. Table 7 shows the new selected runs and Table 8, the new pair matrix. In the pair matrix, 23 positions now have two values: in those cases, values correspond to deposition before and after welding of the 3-4 pair.

Table 7: Selected runs for pipes (modified).

Table 8: Pair matrix for pipes (modified).

Run 1 Run 2 Run 3 Run 4 Run 5 Run 7 Run 8 Run 9

3 1 3 1 3 1 1 3

4 2 2 4 4 2 4 3 4 1 2 2 2 2 2 2

1 4 1 3 7 3 2 4 7 2 4 4 4 3 4 1

2 5 6 2 2 5 6 2 8 5 7 6 3 6 8 5

5 6 4 7 8 4 5 6 2 4 1 3 6 5 1 4

8 3 5 5 1 8 3 7 1 6 8 5 7 4 7 7

6 8 7 6 6 7 7 8 5 7 5 7 5 7 5 8

7 7 8 8 5 6 8 5 6 8 6 8 8 8 6 6

1

2

3 1 2 0 2 1 2 0 0

4

5

6

7 1 1 1 5 2 4 0 2

8 1 1 1 2 2 1 8 0

1

0 6/1

2 1/1 1/1 5 1/1 1/1 4 1/1 1/1 0 1/1 1/1

2 2/1

0

3

1 4/1

4 0/1 1/1

Run 6 1

5 6

0 0

0 0

4

0 1/4

1 2/1

0

7 0/1 0/1 8 0/2 0/1

0 0/3 0/1 0 0/2 0/2

Run 10 3 Run 11 3 Run 12 1 Run 13 1 Run 14 1 Run 15 3 Run 16 3