PSI - Issue 3

F. Berto et al. / Procedia Structural Integrity 3 (2017) 85–92 F. Berto et al./ Structural Integrity Procedia 00 (2017) 000–000

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level. At the end of the fatigue tests, the notch root and the fracture surfaces were examined using optical and electronic microscopy. Some examples of fracture surfaces are visible in Fig. 2a and 2b for in phase and out of phase multiaxial loading.

Fig. 2. Fracture surfaces of (a) in phase and (b) out of phase multiaxial loading.

The results of statistical analyses carried out by assuming a log-normal distribution are summarised in Table 3. In particular, it is summarised the mean values of the nominal stress amplitudes at different number of cycles, the inverse slope k of the Wöhler curves and the scatter index T , which quantifies the width of the scatterband included between 10 and 90% probability of survival curves. All failures from 10 4 to 5x10 6 have been processed in the statistical analysis whereas the run-outs were excluded.

Table 3. Results from fatigue tests. Mean values, P s =50%. Stresses referred to the net area.

σ a or τ a

Series Load

T σ or T τ

N

k

10 6

2 10 6

5 10 6

1 2 3 4

Torsion R = -1

16 14 13 12

13.33 1.144

281.16 266.91 249.18

τ

9.39 7.67 8.42

1.176 1.428 1.335

182.6

169.91 153.84

σ σ σ σ τ σ τ

Multiaxial R = -1, φ  = 0°,   = 1.0 Multiaxial R = -1, φ  = 90°,   = 1.0

145.83 133.23 118.23 253.69 233.64 209.55 214.61 200.10 182.41 128.77 120.06 109.45 187.45 171.17 151.8

Tension R = -1

12

9.9

1.202

5

Multiaxial R = -1, φ = 0°,   = 0.6

6

13

7.63

1.485

Multiaxial R = -1, φ = 90°,   = 0.6

112.47 102.7

91.08

7

Torsion R = -1 Semicircular

11

14.27 1.160

τ

367.17 349.76 328.01

15 17

7.68

1.268

239.57 218.9 263.82 247.4

194.28 227.26

8 9

σ σ σ

Multiaxial R = -1, φ = 0°,   = 1.0 Semicircular Multiaxial R = -1, φ = 90°,   = 1.0 Semicircular

10.79 1.260

332.64 313.10 289.02 199.58 187.86 173.41

12

11.45 1.307

10

Multiaxial R = -1, φ = 0°,   = 0.6 Semicircular

τ

Fig. 3 depicts fatigue data from V-notched specimens tested under tension, torsion and combined tension and torsion with different load phases. In this case the biaxiality ratio is equal to 1. It is visible from the figure that the out of phase loading causes a detrimental effect on the fatigue life of the V-notched specimens. This effect is not observed in the case of semicircular notches where the most damaging case is the in-phase loading. By changing the biaxiality ratio from 1 to 0.6 on V notched specimens tests, the effect of out of phase is again present. Summarising this aspect