Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 8-15; DOI: 10.3221/IGF-ESIS.41.02 14 Leaving aside the physical definition of crack initiation, Fig. 6a shows that the ratio between crack initiation life according to the operational definition (open markers) and total life (filled markers) is equal to approximately 0.50÷0.60 for the sharp V-notch (  = 0.1 mm), while it is equal to about 0.90 for the blunt notch (  = 4 mm). Fig. 6b shows a comparison between the averaged SED-based scatter-band calibrated previously by Berto el al. [24] on about 160 uniaxial and multiaxial experimental data obtained from sharp V-notched Ti-6Al-4V specimens and the new fatigue results relevant to pure bending loading, obtained in the present contribution and expressed in terms of total fatigue life. It can be observed from Fig. 6b that all new data fall within the SED-based scatter band showing a good agreement between theoretical estimations and experimental results. However, it should be noted that the averaged SED- based scatter-band reported in Fig. 6b is characterized by a rather wide scatter (T W = 2.50, i.e. an equivalent stress-based scatter index T  = w T = 1.58), being calibrated on experimental data expressed in terms of total fatigue life. It is worth noting that Fig. 6a suggests that the scatter index could be reduced if the averaged SED-based scatter-band was calibrated on crack initiation data rather than total fatigue life data. Figure 6 : (a) Crack initiation (open markers) and fatigue life to failure (filled markers) of Ti-6Al-4V notched specimens expressed in terms of averaged SED. (b) Comparison of new fatigue data in terms of total fatigue life with the averaged SED-based scatter-band calibrated previously [24] on about 160 uniaxial and multiaxial experimental fatigue results obtained from sharp V-notched specimens. C ONCLUSIONS n the present contribution, some preliminary experimental fatigue results, obtained from circumferentially notched specimens made of titanium alloy, Ti-6Al-4V, with different notch root radii and subjected to pure bending fatigue loadings, have been reanalysed by means of the averaged strain energy density (SED) approach . Agreement has been found between new fatigue data and a SED-based scatter band previously calibrated on uniaxial and multiaxial fatigue results in terms of total fatigue life. However, if the crack initiation life had been considered, even though defined in an operational manner, reduction of scattering in the SED-based synthesis of notches of different severity is likely to be obtained and it will be the subject of future investigations. R EFERENCES [1] Tanaka K, Hashimoto A, Narita J, Egami N. Fatigue life of circumferentially notched bars of austenitic stainless steel under cyclic torsion with and without static tension. J Soc Mater Sci Jpn 2009;58:1044–1050. [2] Tanaka K. Small fatigue crack propagation in notched components under combined torsional and axial loading. Procedia Eng 2010;2:27–46. doi:10.1016/j.proeng.2010.03.004. [3] Ohkawa C, Ohkawa I. Notch effect on torsional fatigue of austenitic stainless steel: Comparison with low carbon steel. Eng Fract Mech 2011;78:1577–89. doi:10.1016/j.engfracmech.2011.01.015. [4] Berto F, Lazzarin P, Yates JR. Multiaxial fatigue of V-notched steel specimens: A non-conventional application of the local energy method. Fatigue Fract Eng Mater Struct 2011;34:921–43. doi:10.1111/j.1460-2695.2011.01585.x. I 0.5 5.0 50.0 1.00E+04 1.00E+05 1.00E+06 1.00E+07 SED range, ΔW [MJ/m 3 ] Number of cycles to failure 0.1 mm 4 mm R 0,I = 0.051 mm ΔW A,50% = 3.08 MJ/m 3 N A = 2 ∙10 6 cycles Scatter Index (10%-90%): T W = 4.87/1.95 = 2.50 Slope k = 5.90 Ti-6Al-4V: scatter band calibrated in [24]  mm  mm 4.87 1.95 3.08 run out (b) 1.0 10.0 1.00E+04 1.00E+05 1.00E+06 1.00E+07 SED range, ΔW [MJ/m 3 ] Number of cycles 0.1 mm innesco 4 mm innesco 0.1 mm 4 mm  mm  mm run out (a) R 0,I = 0.051 mm  mm  mm fatigue crack initiaton life total fatigue life