PSI - Issue 23

Barbara Nasiłowska et al. / Procedia Structural Integrity 23 (2019) 577– 582 Nasiłowska B./ Structural Integrity Procedia 00 ( 2019) 000 – 000

580

4

A special attention should be paid on development of secondary subsurface fractures perpendicular to action of the shot peening force. The research on crystallographic orientation of austenite grains after shot peening, presented in work (Nasiłowska et al. (2015)), showed occurrence of surface squeeze of the grains in the surface layer. This process favourably influenced the retardation of the fatigue fracture initiation. The strengthened layer constituted a barrier for fracture development while orienting the fracture front deep into the subsurface layers. Hence, the secondary fractures were located on the occurrence boundary. In the joints welded with a laser beam (non-shot peened), the largest tensile strain occurred in the weld axis S (117.1 MPa) and in the heat affected zone HAZ (296.3 MPa), where a fatigue fracture initiated (Nasiłowska et al. (2015)). The fatigue fracture in the non-shot peened joints occurred in the weld coming into the fusion line and the heat affected zone (Fig. 3 b).

a

b

5 00 µm

500 µm

Fig. 3. (a) transverse microsection of the fatigue fracture of the weld made with a laser beam and shot peened as well (b) as non-shot peened

Under an influence of shot peening, there occurred an increase in compressive stress equal to (-730,6 MPa (SWC), -713,7 (S) (Nasiłowska et al. (2015) ). Analysing the fatigue fractures, it was observed that a fatigue fracture in the shot peened joints ran from the face side on the fusion line with propagation towards the ridge (Fig. 3 a). The initial fracture in the specimen made with TIG method – non-shot peened usually developed in the weld (Fig. 4 a), from the face side and then initiated from the ridge side. However, there are also generated secondary fractures in the heat affected zone and at this place the specimen fracture occurred.

b

a

1 mm

1 mm

Fig. 4. Transverse microsections of the specimens together with a developing initial fracture within the weld made with TIG method (a) non-shot peened, (b) shot peened

A number of cycles to damage the welded joints after shot peening increased by approximately 24% for a laser weld and by approximately 59% for a weld made with TIG method compared to non-shot peened specimens. The results of experimental tests unequivocally show the purpose of surface machining in the form of mechanical shot peening resulting in an increase in fatigue durability of the elements subjected to this process. Analysis of the results of the research on the incubation period and development of fatigue fractures is presented in Table 1.