PSI - Issue 68

Giuseppe Macoretta et al. / Procedia Structural Integrity 68 (2025) 974–980 G. Macoretta et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 6. Resonant testing machine operating frequency monitoring. Examples for a smooth (a), k t 2 (b) and k t 3.5 (c) specimens.

3.3. Fractographic investigation Irrespective of the specimen geometry, all the specimens featured the fracture onset in proximity to the specimen surface, Fig. 7. As the fracture surface was subject to oxidation during the crack propagation and after failure due to the permanence in the furnace chamber, the crack onset and first propagation area can be detected by observing the area characterized by a bright blue color. The only exception is represented by the k t 2 specimens that failed over 0.1 million cycles, where a pore present in proximity to the specimen surfaces acted as a crack nucleation site. While in the case of the smooth specimens the fracture presented a unique nucleation site, the notched specimens' fracture surfaces presented several onset sites and the fracture surface is the result of a combination of several fatigue cracks propagation. As observed at room temperature in previous works [8], for the smooth specimens the fracture arose from the root of surface roughness valleys, often in proximity to an additional sporadic undulation in the circumferential direction, Fig. 7 (a). In the case of the sharper notch, the fracture nucleation happened on a ring covering about half of the specimen circumference and several ratchet marks can be observed where the different crack propagation planes joined, Fig. 7 (b) and (c).

Fig. 7. Fracture surfaces of smooth (a), and notched specimens featuring a k t of 1.5 (b) and 3.5 (c). White arrows point out the location of the crack onset. 4. Conclusions Smooth and V-notched specimens, designed to feature a stress concentration factor between 1.5 and 3.5, were tested in the HCF regime at a temperature of 650°C to study the material notch sensitivity and effects of a local multiaxial stress state introduced by a geometric macroscopic notch. The HCF test showed that both the smooth and notched specimens exhibit a fatigue behavior characterized by the occurrence of a fatigue limit before 0.1 million cycles. Despite the scatter of the fatigue data in proximity to the knee