Issue 30

P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 30 (2014) 244-251; DOI: 10.3221/IGF-ESIS.30.31 246 Figure 3 : Experimental setup employed in this work. E XPERIMENTS he initiation stage of the crack growth was studied under combined proportional tension-compression and torsion tests [10]. All experiments were conducted under strain control mode with the help of the biaxial extensometer, with cyclic sinus signal with zero mean strain (R = -1). The aim of the work was to study a wide range of biaxial loads, with angle between shear and axial strains, φ , going from 0 to 90º in 15º increments (see Fig. 4). Imaging of the crack during initiation was possible by drilling a 150 μm diameter hole on the outer surface of the specimen [11]. The long-distance microscope was then focused on the hole, so that the crack initiation could be acquired. The hole on the surface looked like those shown in Fig. 5. The hole served as a stress raiser thus increasing the chances of the crack nucleating there. The size of the hole was chosen so that fatigue behaviour remains the same as that of the specimen without the hole, but large enough so that nucleation of the crack occurs at the hole. The hole influence on the fatigue life is negligible, as shown previously [12]. The setup allowed detection of the crack a few microns away from the edge of the hole. On average, detection of the crack was possible at distances of 6 μm from the edge of the hole. In practice, the crack did not nucleate at the hole in around 50% of the specimens tested. Only the samples where crack nucleated at the hole are reported here. When the shear component was increased beyond φ = 60º, it was not possible to nucleate the crack around the hole. Instead, the crack appeared either on the inside of the specimen or elsewhere. Consequently, it was not possible to conduct experiments with angle greater than φ = 60º. Tab. 2 summarises the different tension-compression and torsion strain combination studied. Figure 4 : Definition of angle between axial and shear strain amplitudes. T