PSI - Issue 75

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

551

6

4.2. In-plane bending The sample was inspected at reception by DPI. No indication of defect was found, some grinding marks were present at the weld toe. As for the previous sample, a frequency sweep was performed, leading to the determination of resonance frequencies of 21.5Hz in-plane and 21.1Hz out-of-plane. The target hot spot strain from the perpendicular strain gauges was 360 micro-strain, measured on the left brace at 6h and on the right brace at 12h. The first inspection was carried out after 350 000 cycles. At that moment, small cracks were found at the weld toe on the left and right braces. On the left brace, an indication of 30mm length was present at 6h, and another of 120mm at 5h30. On the right side, an indication of 40mm length was found at 11h30, at the weld toe on the chord. The test was then restarted, with the frequency regularly lowered in order to generate beach marks after 350 000, 450 000 and 550 143 cycles. A breakthrough crack was observed at 616 597 cycles by a decrease of the pressure in the left brace. The test was interrupted soon after. A deviation of strain amplitude measured to the strain gauge installed close to the weld toe (indicating a growing crack) was found on the left side at 6h after 340 000 cycles, while on the right side any deviation on the brace was only measured after 581 000 cycles. At the end of the test, a final inspection was carried out. It revealed on the left brace a crack at the heel on the brace starting from 5h15 and going up to 7h30 (length of 450 mm). Small cracks were present on the brace toe left also at 7h45 (20 mm) and 8h (30 mm). Indications were also at the weld toe on the chord at 6h30 (20 mm), 7h (70 mm) and between 7h30 and 8h (90 mm). On the right side, a long crack was present between 11h30 and 9h30 (450 mm) at the chord weld toe (see figure 6). The presence of a crack on the chord is probably relieving the stresses on the toe on the brace side, decreasing the probability of crack initiation and growth on the brace side in the vicinity of the crack on the chord side. At the start of the test, the excitation frequency was set at 19.9Hz to achieve the desired hot-spot stress. The excitation frequency remained constant during most of the test, decreasing only to 19.7Hz at the end. Similarly, the displacement amplitude in the middle of the sample remained constant during most of the test, at 2.7mm. Both measurements indicate that the stiffness of the sample did not change significantly during the test (figure 5). The strain amplitudes were measured at the crown heel positions and at +/-30° from that position. It was observed that there was a minimum at the crown position, both on the brace and on the chord. While the presence of a minimum stress on the chord was expected, it wasn’t on the brace. The hot spot stress amplitude was estimated between 95 and 102MPa on the left brace and between 84 and 88MPa on the right brace. One strain gauge was defective during the test, leading to the absence of data at the right brace at 11h (Table 1).

Table 1. Hot spot stress amplitude during the in-plane test Position

Hot spot stress (MPa)

Brace left 5h Brace left 6h Brace left 7h

101.6

94.5

101.8

Brace right 11h Brace right 12h Brace right 1h Chord left 5h Chord left 6h Chord left 7h Chord left 11h Chord left 12h Chord left 1h

No record

84.4 87.7 81.6 41.1 71.5 72.4 43.4 78.2