PSI - Issue 75

Francesco Collini et al. / Procedia Structural Integrity 75 (2025) 375–381 F. Collini et al. / Structural Integrity Procedia 00 (2025) 000–000

379

5

Fig. 3. (a) Axial and torsional fatigue test results expressed in terms of remote stress range applied; and (b) combined axial and torsional fatigue tests results expressed in terms of remote axial stress range applied.

Test Manager. All tests were conducted with a load ratio R = − 1. The multiaxial fatigue tests were carried out with a biaxiality ratio λ σ g = 0 . 7, and two di ff erent phase shift angles ϕ were studied: 0 ◦ (in-phase axial and torsion) and 90 ◦ (out-of-phase axial and torsion). The maximum testing frequency of the machine is 100 Hz, and for the tests performed, it ranged from 25 Hz to 100 Hz depending on the load level and loading condition. The failure criterion was based on the complete separation of the specimens before reaching 10 7 cycles. For the calibration of R c , 1 and R c , 3 , the fatigue limit was estimated using the up-and-down method for small samples, as proposed by Dixon (1965). 4. Experimental results and validation Fig. 3 shows the preliminary fatigue test results under axial, torsional, and combined axial–torsional in-phase and out-of-phase loading conditions. Data from the axial and torsional high-cycle fatigue regimes were used for a first approximation of the structural control volume for the two loading keywords. All parameters used to calibrate R c , 1 and R c , 3 , such as the relevant fatigue limits of the V-notched specimens estimated using the procedure by Dixon (1965), as well as ∆ w th , are reported in Tables 1 and 2.

Table 1. Material properties of L-PBF Ti6Al4V. Material HV 0 . 5 [kgf / mm 2 ]

3 ]

E [GPa]

∆ σ 0 [MPa]

c w [Nmm / mm

∆ w th

ν

Ti6Al4V

338

116

0.34

1081.6

0.5

2.521

Table 2. Parameters and fatigue limits of axial and torsional fatigue tests. Loading condition e 1 , e 3 γ 1 , γ 3 α γ 1 , α γ 3

v eq ∆ w , th

[MPamm γ ]

∆ σ g , th , ∆ τ g , th [MPa]

R c , 1 , R c 3 [mm]

∆ K

Axial

0.138 0.318

0.455 0.333

1.615 2.475

53.5

5.46E-03 6.57E-02

28.8

Torsion

105.7

Fig. 4 reports all data related to the multiaxial fatigue test results in the ALM diagram. Note that, when all fatigue data are plotted as in this case, Eq. (6) should ideally separate failures from run-out tests (i.e., open and filled markers). In addition, all multiaxial test series share the same a v eq ∆ w according to Eq. (6), thus most of the data points overlap. To better appreciate the correlation, a detailed view of each test series is also provided in Fig. 4. From this preliminary investigation, which is based on the calibration of R c , i using a limited dataset, it can be noted that the estimation based on Eq. (6) shows a good correlation with the fatigue thresholds obtained from multiaxial