Issue 33

F. Berto et alii, Frattura ed Integrità Strutturale, 33 (2015) 229-237; DOI: 10.3221/IGF-ESIS.33.29 230 very good static and fatigue properties, an high strength-to-mass ratio, with an excellent wear resistance, also at high temperature and in corrosive environments. The uniaxial fatigue resistance of smooth and notched specimens made of Ti- 6Al-4V has been extensively investigated in the recent literature. A complete set of data from sharply V-notched specimens under torsion and combined tension and torsion loadings, both in-phase and out-of-phase, is not available in the literature for Ti-6Al-4V. With the aim to fill this lack, a complete set of experimental data from a severely notched titanium alloy under multiaxial loading is provided. Circumferentially V-notched specimens are tested under combined tension and torsion loading, both in-phase and out-of- phase, with two nominal load ratios, R = -1 and R = 0. All axisymmetric specimens are characterized by a notch tip radius less than 0.1 mm, a notch depth of 6 mm and a V-notch angle equal to 90 degrees. The experimental data from multiaxial tests are discussed together with those from pure tension and pure torsion tests on un-notched and notched specimens, carried out at a load ratio ranging from R = -3 to R = 0.5. Altogether more than 160 new fatigue data (15 Wöhler curves) are analyzed in terms of nominal stress amplitudes referred to the net area. The application of a multiaxial loading reduces the fatigue life compared to the pure tension loading case, with reference to the same normal stress amplitude, but the reduction results to be quite limited for the specific value of the biaxiality ratio (  = 0.6). Stronger is the reduction of the multiaxial fatigue strength due to the nominal load ratio R. The phase angle effect is found to be weak for R = -1, being the mean values of the normal stress amplitude about the same at 2·10 6 cycles. More clear is the effect of the phase angle when R = 0: the out-of-phase loading is slightly beneficial with respect to in-phase loading at high cycle fatigue regime whereas the fatigue strength is almost the same at low cycle regime. The sensitivity of this titanium alloy under multiaxial fatigue loading to the phase angle effect results to be quite limited, being lower than +15 percent for the R = 0 case and negligible for the R = -1 case. The fracture surfaces of the specimens tested under multiaxial conditions have been analyzed. The fracture surface morphology seem to be affected by the phase angle. Some signs of micro abrasions could be observed on all fracture surfaces and the extent to which the rubbing occurred depends on phase angle. Generally, a limited but distinguishable quantity of debris and powder was emanated from the notch tip when a visible crack started to propagate. Afterwards, all fatigue strength data are summarized in terms of the local strain energy density (SED) averaged over a material-dependent control volume surrounding the V-notch tip [23-30]. The dependence of the control volume size on the loading mode is investigated showing the need to use for the titanium alloy Ti-6Al-4V a different size of the control volume under tension and under torsion, due to a very different notch sensitivity for tension and torsion. The expressions for estimating the control radii, thought of as material properties, are obtained imposing at 2·10 6 cycles the constancy of the SED from smooth and V-notched specimens, which depends on the notch stress intensity factors and the radius of the control volume. In particular, a control volume of radius R 1c are used to evaluate the averaged contribution to local stress and strains due to tensile loading, whereas a radius R 3c are used to assess the averaged contribution due to torsion loading. The control radii result to be R 1c = 0.051 mm and R 3c = 0.837 mm. The size of R 3c radius is highly influenced by the presence of larger plasticity under torsion loading with respect to tensile loading and by friction and rubbing between the crack surfaces, as discussed extensively for different materials [31]. The SED is called ‘apparent linear elastic SED’ to remember that the evaluation of the strain energy in two different volumes (for tension and torsion, as determined from experimental data) allows us to overcome the problem tied to shielding mechanisms maintaining a linear elastic model. The unifying capacity of the SED approach is highlighted, in fact the synthesis based on the local strain energy density allows to obtain a quite narrow scatter-band, which has an equivalent stress-based scatter index T  equal to 1.58, considering all the data from V-notched and smooth specimens under pure tension, pure torsion and multiaxial loading, independent of the load ratio and the phase angle. The results of the synthesis in terms of SED are also compared with those from other materials already reported in the literature by the same authors. M ATERIAL PROPERTIES AND GEOMETRY OF THE SPECIMENS he material under investigation is a grade 5 titanium alloy, also known as Ti-6Al-4V. The geometries of the un- notched and V-notched specimens are shown in Fig. 1 together with some details of the notch tip. The hourglass un-notched specimens (Fig. 1) were characterized by a diameter of the net transverse area equal to 12 mm and by a connecting radius (  = 100 mm) between the net and gross sections large enough to avoid any effect of stress concentration. The cylindrical notched specimens (Fig. 1) were characterized by a V-notch depth d = 6 mm and an opening angle equal to 90 degrees, whereas the notch root radius,  , was always lower than 0.1 mm. The experimental measurements of the notch tip radius, carried out by means of an optical microscope and the dedicate software LAS T