• Determine if the fatigue strength of the microstructure is reduced by overlapping the heat-affected zones of two parallel welds.
• In order to focus on the microstructure, it is necessary to design the fatigue test specimen such that the test excludes the influence of geometrical stress concentrations, weld discontinuities as well as linear and angular misalignments.
• Determine the location of crack initiaion.
Experimental procedure
The fatigue specimens were extracted from the welded production plates in chapter 7. The objective of the test was to see if overlapping heat-affected zones influences the fatigue behavior in the absence of stress concentrations due to geometry, weld discontinuities, angular and linear misalignments. In order to achieve this, it was decided to remove any influence from stress concentrations. It was decided to machine the top and bottom surface of the specimens before fatigue testing, mainly from the bottom face. The fatigue machine grip limit was 11 mm and the thickness of the plate was 15 mm so 4 mm had to be removed from the bottom face.
This also removed the weld cap and introduced compressive stresses in the weld surface.
Some angular misalignment due to distortion was observed after welding as documented in Table 9-5 and in figures below.
Table 9-5 - Welding distortion in production plates
Welding Distortion in [°]
Plate ID 8612-2 PL3 PL4 8612-3 PL5 PL6 8612-4 PL7 PL8
Bottom Weld A 3 3 3 3 -1.5 2
Bottom Weld B 1 0.5 3 2 -2 0
Top Weld A 3 3 2 3 -1 1
Top Weld B 2 0 0.5 1 -3 0
Figure 9-9 – Weld distortion after second weld PL3.
Figure 9-10 - Weld distortion after second weld PL5.
Figure 9-11 - Weld distortion after second weld PL7.
The specimen was machined in sequence as shown in Figure 9-12. Initially the top face was machined down until parallel and subsequently the bottom face was machined until the surface was parallel to the top. In order to minimize the effect on the microstructure the machining was performed at lower speeds and with an increase amount of coolant.
Figure 9-12 – Machining of fatigue specimens from welded S420G2+M plates in order to remove stress concentrations and misalignments the top face was initially machined until parallel followed by the machining of the bottom face.
The specimen dimensions were the same as for the unwelded pure base material specimens as seen in Figure 9-6 and method of preparation was also the same.
Results and discussion
The sample size from the fatigue testing of the welded production plates are presented in Table 9-6. Due to the distortion of the plate 8612-4 PL7 DW5 the machining of the top face resulted in the removal of the critical area of the conjoining HAZ. Due to time constraints it was not possible to machine more specimens that had this critical distance between the welds.
Table 9-6 – Fatigue test sample size production plate.
Fatigue test samples
Specimen ID Sample size Distance between welds
8612-2 PL3 DW50 2 44 mm
8612-3 PL5 DW15 2 12 mm
8612-4 PL7 DW5 1 1.2 mm
The results from the fatigue tests are shown in Table 9-7 and
Table 9-8.
Table 9-7 - Results from fatigue test of the production plate specimens.
Fatigue test R=0.1 Production plate specimens
Specimen ID 8612-2 2A (B) 8612-2 2A (B) 8612-2 2A (BB) 8612-3 A3 8612-3 A3
Weld distance 44 mm 44 mm 44 mm 12 mm 12 mm
Test number 12 13 17 14 14
Run sequence 1st run 2nd run 1st run 1st run 2nd run
Stress range [MPa] 375 400 400 375 400
Area [𝑚𝑚^2] 232.0 232.0 243.1 235.25 235.25
Max load [MPa] 416.7 444.4 444.4 416.7 444.4
Min Load [MPa] 41.7 44.4 44.4 41.7 44.4
Mean load [MPa] 229.1 244.42 244.42 229.1 244.42
Machine Load [MPa] 229.1 ± 187.5 244.42 ± 200.0 244.42 ± 200.0 229.1 ± 187.5 244.42 ± 200.0
Cycles 5 000 000 439 404 322 187 4 953 349 483 574
Displacement range 0.69 - 0.80 0.70 0.76
Run out/Fracture Run out Fracture Fracture Run out Fracture
Specimen ID Description References
8612-2 2A (B) Run out at 375 MPa after 5 000 000 cycles.
Continued second run at 400 MPa and fractured after 439 404 cycles.
Fracture occurred in the base material at a previously detected surface defect.
8612-2 2A (BB) Fractured at 400 MPa after 322 187 cycles.
Fracture occurred in the base material on the side face.
8612-3 A3 Run out at stress range 375 MPa after 4 953 349 cycles.
Continued second run at 400 MPa and fractured after 483 574 cycles.
Fracture at surface defect in base material. An additional crack initiated in the base material at another surface defect.
Figure 9-13, Figure 9-14, Figure 9-15
Table 9-8 - Results from fatigue test of the production plates.
Fatigue test R=0.1 Production plate specimens
Specimen ID 8612-3 3A 8612-4 8612-4
Weld distance 12 mm Initial distance was 1.2 mm. Initial distance was 1.2 mm.
Test number 18 15 15
Run out/Fracture Fracture Run out Fracture
Specimen ID Description References
8612-3 3A Fractured at 400 MPa after 306 418 cycles.
Fracture initiated in the base material.
8612-4 Initial run out at 375 MPa after 4 938 994 cycles.
Fracture at 2 084 237 cycles at 400 MPa.
The fracture initiated between the welds inside the specimen in what seemed to be a weld defect in weld B.
This sample was also subjected to a lower load level due to miscalculation of the cross-sectional area.
The results from the fatigue testing indicates that the welded joints had a higher fatigue strength than the base material. This agrees with Maddox, that writes [11] that if a weld joint would be machined and grinded flush, the most normal crack initiation location would be the base material. This statement is based on one single weld. The results from the fatigue test showed that this was the case for 12 mm and 44 mm between two parallel butt-welds.
The preparation procedure might have adversely affected the results due to introducing compressive residual stresses that inhibited crack initiation.
The fatigue test of the sample with 1.2+ mm between the welds failed in what seemed was a weld defect on the side face of the specimen, so the result from this was inconclusive.
The effect of residual stresses did not result in a fracture in the weld joint.
The number of cycles needed to fracture the specimen was very similar to that of pure base material. This may be due to the careful preparation process that all specimens underwent and since the base material was the weakest part of the specimen.
A visual inspection of each specimen was performed before every test, and some (Figure 9-13,Figure 9-14 and Figure 9-15) visible surface defects was detected and recorded. After fracturing, these specimens where examined and in several cases these locations were the source of crack initiation.
Figure 9-13 – Final fracture and location of crack initiation of 8612-3 A3 specimen.
Figure 9-14 – Location of a small defect in the base material surface where fracture occurred.
Figure 9-15 - Location where the crack initiated was at the location of the defects in the base material.
Summary
The results from the fatigue test indicates that having two parallel buttwelds, in the absence of stress concentrations and welds discontinuities, does not negatively impact the fatigue performance of the weld joint at the distances of 12 mm and 44 mm.
The specimen with an initial distance between the welds of 1.2 mm failed due to a weld defect, and therefore was inconclusive.
The preparation method might have adversely affected the results due to introducing compressive residual stresses that inhibited crack initiation.
The main fatigue life of the “production plate specimens” composed of the crack initiation stage.