numero25

B. Moreno et alii, Frattura ed Integrità Strutturale, 25 (2013) 145-152 ; DOI: 10.3221/IGF-ESIS.25.21 149 test in both axial and torsion components. The very good agreement observed in Fig. 4 between DIC and extensometer data demonstrate the validity of the experimental proposed method. Figure 4 : (LHS) Axial strain and (RHS) angular strain evolutions along 5 cycles measured by DIC and biaxial extensometer. S TRAIN RESULTS IN CRACKED SPECIMENS n-cracked specimens were used to validate the methodology combining DIC and long-distance microscope on biaxial samples. Once the methodology was validated, it is applied to study the crack-tip field around fatigue cracks. A 0.15 mm diameter hole was drilled in a sample. The sample was then subjected to ± 15 kN axial load and ± 70 Nm torsion torque at a 3 Hz frequency. Axial load and torsion torque were in-phase. However, due to the small stress concentrator effect of the 0.15 mm diameter hole, the crack was initiated in a region far from the hole and thus it could not be detected nor measured with DIC. It was then decided to drill a larger hole (0.5 mm diameter) to enforce the crack initiation from the hole (see Fig. 3 RHS). Every 2500 cycles, the frequency was decreased from 3 to 0.2 Hz so that each cycle could be studied with sufficient number of images (65 images per cycle). 5 complete cycles were captured by acquiring images for 30 seconds. Subsequently, the frequently was set to 3 Hz again and 2500 cycles were applied before the following measurement. Fig. 5 shows the vertical displacement field, as well as a displacement profile along a vertical line after 100 cycles were applied on the specimen. No crack was detected in Fig. 5. A linear displacement distribution along the vertical direction is also observed in Fig. 5. Very similar results were obtained in un-drilled samples, thus suggesting the little stress concentrator effect of such hole. Figs. 6 LHS and RHS show the strain field after applying 27500 and 30000 cycles respectively. The stress concentrator effect can be easily appreciated in Figs. 6 in two diametrically opposite regions around the hole. The growth direction of the crack can be estimated from Fig. 6 to be approximately 30º with respect the longitudinal (vertical) axis of the specimen. By carefully examining the hole, the crack can be only appreciated on the top right area and bottom left area of the hole. However, it is not possible to distinguish the crack in the speckle-painted surface. Nevertheless, the strain maps in Figs. 6 clearly show evidence of a crack. Figure 5 : (LHS) Vertical displacement field and (RHS) displacement profile along a vertical line (shown in white) after applying 100 cycles on the specimen. U