PSI - Issue 26

Sabrina Vantadori et al. / Procedia Structural Integrity 26 (2020) 106–112 Vantadori et al. / Structural Integrity Procedia 00 (2019) 000 – 000

110

5

3. Comparison between experimental and computed results Now the above criterion is applied to simulate the experimental high-cycle fatigue tests described above. Figure 2 compares the experimental fatigue lives with the calculated ones. The solid line indicates N cal = N exp (results above this line are conservative), the dashed lines correspond to N cal = N exp equal to 0.5 and 2, and the dash-dot lines correspond to N cal = N exp equal to 1/3 and 3.

10 6

Conservative

AXIAL LOADING INNER PRESSURE PUSH-PULL AXIAL L. + IN. PRESS.

10 5

10 4

N exp [cycles] 10 3

Scatter band 3X Scatter band 2X

10 2

10 1

10 5

10 1

10 6

10 2

10 4

10 3

N cal [cycles]

Fig. 2. Comparison between experimental and computed fatigue lives for 316 stainless steel.

The results determined are quite satisfactory for the stress state being analysed since 85% of the results fall within the scatter band 2 and 100% within the scatter band 3. Then, inner and outer surface cracks in the fatigued 316 stainless steel specimens were examined in Ref. by Morishita et al., 2018. Through-the-thickness cracks were observed in all tests and, since the crack length in the inner surface was longer than that on the outer one for each test, it was concluded that crack initiated from inside and propagated to the outer surface through the thickness. Under cyclic axial loading and push-pull , cracks presented an Y-shape due to changing of the propagation direction from the circumferential one towards branches inclined at 45° with respect to the specimen axis (see tests T3 and T11 in Figure 3(a) and 3(b), respectively). Under cyclic inner pressure and combined cyclic axial loading and inner pressure , cracks presented an Y-shape due to changing of propagation direction from axial direction towards branches inclined at 45° with respect to the specimen axis (see tests T6 and T12 in Figure 3(c) and 3(d), respectively).

4. Conclusions

The behaviour of 316 stainless steel subjected to high-cycle fatigue loading with constant amplitude has been herein examined. Experimental tests under HCF triaxial loading had been carried out combining cyclic axial loading and cyclic inner pressure, using a three-actuator testing machine designed by some of the present authors. The comparison between such experimental results and the analytical ones computed through the multiaxial critical plane-based criterion by Carpinteri et al. is satisfactory since all results related to fatigue life fall into the scatter band 3, and a high