Issue 35

A. Nikitin et alii, Frattura ed Integrità Strutturale, 35 (2016) 213-222; DOI: 10.3221/IGF-ESIS.35.25 213 Focussed on Crack Paths Crack path in aeronautical titanium alloy under ultrasonic torsion loading A. Nikitin, C.Bathias LEME, University Paris Ouest Nanterre La Defense, 50, rue de Serves, Ville-d'Avray, 92410, France nikitin_alex@bk.ru T.Palin-Luc Arts et Metiers Paris Tech, I2M, CNRS University of Bordeaux, Esplanade des Arts et Metiers, Talence, 33405, France thierry.palin-luc@ensam.eu A. Shanyavskiy SCCAFS, Air. Sheremetevo-1, PO Box 54, Moscow reg., Chimkovskiy state, 141426, Russia shananta@mailfrom.ru A BSTRACT . This paper discusses features of fatigue crack initiation and growth in aeronautical VT3-1 titanium alloy under pure torsion loading in gigacycle regime. Two materials: extruded and forged VT3-1 titanium alloys were studied. Torsion fatigue tests were performed up to fatigue life of 10 9 cycles. The results of the torsion tests were compared with previously obtained results under fully reversed axial loading on the same alloys. It has been shown that independently on production process as surface as well subsurface crack initiation may appear under ultrasonic torsion loading despite the maximum stress amplitude located at the specimen surface. In the case of surface crack initiation, a scenario of crack initiation and growth is similar to HCF regime except an additional possibility for internal crack branching. In the case of subsurface crack, the initiation site is located below the specimen surface (about 200 µm) and is not clearly related to any material flaw. Internal crack initiation is produced by shear stress in maximum shear plane and early crack growth is in Mode II. Crack branching is limited in the case of internal crack initiation compared to surface one. A typical ‘fish-eye’ crack can be observed at the torsion fracture surface, but mechanism of crack initiation seems not to be the same than under axial fatigue loading. K EYWORDS . Very-High Cycle Fatigue; Titanium alloy; Torsion; Ultrasonic; Crack initiation; Crack growth. I NTRODUCTION t was outlined by many authors that study on fatigue properties of structural materials, such as steels, aluminum and titanium alloys, under torsion loading is an important subject for industrial applications [1-2]. Many engineering elements are subjected to complex loading involving bending, tension or torsion load modes. Moreover, in case of modern applications, such as cars, high-speed trains and aircraft motors, a significant fatigue life for components can be I