numero25

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 25 (2013) 138-144; DOI: 10.3221/IGF-ESIS.25.20 142 from the horizontal axis. Compare the values of 10, 15 and approximately 30 degrees obtained for notches of radii 2, 1 and 0.5 mm respectively. Figure 4 : Crack length in relation to number of cycles. Figure 5 : Crack growth in relation to crack length. Conversely, bigger notch radii tend to concentrate the possible initiation area bringing it closer to the horizontal axis. Of course, for materials with "normal" microstructural sizes of the order of a few tens of microns, and "normal" notch sizes of a few millimeters, where the notch is several times larger than the microstructure, this indicates that cracks would normally initiate in the expected horizontal symmetry axis location, which is just as well. This observation clearly reveals that the application of the usual techniques of notch analysis (viz critical distance [3,4] or notch sensitivity index concepts [5,6]) may not be applicable in situation where the notch size is of the same order or even smaller that the microstructure and this points out that this region of notch size may be a good testing ground to discriminate among different notch fatigue calculation methods. Finally, we would just like to present some preliminary results obtained by using Digital Image Correlation techniques in our experiments with the big grains material. Figure 7 shows the resulting deformation areas for a) Specimen with a microstructure of 0.06 mm, 2 mm notch size and subjected to a 50 MPa tension-tension (R=0.1) fatigue loading, with N=7.9x10 5 cycles and b) microstructure of 8.22 mm, 2 mm notch size and subjected to a 45 MPa tension-tension (R=0.1) fatigue loading, with N=2.38x10 6 cycles. For carrying out the DIC analysis the fatigue test is interrupt every 10000 cycles and a photograph of the speckle pattern is taken. Later these images are analyzed with the VIC 2D software in order to obtain the evolution of the strain throughout the test.