DECLARATION 2 PUBLICATIONS
7.3 Residual Stress Measurement Results
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Figure 7-15: Stress Profiles for Weld Specimens
The three components of stress (i.e. hoop, axial and radial) that were measured in each welded sample are listed in Table 7.3. The stress values given were recorded at the point closest to the weld centreline (i.e. 6 mm away from point 0,0,0 on the flange side, and 13.6 mm away from point 0,0,0 on the pipe side, as indicated in Figure 4.5 above). It can be observed from the table that hoop stress is the highest of all the three dimensions of stress. The highest observed hoop stress on the flange side is 425 MPa in Sample 9, whereas the lowest hoop stress is observed in Sample 8 at 292 MPa. On the pipe side the highest hoop stress is 558 MPa in Sample 15, and the lowest is 295 MPa in Sample 7. The yield stress of the filler metal is approximately 440 MPa; the recorded tensile hoop stresses are therefore very close to or above the yield strength.
-400 -200 0 200 400 600
0 10 20 30 40 50
Stress in MPa
Distance from WCL
Sample 12 Flange
Hoop Axial Radial
-200 0 200 400 600
0 20 40 60 80
Stress in MPa
Distance from WCL
Sample 12 Pipe
Hoop Axial Radial
-600 -400 -200 0 200 400 600
0 10 20 30 40 50
Stress in MPa
Distance from WCL
Sample 15 Flange
Hoop Axial Radial
-200 0 200 400 600
0 20 40 60 80
Stress in MPa
Distance from WCL
Sample 15 Pipe
Hoop Axial Radial
-500 0 500
0 10 20 30 40 50
Stress in MPa
Distance from WCL
Sample 16 Flange
Hoop Axial Radial
-400 -200 0 200 400 600
0 20 40 60 80
Stress in MPa
Distance from WCL
Sample 16 Pipe
Hoop Axial Radial
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Table 7-3: Residual Stress Results at WCL Sample I
(Amp) V
(Volts S
(mm/s) FR
(mm/s) Mode Flange (MPa) Pipe (MPa)
Hoop Axial Radial Hoop Axial Radial
4 380 30 10 18.3 CA 371 -11 37 431 106 61
6 380 25 8 18.3 CA 402 52 78 423 85 58
7 360 25 8 18.3 CA 359 12 36 295 -74 -16
8 360 25 10 18.3 CA 292 64 82 357 150 1
9 380 25 10 18.3 CA 425 66 130 507 132 69
12 360 30 8 16.7 CW 407 51 80 409 41 53
13 360 30 8 18.3 CW 375 49 42 381 -11 -26
15 360 25 8 18.3 CW 355 27 65 558 203 148
16 360 25 8 16.7 CW 412 70 117 507 76 41
Hoop stress is also the most dangerous as it is in tensile form and it contributes to stress-induced cracking and hence reduction of fatigue life of the welded structure. Hemmesi et al. (2014) showed that the behaviour of residual stresses that are exposed to multiaxial loading is completely different from those exposed to uniaxial loading. Farajian et al. (2014) also noted that residual stresses are multiaxial in nature. Hence, the superpositioning of WRS and load stress in one direction is inadequate to properly account for stresses experienced by the welded structure under fatigue loading. The authors showed that in circumferential welds, the principal residual stresses in the WCL and weld toe are parallel and normal to the weld bead, respectively. It is for these reasons that in the present study the effect of each of the three components of stress is not considered to be linear. The study by Lu (2002) also showed that the dominant components of residual stress in a circumferential weld are hoop and axial stresses. It was therefore decided to rank the two components of residual stress as follows:
Radial component has a factor of 1.0 Axial component has a factor of 1.1 Hoop component has a factor of 1.25
Table 7.3 was then updated with the ranking shown above, and the resultant stresses are shown in Table 7.4.
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Table 7-4: Ranked Residual Stress Results at WCL
Sample Mode Flange (MPa) Pipe (MPa)
Hoop Axial Radial Net WRS
Hoop Axial Radial Net WRS
4 CA 464 -12 37 489 539 117 61 717
6 CA 502 57 78 637 529 94 58 681
7 CA 449 13 36 498 368 -81 -16 271
8 CA 365 70 82 517 446 165 1 612
9 CA 531 73 130 734 634 145 69 848
12 CW 509 56 80 645 511 45 53 609
13 CW 469 54 42 565 476 -12 -26 438
15 CW 444 30 65 539 698 223 148 1069
16 CW 515 77 117 709 634 84 41 759
The lowest net stresses are highlighted in Table 7.4. Samples 4, 7 and 8 (in that order) have the lowest net stresses from the plate side. Samples 7, 12 and 13 have the lowest net stresses from the pipe side.
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Figure 7-16: Flange Side Residual Stress Distribution for Samples 4, 7 and 8 -400
-200 0 200 400 600
0 10 20 30 40 50
WRS MPa
Distance from WCL in mm
Hoop Stress - Flange
Sample 4 Sample 7 Sample 8
-100 -50 0 50 100 150
0 10 20 30 40 50
WRS MPa
Distance from WCL in mm
Axial Stress - Flange
Sample 4 Sample 7 Sample 8
-100 -50 0 50 100 150 200 250
0 10 20 30 40 50
WRS MPa
Distance from WCL in mm
Radial Stress - Flange
Sample 4 Sample 7 Sample 8
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Figure 7-17: Pipe Side Residual Stress Distribution for Samples 4, 7 and 8 -200
-100 0 100 200 300 400 500
0 10 20 30 40 50 60 70 80
WRS MPa
Distance from WCL in mm
Hoop Stress - Pipe
Sample 4 Sample 7 Sample 8
-400 -300 -200 -100 0 100 200 300
0 10 20 30 40 50 60 70 80
WRS MPa
Distance from WCL in mm
Axial Stress - Pipe
Sample 4 Sample 7 Sample 8
-200 -150 -100 -50 0 50 100
0 10 20 30 40 50 60 70 80
WRS MPa
Distance from WCL in mm
Radial Stress - Pipe
Sample 4 Sample 7 Sample 8
131 | P a g e The stress distribution of the “lowest three” specimens is shown in Figure 7.16 and Figure 7.17.
The stress profiles for hoop stress in both the flange and pipe sides are very similar for all three specimens. Axial stresses for the flange side show very different readings closer to the weld region, and they seem to become more steady as the distance away from WCL increases. Axial and radial stresses for Sample 4 are highly compressive on the pipe side at distance of about 20 mm from the starting point. This distance coincides with the location of the weld toe region.
A number of studies discussed in this thesis, as shown in Chapter 2, involved fatigue failures that originated from the weld toe, wherein the weld toe experienced tensile mean stresses as a result of WRS (Rading, 1993; Lu, 2002; Rosenfeld & Kiefner, 2006). However, some researchers have shown that weld toes of circumferential welds can be under compressive stress (Pasta & Reynolds 2007). Although the multiaxiality of residual stress is such that one component of stress doesn’t unilaterally determine the position of the maximum principal stress in a welded structure, such significant compressive stresses as observed in this case would make it unlikely for failure to occur at the weld toe. Apart from Sample 4 early readings, the rest of the samples seem to have similar distributions of radial residual stresses across the measured distance.
Table 7-5: Parametric Combination for Samples 4, 7, 8, 12 and 13 Sample I
(Amp) V
(Volts S
(mm/s) FR
(mm/s) Mode Heat Input (kJ/mm)
4 380 30 10 18.3 CA 969
7 360 25 8 18.3 CA 956
8 360 25 10 18.3 CA 765
12 360 30 8 16.7 CW 1148
13 360 30 8 18.3 CW 1148
The identified samples are chosen as the best using the applied criteria of lowest net WRS;
however, it may still happen that fatigue properties of the said samples are not necessary in agreement with this finding given other factors that influence the latter. Furthermore, since the parametric combination of the identified three samples is known, this could still be used further to identify welding parameters that produce the most optimal welds in terms of WRS distribution.
Table 7.4 presents the welding combination for the three “best performing” samples in terms of residual stress distribution from the flange (or plate) side, and the three from the pipe side. It can be seen that the three best samples on the flange side all have the same wire feed rate (FR) of 18.3 mm/s, and they all have the same mode setting of constant amperage (CA). Sample 7 appears both on the flange and pipe sides. The rest of the samples from the pipe side have the same mode setting of constant wire (CW) and have the same power input. The settings of current, voltage and speed is also the same on the pipe side samples. This effectively means that both mode
132 | P a g e settings can produce optimal parametric combinations depending upon which residual stress distribution is used as criteria between the flange side and the pipe side. The choice of residual stress distribution will be influenced by the point and direction of loading of the welded structure.
For example, a case where the cantilever load is applied at the tip of the free side of the pipe, with the flange fixed, would require the flange side stresses to be considered since the forces will be acting in that direction.
The parametric combination of the five samples that are highlighted in Table 7.4 is given in Table 7.5 together with the heat input of each iteration. The heat input of Sample 4 and Sample 7 are very similar in magnitude. Both these samples were generated using the CA mode setting of the welding machine. The heat input of Sample 12 and Sample 13 are exactly the same in magnitude, even though the wire feed rate of the two is different. The two samples were generated using a CW mode setting of the welding machine. It should be noted that the heat input of the two CA mode samples differ significantly from that of the two CW mode settings. It is clear, therefore, that if the four samples with different heat inputs can all have favourable residual stress properties compared to the rest of the samples, then the mode setting clearly plays a role in determining the resultant conditions of the residual stress fields in the welds.