PSI - Issue 75

Philippe Thibaux et al. / Procedia Structural Integrity 75 (2025) 546–554 P. Thibaux et al. / Structural Integrity Procedia (2025)

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2.2 while it is 1.5 on the chord. On the chord, there is a local minimum of the SCF at the heel (270°). This minimum is not present on the brace. However, while there is no such minimum for the hot spot on the brace, there is a similar minimum for the stress measured at some distance of the chord / brace intersection. In a real specimen with a weld reinforcement, it is possible that a minimum is present. This has implications for the instrumentation of the specimens, as the strain gauges should be placed at the positions where the failure is expected to initiate.

In-plane bending - brace -HS1 -HS2 -HS12

Out-of-plane bending - chord -HS1 -HS2 -HS12

0,0 0,5 1,0 1,5 2,0 2,5

0,0 0,5 1,0 1,5 2,0 2,5

-2,5 -2,0 -1,5 -1,0 -0,5

-2,5 -2,0 -1,5 -1,0 -0,5

0

90

180 270 360

0

90

180 270 360

SCF

SCF

angle (°)

angle (°)

Fig. 2. Stress concentration computed on the brace (left) and the chord (right) for in-plane bending. The HS1 curve is the principal stress at 18mm from the weld toe, HS2 is the value at 56mm (brace) / 49mm (chord) from the weld toe, and HS12 is the extrapolated structural stress.

4. Experimental results 4.1. Out-of-plane bending

The sample was inspected at reception by dye penetrant and no defect was found. The weld toes were appearing rather clean, with grinding marks present at different locations. A frequency sweep was performed to verify the resonance frequency of the sample. The resonance frequencies were found to be 21.4 Hz in-plane and 21.2 Hz out of-plane, very close to the finite element simulation results of 21.3 and 21.8 Hz. The hot spot strain amplitudes estimated from the perpendicular strain gauges were between 419 and 472 micro strains, with a target around 448 micro-strains, corresponding to hot spot stresses of 100 to 110MPa. During the test, inspections were performed at specific moments, triggered by the observation of strain amplitude changes measured by strain gauges. The first inspection (DPI) was performed after 100 118 cycles. Three small indications were found at the left side on the chord between the 9 and 10h positions. On the right side, 4 small indications were found, three on the chord and one between weld beads, between 2h45 and 4h. The largest indications were 70mm long. The second inspection was performed after 200 129 cycles. On the left side, four indications were distributed between 8 and 10h. One indication was present at 2.5h. On the right side, three indications were present between 2 and 4h and three other indications between 8 and 9h30. Two of these indications were between weld beads. At that moment, the largest crack was 300mm long. A breakthrough crack was found after 365 092 cycles, on the right side at 3h. A final inspection was performed. On the left side, a very long crack was present at 9h (820mm), and two medium cracks at 3h (less than 400mm). On the right side, two very long cracks were present (1150mm at 3h and 950mm at 9h, see for example figure 4). The test was then stopped. Beach marking was applied after both inspection and after 300 000 cycles. An analysis of the strain gauge signals installed close to the weld toes was done to provide supplementary information about the initiation of cracks. A deviation on the strain gauge signals on the chord, on the right side at 3h is identified after approximately 80 000 cycles, at 9h after 100 000 cycles and on the left side 9h after 85 000 cycles.