PSI - Issue 42

4

Author name / Structural Integrity Procedia 00 (2019) 000–000

J.R. Antunes et al. / Procedia Structural Integrity 42 (2022) 588–593

591

Figure 5 - Residual stress measurements using the ICHDM for an unpeened sample.

As can be observed, for an as-welded FSW sample before any peening treatment, the residual stresses are tensile in nature for both the longitudinal and transverse directions. The magnitude of the stresses is significantly high (90 to 130 MPa) in the longitudinal direction of the weld. However, in the transverse direction, the residual stresses are close to 15 MPa on average. This stress direction is the most significant since it coincides with the loading direction of these samples during fatigue testing. The residual stresses after laser peening can be observed in Figure 6, according to the setup explained in section 3.

Figure 6 - Residual stress measurements using the ICHDM on peened samples. a) Longitudinal direction to the weld; b) Transverse direction to the weld.

At the weld centreline (measurement A), the superposition of stresses resulted in a peak compressive stress of approximately 230 MPa on both the shoulder and root sides of the weld for the longitudinal direction of the weld, and 180 MPa for the transverse direction. The balancing tensile stresses were found at the centre of the thickness, with a peak magnitude of 50 and 110 MPa. The penetration depth was approximately 750 and 500 µ m on both sides. At the interface of the peening region (measurement B), the stress profile has a similar shape compared to measurement A, although there is a general decrease in peak compressive stress of about 50 Mpa in both directions. Finally, for measurement C outside of the peening region, the stresses are constant and close to zero in both directions. 6. Discussion 6.1. Residual stresses and fatigue behaviour of unpeened samples As observed in section 5.2, the baseline stresses of the FSW joint before any peening treatment are subject to tensile residual stresses at the welding line. This is detrimental to fatigue life as was verified by the S-N results, given that they will promote