Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 8-15; DOI: 10.3221/IGF-ESIS.41.02 11 Figure 3 : (a) Calibration curves ΔV/ΔV 0 obtained numerically as a function of the normalized crack depth a /r net , for Ti-6Al-4V notched specimens. ΔV 0 represents the electrical potential of the reference un-cracked specimen ( a = 0). (b) Definition of crack initiation life in terms of electrical potential drop. Tanaka [6]. According to the zoom reported in Fig. 3b, it is seen that a ratio  V/  V 0 equal to 1.01 corresponds to a crack depth a in the range 0.8÷1.4 mm. A VERAGED S TRAIN E NERGY D ENSITY A PPROACH he strain energy density (SED) averaged over a control volume the radius of which is thought of as a material property according to Lazzarin and Zambardi [14], proved to efficiently account for notch effects both in static [14,21] and fatigue [14,22,23] structural strength problems. The idea is reminiscent of the stress averaging to perform inside a material dependent microstructural length, according to the approach proposed by Neuber. Such a method was formalized and applied first to sharp, zero radius, V-notches [14] and later extended to blunt U and V- notches [15]. When dealing with sharp V-notches, see for example the case with  = 0.1 mm in Fig. 1, the control volume is a circular sector of radius R 0 centered at the notch tip [14] as shown in Fig. 4a. For a blunt V-notch, see for example the case with  = 4 mm in Fig. 1, the volume assumes the crescent shape shown in Fig. 4b [15], where R 0 is the depth measured along the notch bisector line. The outer radius of the crescent shape is equal to R 0 + r 0 , where r 0 depends on the notch opening angle 2  and on the notch root radius  according to the following expression:    q 1q r 0 (1) with q defined as:    2 2 q (2) The control radius R 0 for fatigue strength assessment of notched components made of titanium grade 5 alloy has been previously estimated by Berto et al. [24] by equating the averaged SED in two situations, i.e. the fatigue limit of un- notched and notched specimens, respectively. Therefore, R 0 combines two material properties: the high-cycle fatigue strength of smooth specimens, referred to N A = 2·10 6 cycles, and the value of the Notch Stress Intensity Factor (NSIF) range for sharp V-notches with opening angle equal to 90 degrees, referred to the same number of cycles N A , according to T 1.00 1.01 1.02 0.00 0.05 0.10 0.15 0.20 0.25 ΔV/ΔV 0 a /r net c/a = 2 c/a = 3 c/a = 4 c/a = 2 c/a = 3 c/a = 4 I = const. = 30 A r net = 6 mm  = 0.1 mm  = 4 mm crack initiation life → ΔV/ΔV 0 ≈ 1.01 crack initiation life → 0.14 < a /r net < 0.24 (b) 1.00 1.10 1.20 1.30 0.00 0.30 0.60 0.90 ΔV/ΔV 0 a /r net c/a = 2 c/a = 3 c/a = 4 c/a = 2 c/a = 3 c/a = 4 I = const. = 30 A r net = 6 mm  = 0.1 mm  = 4 mm (a)