PSI - Issue 53

3

Camilla Ronchei et al. / Procedia Structural Integrity 53 (2024) 112–118 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

114

All fatigue tests were carried out on cylindrical specimens, with a gauge section diameter and length of 10mm and 5mm, respectively. The fatigue tests were performed under load-controlled mode and the experimental loading conditions consisted in: tensile loading with the fatigue ratio R equal to -1 and torsional loading with R equal to both -1 and 0.1. The results in terms of stress range and number of loading cycles to failure, exp N , are reported in Fig. 1, where the arrows are referred to run-out conditions (defined at a number of loading cycles equal to about 5·10 6 cycles). The parameters of the S-N and T-N curves for fully reversed stress are listed in Tab. 1 together with the experimental fatigue limits under normal loading, , 1 af  − , and shear loading, , 1 af  − .

Table 1. Parameters of S-N and T-N curves for fully reversed stress. S-N curve parameters T-N curve parameters

m [-] -0.116

0 N [cycles]

* 0 N [cycles]

, 1 af  − [MPa]

, 1 af  − [MPa]

* m [-]

102.00

1.49·10 6

85.00

-0.088

3.63·10 6

After fatigue tests, the fracture surfaces of all the broken specimens were examined by using SEM, showing that all the specimens failed due to fatigue cracks nucleated in correspondence of a defect located close to the external surface. Such cracks, after their propagation, led to the fatigue failure of the specimens. Two types of defects were found: lack of fusion and pore. The size of the crack initiating defect, in terms of area , was measured for each specimen and reported in the original work (Sausto et al. (2022)). 3. Proposed procedure The novel analytical procedure (Vantadori et al. (2023)) is hereafter applied to the experimental data presented in the previous Section. In particular, the fatigue limit under cyclic normal loading, w  , and that under cyclic shear loading, w  , are calculated by exploiting the Kitagawa-Takahashi diagram together with a defect content analysis. For cyclic axial loading, the Kitagawa-Takahashi diagram in terms of the modified El-Haddad model requires the following input parameters: - the fatigue limit of the defect-free material, 0 w  , here estimated by using an empirical relationship (that is, 0 0 5 w u .   =  = 190.75MPa, being u  the material ultimate tensile strength); - the threshold stress-intensity factor range, th K  , here taken from the literature (Romano et al. (2018)). The Kitagawa-Takahashi diagram in terms of the modified El-Haddad model is plotted in Fig. 2, according to the following equation:

2

0, area area area + EH

 

  

K

1



th

with

(1)

area

= 

w w   =

0,

0

EH

0.65 2 





0

w

0,

n

EH

0, EH area is the El-Haddad parameter, which is here assumed to be equal to 87 µm.

where

10 3

 w , [MPa]

10 2

10 0

10 1

10 2

10 3

 area n , [  m]

Fig. 2. Kitagawa-Takahashi diagram in terms of the modified El-Haddad model for cyclic axial loading.