PSI - Issue 7

F. Fomin et al. / Procedia Structural Integrity 7 (2017) 415–422 Fedor Fomin and Nikolai Kashaev / Structural Integrity Procedia 00 (2017) 000–000

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shifts the S-N curve toward higher stresses. Thus, coarse lamellar microstructure, shown in Fig. 1(b), is more beneficial for the HCF performance than the martensitic morphology, shown in Fig. 1(a).

Fig. 1. (a) FZ microstructure in the as-welded condition; (b) FZ microstructure after PWHT; (c) effect of PWHT on the fatigue behaviour.

4. Details of fracture surface morphology Fracture surfaces of the specimens after fatigue testing were studied using an optical microscope (OM) and scanning electron microscope (SEM). All fractures occurred from internal pores or clusters of pores. These pores are located at the depth from the surface of 510 ± 145 µm (average value with standard deviation from 30 specimens). Fig. 2(a-b) shows an example of the fracture surface of the specimen that endured N f = 798,200 cycles at the maximum stress σ max = 600 MPa. A bright circular area around the crack initiation site was typically observed on the fatigue fracture surface, as shown in Fig. 2(a). This white circular mark, named “fish-eye”, is a common attribute of fatigue fracture originating from internal defects. Usually, a pore or a cluster of pores is located at the centre of the fish-eye. In the present study, the radius of the fish-eye was always equal to the distance from the specimen surface to the pore. Apparently, the bright appearance of the fish-eye arises from the relatively smooth fracture surface and consequently increased light reflection in the OM (Fig. 2(a)). The authors believe that the most plausible explanation for this effect was given by Murakami (2002) for steels and is based on cyclic contact of the fracture surfaces in the absence of atmospheric effects within the smooth area and by cyclic contact in the presence of atmosphere outside of the fish-eye.

Fig. 2. Fish-eye fracture surface of the laser beam welded Ti-6Al-4V butt joint, σ max = 600 MPa, N f = 798,200 cycles. (a) Overview OM image; (b) SEM image; (c) magnified view of the ODA region; (d) magnified view of the smooth area. In most cases, a dark area in close proximity to the pore can be found in the optical macrograph of the fracture surface (Fig. 2(a)). This particular area is darker in the OM and brighter in the SEM owing to the significantly different morphology. The region near the internal crack initiation site was called optically dark area (ODA) by Murakami