Issue 33

F. Berto et alii, Frattura ed Integrità Strutturale, 33 (2015) 229-237; DOI: 10.3221/IGF-ESIS.33.29 229 Focussed on multiaxial fatigue Multiaxial fatigue strength of severely notched titanium grade 5 alloy F. Berto, A. Campagnolo University of Padua, Italy berto@gest.unipd.it, campagnolo@gest.unipd.it A BSTRACT . The multiaxial fatigue strength of severely notched titanium grade 5 alloy (Ti-6Al-4V) is investigated. Experimental tests under combined tension and torsion loading, both in-phase and out-of-phase, have been carried out on axisymmetric V-notched specimens considering different nominal load ratios (R = -1, 0). All specimens are characterized by a notch tip radius less than 0.1 mm, a notch depth of 6 mm and a notch opening angle equal to 90 degrees. The experimental data from multiaxial tests are compared with those from pure tension and pure torsion tests on un-notched and notched specimens, carried out at load ratio ranging from R = -3 to R = 0.5. In total, more than 160 new fatigue data are examined, first in terms of nominal stress amplitudes referred to the net area and then in terms of the local strain energy density averaged over a control volume surrounding the V-notch tip. The dependence of the control radius on the loading mode is analysed showing a very different notch sensitivity for tension and torsion. For the titanium alloy Ti-6Al-4V, the control volume is found to be strongly dependent on the loading mode. K EYWORDS . Titanium alloy; Ti-6Al-4V; Multiaxial fatigue; V-notch; Control volume; Strain Energy Density. I NTRODUCTION ultiaxial fatigue of metallic materials is a topic of active research. For a comparison between different criteria we mention here a recent overview [1] and the report [2] based on a large body of experimental data from notched specimens. A central position is occupied by the critical plane approach [3, 4] and by some interesting variants [5-7]. In this ambit, energy-based criteria find important applications [8]. Recently it has been proposed a multiaxial fatigue criterion based on a frequency-domain formulation of a stress invariant, called “Projection by Projection” (PbP) approach [9]. A frequency-domain formulation of the critical plane-based C-S criterion has recently been presented in [10]. In 1923 Jasper [11] first used an energy-based parameter to analyse fatigue strength under tension–compression loadings. Afterwards dealing with multiaxial fatigue loading Ellyin proposed an approach based on the combination of both the plastic and elastic strain work [12-13]. A wide review of energy-based multiaxial fatigue life formulations was carried out in [14]. Theoretical and experimental problems tied to multiaxial fatigue were discussed by several researchers [15-22]. The present work deals with multiaxial fatigue strength of severely notched titanium grade 5 alloy (Ti-6Al-4V). This titanium alloy is widely used for advanced military, civil aerospace and naval applications. The in-service conditions are usually characterized by a complex stress state combined with aggressive environments. The Ti-6Al-4V titanium alloy has M