Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 8-15; DOI: 10.3221/IGF-ESIS.41.02 10 was the fatigue tested specimen, while the other one was the reference unloaded specimen. The specimens were connected in series so that a 30-A-constant-current (I in Fig. 2a) flowed through both of them. During each experimental fatigue test, the electrical potential drops  V, for the tested specimen, and  V 0 , for the reference one (see Fig. 2b), have been measured between two 3-mm-diameter copper pins, glued close to the notch edges by using Ag-filled epoxy. Accordingly, the technical fatigue crack initiation life can be defined as the number of loading cycles at which the normalized electrical potential  V/  V 0 shows a given increase, assumed here about equal to 1%. Figure 2 : (a) Flexible test bench adopted for pure bending fatigue tests and (b) experimental setup of the adopted electrical potential drop measurement system. C ALIBRATION CURVES OF THE POTENTIAL DROP METHOD ccording to the pioneering contributions by Ritchie and Bathe [17] and Aronson and Ritchie [18], the finite element method (FEM) is an accurate tool to derive the calibration curves of the electrical potential drop method, i.e. the potential drop as a function of the initiated crack depth in a specific specimen geometry, which is more simple and fast than experimental or theoretical calibrations. The calibration curves relevant to Ti-6Al-4V notched specimens (reported in Fig. 1) have been derived by means of electrical FE analyses. The numerical analyses have been carried out using Ansys® code, by adopting a 3D, 10-node, tetrahedral, electric solid element (SOLID 232 of Ansys® element library). The boundary conditions have been applied to the FE model following the procedure extensively described in a previous contribution [19], which is not repeated here. However, in this case due to application of bending loadings in the fatigue tests, the fatigue crack has been assumed to initiate and subsequently to propagate with a semi-elliptical shape from the circumferential notch toward the center of the bar, and not concentrically as assumed in [19], in which axial and torsion loadings were considered. The results obtained by employing a full 3D FE analysis relevant to Ti-6Al-4V notched specimens are reported in Figs. 3a in terms of ratio  V/  V 0 as a function of the normalized crack depth a /r net , r net being the radius of the net-section. A range of the ellipse semi-axes ratio c / a has been analysed, since it could be not the same in all specimens and it could also change during crack propagation as highlighted by Doremus et al. [20]. However, it is worth of mentioning that a saturation effect occurs on the calibration curves with increasing the c / a ratio, so that ratios higher than 4 have not been considered. The calibration curves reported in Fig. 3a were used to define the crack initiation life in an operational manner, i.e. at a relative potential drop increase  V/  V 0 equal to 1.01. Such operational definition is consistent with that adopted by A 40 a 16 ΔV 0 ΔV imposed electrical current Matelect ® DCM-2 DCPD crack growth monitor fatigue tested specimen I reference specimen I screw for current I/O (b) Matelect ® DCM-2 DCPD crack growth monitor fatigue tested specimen reference specimen hydraulic actuators (a)