Issue 53

C. Navarro et alii, Frattura ed Integrità Strutturale, 53 (2020) 337-344; DOI: 10.3221/IGF-ESIS.53.26 341 From an initial view, it can be seen that HIP and AS BUILT groups are clearly less resistant than other groups. The SP group shows a higher strength, especially for high cycle fatigue; CASE increases this strength with near ten times the lives presented by SP. Finally, LP group shows the best results among all groups. Considering the initiation point of the specimens, all specimens of AS BUILT group initiated the crack on the surface, as well as the HIPed specimens. In the case of shot peened and LP, all specimens failed from the interior, but one, which failed from a defect on the surface, for SP, and on the rounded corner, for LP. All SP+CASE specimens failed with cracks initiated in the interior. The results for HIPed specimens are very similar to those obtained by Kasperovich and Hausmann [28] with as built surface but HIPed specimens. This low strength can be understood considering that HIP does not eliminate or reduce the surface defects [11,15,29,30]. So the crack initiates similarly than in the case of AS BUILT specimens. Tests with HIPed and surface finished specimens are planned for the future, where a high improvement of the fatigue strength is expected, as occurred to Kasperovich and Hausmann with HIPed and machined specimens. Figure 3: Fatigue test results. Each group is identified with a different symbol. Symbols are shown as solid for failures initiated from the interior and hollow for failures initiated from the surface Regarding the shot peened specimens, results are in the same line that those obtained by Bagherifard et al. [20] for Al10SiMg, with a clear improvement of the strength. In Ti6Al4V specimens the results are also similar to those obtained by Wycisk et al [19] under axial loads. As far as authors know there are not results in the literature for the other twe surface treatment considered here. Figure 4 depicts two fracture surfaces with the initiation points marked by arrows. Figure 5 shows SEM images of the initial defects of two of the specimens tested, one produced by lack of fusion and the other a gas pore. Figure 6 represents the position of each initiation point for all specimens that failed with an initiation point at the interior. It can be seen that the initiation point for SP is the surface or at any defect beneath it, while for SP+CASE all cracks initiate beneath the surface. The improvement of the strength produced by SP+CASE compared to SP could be explained because the roughness and the deleterious effect of shot peening on the surface, which also produces small micro cracks, while the CASE treatment reduces the roughness and eliminates those micro cracks. The improvement of the surface finish together with the residual compressive stress field close to the surface eliminates the crack initiation produced by the surface defects. Also, the elimination or reduction of the microcracks due to the CASE treatment, as well as the reduction of the effect of roughness on the stresses very near to the surface make the cracks to initiate deeper, where the stresses are lower because the stress gradient produced by bending and the residual compressive stresses in that zone, which are higher. Regarding laser peening, the increase of the fatigue strength relative to the other two treatments could be explained by two reasons. On one side, the increase of the depth and width of the residual compressive stress field make the surface initial cracks to stop, allowing growing only cracks initiated below the residual stress field. On the other side, the deeper initiation points means lower stresses in the initiation zone and, so, higher fatigue lives.