PSI - Issue 39

286 6

Gonzalo M. Domínguez Almaraz et al. / Procedia Structural Integrity 39 (2022) 281–289 Author name / Structural Integrity Procedia 00 (2019) 000–000

Fig. 4. (a) Comparative results of ultrasonic fatigue tests on I-718 alloy (load rate R=-1 and room temperature):  Present work , 20 KHz [28] , ■ 19.5 KHz [2] , 52.5 Hz [1], (b) Fracture surface of a specimen tested with 746 MPa, (c) Fracture surface of a specimen tested with 511 MPa. Concerning the fracture surface, Figure 4(b) shows the fracture surface for the high level of loading (746 MPa); the general appearance shows two principal topographies: the first one is at the top of fracture surface presenting a quasi-flat surface with some striations, where are located the crack initiation and the slow crack propagation. The second topography is characterized by fast crack propagation, presenting rough surface and containing the final fracture zone. On the other hand, Figure 4(c) shows the fracture surface of a specimen loaded at 511 MPa; this time the fracture surface shows fast crack propagation zone with a smaller area with respect to the previous case; this behavior may be attributed to the fact that at lower load levels the slow crack propagation is amplified and covers larger area of the total crack propagation area. In Figure 5 is depicted the fracture surface for the high stress level (746 MPa). The crack initiation is located at subsurface and associated with slip bands: initiating under trans-granular ductile behavior; then, to a dominant cleavage fracture for crack propagation. All specimens tested in this study below 746 MPa presented subsurface crack initiation; whereas specimens tested at 746 MPa, one of them presented surface crack initiation and the rest shown subsurface crack initiation. In Table 2 are listed the tests carried out for the four stress levels and the corresponding crack initiation modality.

Table 2. Ultrasonic fatigue results and crack initiation modality. Test number Applied load (MPa) Fatigue life (millions of cycles)

Temperature at neck section (  C average)

Average humidity (%)

Crack initiation modality

1 2 3 4 5 6 7 8 9

746 746 746 670 670 670 585 585 585 510 510 510

0.9 0.7 0.8

68 73 70 60 63 62 56 58 55 54 52 50

48 47 45 48 43 46 44 46 48 45 47 43

subsurface

surface

subsurface subsurface subsurface subsurface subsurface subsurface subsurface subsurface subsurface subsurface

1.01 1.02 1.02 1.08 1.1 1.09

10 11 12

1.4 1.8 2.1

A critical stress intensity factor delimitates the transition from the initial trans-granular ductile fracture to cleavage