Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 301 Figure 8 : Micrograph of weld-480 in Area I of Fig. 3(a) around tensile fracture site. Fatigue fracture behavior S-N data of the FSW joints are drawn together with that of the base material in Fig. 9. Since the welds showed softening in all conditions, their fatigue lives were lower than the base material. Open marks indicate that fatigue crack initiation took place at weld defects. More details of the fatigue behavior of the FSWed Al-Mg-Sc alloy have been discussed somewhere else [21]. In this study, the focus is limited to the influence of JLR on the fatigue behavior. In weld-720, JLR didn’t play any roles in either initiation or propagation behavior: i.e. fatigue cracks in those welds were initiated independently from the JLR distribution and their growth was not affected by JLR. In weld-480, fatigue crack initiation sites scattered around the weld both on the advancing side (AS) and retreating side (RS), as marked in Fig. 10. When a crack initiated on AS (at stress amplitudes  a = 240 MPa and 200 MPa), its propagation was also within AS and no influence of JLR was observed. Similarly, in the case of the crack initiation on RS (at stress amplitudes  a = 220 MPa and 180 MPa), the crack propagation was also in RS. Fig. 11 shows SEM image of the fracture surface of the sample tested at  a = 220 MPa. The crack origin was located on the bottom surface as indicated with an arrow, and the crack grew from this origin radially towards the opposite top surface. Macroscopic crack growth directions according to the striation patterns on the fracture surface are indicated with several white arrows. Even after the crack reached the opposite surface of the specimen, the crack front trace was not parallel to the through-thickness direction as it is typically observed for through-thickness cracks in homogeneous material under tensile cyclic loading. Figure 9 : Fatigue data of friction stir welds and base material ( R =-1).