PSI - Issue 42

3

J.R. Antunes et al. / Procedia Structural Integrity 42 (2022) 588–593 Author name / Structural Integrity Procedia 00 (2019) 000–000

590

Figure 4 – Residual stress measurement locations for a) unpeened sample b) peened sample. Figure 3 – Residual stress measurement locations for: a) unpeened sample; b) peened sample.

4. Fatigue tests Tension-tension fatigue tests were conducted at room temperature, on a load-controlled 250 kN Instron 8800MT testing machine controled with Wavematrix software. The specimens were clamped using bolted grips and tested at a frequency of 5 Hz and load ratio, R = 0.1. The samples were tested at different load levels with respect to the norm BS EN 6072:2010. The sample preparation involved polishing the sharp edges using a deburring tool and emery paper P2500 grit to minimize the potential of crack initiation from the sides. This was done cautiously in order to avoid reduction of the cross-sectional area and having to use a correction factor on the applied external load. 5. Results 5.1. Fatigue test results The S-N results from the constant amplitude fatigue tests are provided in Figure 4. These results show that the fatigue life of the welded specimens is reduced by an average of 68% when compared to the reference base material . After failure, the FSW samples were visually inspected and the preferred fracture locations in the weld plane and perpendicular to it were investigated. For the reference samples, failure occurred in the region of the smallest cross-section, as expected. For the as welded samples, failure occurred consistently on the advancing side of the weld. The results for as-welded samples after the laser peening treatment can also be observed in Figure 4. The data shows a clear increase in fatigue life for the treated samples, particularly for the lower stress range. For the two lowest applied stresses (200 and 240 MPa), the number of cycles increased by a factor of 2.1 to 2.4 respectively. For the higher end of the load spectrum, this increase becomes less significant, with the data overlapping at 350 MPa.

Figure 4. S-N fatigue test results for peened and unpeened samples

5.2. Residual stresses through the thickness direction The residual stresses of unpeened samples were investigated using the ICHDM on the centreline of the weld in the middle of the section. The results can be observed in Figure 5.