PSI - Issue 42

Radomila Konecna et al. / Procedia Structural Integrity 42 (2022) 857–862 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

860

4

 max = A N b . The fits are shown as the full lines in Fig. 4. The fitted equations are presented in Tab. 1. The fatigue limits for 2 x 10 6 cycles defined by the fitted equations differ by a factor of 2 with orientations A- and C exhibiting the lowest fatigue limit (97 and 100 MPa, respectively) and the orientation B the highest fatigue limit (206 MPa).

Fig. 4. S-N data for different notch orientations.

Tab. 1 Reference data for the four specimen orientations.

Residual stress* [MPa]

Directional factor (B ref)

Notch orientation

S-N equation

 max at 2 x 10

6 cycles

Local  a , R=-1 [MPa]

[MPa]

B

 max = 385 N  max = 844 N  max = 3230 N  max = 2029 N

-14

132

1

-0,0432

 

A+

-7

92 55

0.70 0.42 0.83

-0.122

A-

n.a. 167

-0.242



C

109

-0.208



(*) Residual stresses from Beretta et al. (2020); Local fatigue strength at R = -1 according to Eq. 1

4. Discussion The explanation of the observed strong differences in the fatigue life in the high cycle fatigue region can be sought in the surface quality of the notch, details of the microstructure in the notch root, which influences initiation and propagation of short fatigue cracks, and in residual stress state. For a given applied stress, fatigue lives to failure reduce in dependence of the notch severity, which is composed of a macro geometrical contribution (K t ) and a micro geometrical contribution related to notch orientation with respect to the build direction. The macro geometry of the notch root and surface roughness for the orientation B and C is practically identical, Fig. 2. Notch root for orientation B is accurately semi-circular because it was produced by contouring. Notch root for C orientation is also well semi-circular and not apparently influenced by stair stepping. This manifests itself at the edges of the notch, not at the notch root. Nevertheless, the fatigue life for orientations B and C differs substantially. This issue will be later discussed. The surface of the notch root corresponding to the orientation A+ is very similar to the orientations B and C, only small undulation due to the layer thickness of 50  m is apparent. The fatigue life is lower than for B orientation; the maximum stress corresponding to the fatigue life 2 x 10 6 is 144 MPa. The worst notch root surface has the notch in A- orientation where the surface is made by a down-skin region. The fatigue life in the whole applied range of