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B. Moreno et alii, Frattura ed Integrità Strutturale, 25 (2013) 145-152 ; DOI: 10.3221/IGF-ESIS.25.21 146 extensometer Epsilon 3550 was employed for axial and angular strain measurement. The experimental setup employed in this work is shown in Fig. 1. Figure 1 : (LHS) Experimental setup and (RHS) geometry of the specimen employed in the experiments. Dimensions are in mm. M ETHODOLOGY TO STUDY CRACK INITIATION rack initiation stage was studied under combined tension-compression and torsion tests. Strain control mode was used during the experiments with cyclic sinus signal where the mean strain was zero (R = -1). At this stage proportional loading with amplitudes ε a = 0.0015 and γ a = 0.0032 was applied. The objective is to measure the initiation and growth of cracks with size ranging from a few hundreds microns up to 2 mm. This was done by means of a long distance microscope capable of imaging field of view of 2 × 1.5 mm 2 . However, it is very complicated to know a priori the exact spot where the crack will nucleate, given the cylindrical shape of the specimens. Moreover, current biaxial extensometers hide a large portion of the sample, making it extremely difficult to find the nucleation point with the long distance microscope. Therefore the geometry of the specimen needs to be modified so as to force the initiation of the crack at a particular spot and focus the microscope right there. Different possibilities were explored. The first option was to use cylindrical specimens where the thickness was minimum at the central section, similarly to hour glass specimens [1]. However this option would not solve the problem, since although the crack would initiate at a particular section (that of minimum thickness), it could appear anywhere around the circumference. Moreover this geometry would condition the crack to grow in specific orientations. This could be avoided by introducing a very small stress concentrator. By making the concentrator of circular shape, it will not condition the direction of initiation of the crack. Thus, the second option investigated was to insert an indentation with a hardness testing facility. The main advantages of this option are the experimental simplicity and the repeatability that can be achieved by employing the same load and the same penetrator. Three indentations were introduced in a sample with a conical penetrator. The diameter of the mark left by the penetrator was smaller than 200 µm. However, it was observed that after applying 40000 cycles not only the specimen did not fail from any of these indentations, but also no single crack nucleated around the indentations. This is explained by the fact that the material work hardens around the indentation, thus being more resistant to fatigue growth in these spots. The third option explored consisted of drilling a very small hole. The hole should be large enough to force the crack nucleation around it, but small enough to induce a damage equivalent to that of the defects existing already in the material. The effect of holes of different diameter was thus analysed. The fatigue life of different samples with holes of diameters 1.25, 1, 0.6, 0.2 and 0.15 mm and no hole were studied, following Murakami’s ideas [2]. The depth of the hole in all cases was approximately the same dimension as the diameter. The results are shown in Fig. 1. All the experiments were stopped when the axial load or torque dropped 20%. The crack length at the end of each experiment was approximately 20 mm. A crack nucleated around the hole in the sample with 0.15 mm drilled hole and fatigue life of 43500 cycles. However that crack did not propagate and instead the dominant crack did not originate at the hole. The results from Fig. 1 show that the fatigue life is reduced as the size of the hole is increased. For the largest hole tested, the fatigue life was reduced by 90% approximately. The fatigue life increases C