Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 8-15; DOI: 10.3221/IGF-ESIS.41.02 13 SED- BASED REANALYSIS OF CRACK INITIATION AND TOTAL LIFE EXPERIMENTAL DATA he local SED values, averaged over the control volume relevant to mode I loadings (R 0,I = 0.051 mm), have been calculated using the direct approach, W  , according to Eq. (5) (with about 50 finite elements inside the control volume). Due to application of non axis-symmetric loadings, i.e. pure bending, 3D FE analyses of the cylindrical specimens were needed to evaluate the averaged SED. To obtain the 3D mesh, first, a 2D free mesh pattern of quadrilateral 8-node PLANE 183 elements has been generated; after that, the 2D FE mesh has been extruded about Y axis by using 3D 20-node brick elements (SOLID186 of the Ansys® element library) and by setting a proper extrusion step size in order to obtain a control volume V having both radius and depth equal to R 0,I , as suggested for 3D applications of the SED approach [21,23]. An example of the adopted FE mesh along with a detail of the structural volume for averaged SED calculation is reported in Fig. 5. It should be noted that only a quarter of each specimen has been modelled, taking advantage of the anti-symmetry on the YZ plane and of the symmetry on the XY plane (see Fig. 5). To properly simulate the loads and restraints applied by the adopted test bench (see Fig. 2a), the cylindrical surface highlighted in gray in Fig. 5a was fixed, while bending loading was applied by means of concentrated forces at the opposite end. Figure 5 : (a) Coarse FE mesh (about 50 FE inside the control volume) adopted in the 3D numerical analyses. The Y-axis coincides with the axis of the specimen. (b) Details of the FE mesh inside the control volume V. Considered case: sharp V-notched specimen (  = 0.1 mm) under pure bending loading, R 0,I = 0.051 mm. The preliminary experimental fatigue results presented in terms of local SED and relevant to pure bending fatigue loading are reported in Fig. 6a, where the operational definition of crack initiation life (  V/  V 0 = 1.01 according to Fig. 3b) has been applied. In a recent contribution of the present authors [16], it has been highlighted that a physical definition of the crack initiation life in relation to the SED-approach could be defined when the crack depth a equals the structural volume size R 0 , that is the initiated crack depth leads to the structural volume failure. Dealing with the present results, it should be noted that: (i) given a crack depth a , the potential drop method is less sensitive to the initiation of an elliptical crack than to the initiation of a circumferential one (calibration curves of Fig. 3 should be compared with those reported in [19]); (ii) the control radius R 0,I for fatigue strength assessment of Ti-6Al-4V notched components is equal to 0.051 mm. Therefore, the physical definition of the crack initiation life ( a =R 0,I ) corresponds to a relative potential drop increase  V/  V 0 lower than 1‰ (Fig. 3b): such a reduced  V/  V 0 value is not applicable to the present case due to the un-sufficiently high sensitivity of the adopted DCPD crack growth monitor device. T Fixed region V Notch bisector R 0,I R 0,I F/8 X Y Z F/8 (a) (b)