Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 302 Figure 10 : Fatigue crack initiation sites of weld-480. Irregular fracture morphology was observed in the lower and left-hand part of the specimen. In order to investigate the fatigue crack growth behavior on this irregular fracture, a cross section at B-B’ was examined, as shown in Fig. 12. Macroscopically large flexure was observed on this cross section (Fig. 12(a)). The profile C-C’ corresponds to the irregular fracture site (see Fig. 11). JLR was well pronounced in this cross section, and the crack encountered JLR. As seen in the enlarged image (Fig. 12(b)) of the rectangle in Fig. 12(a), it can be observed that the crack propagation partially occurred along JLR. This area was nearly consistent with the fracture site along JLR in the tensile test as mentioned in the previous section (see Fig. 8). Thus, also fatigue crack propagation partially along JLR resulted in macroscopic flexure of the crack path. SEM fractography was performed for more detailed observation in the areas marked by the rectangles II in Fig. 11. Fig.s 13(a) shows Area II where the irregular fracture began to appear. The lower part of the image displays the irregular fracture surface, i.e. JLR-fracture site, see arrow (1). In the upper part fine striations were observed with intervals of a few micrometers. Major crack propagation direction in this area was upwards according to the orientation and the development of intervals of the striations, see arrow (2), and it indicates that the crack propagated from the JLR-fracture. Fig. 13(b) shows higher magnification image of Area III marked in Fig. 13(a). No striation was observed in the JLR- fracture part (lower part of Fig. 13(b)). Instead, very fine asperity was formed. Since the asperity of JLR fracture in the lower part of Fig. 13(b) was equivalent to the average grain size of weld-480 (1.67 µm), it is considered that the crack propagated intergranularly at JLR. Figure 11 : Fatigue fracture surface of weld-480 tested at  a = 220 MPa. Figure 12 : Cross section at B-B’ in Fig. 11: (a) macrograph, (b) magnified view of the area defined by the rectangle in (a).