Issue 30

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44 373 Below are also discussed the results of other studies currently under way at the University of Seville [6–8] on the influence of specimen microstructure on crack propagation. These studies are aimed at confirming the differences in propagation between “short” cracks, which are comparable to the specimen grain size in length, and “long” cracks, which are much longer. Below is commented on the experimental contribution of the proposed technique in this respect. Figure 8 : Image of the specimen immediately before plastic collapse. Grain size = 8.22 mm [8]. Fig. 8 shows the situation for a specimen of grain size 8.22 mm. The specimen was 2 mm thick and the grain size across its thickness coincided with the specimen physical thickness. This allowed the microstructure of the material to be assimilated to a two-dimensional arrangement of aluminium single crystals. We used DIC to examine one side only —the other was used to extract microstructural information (viz., the position of grain boundaries). Thus, the notch, the crack and the grain boundaries —which were outlined to facilitate analysis— are shown at the top, and the deformation fields resulting from propagation of the crack at the bottom. Clearly, this example is one case of “short” cracks as the cracks propagated through active sliding of the grains they spanned. In order to propagate from one grain to the next, the crack must switch to a different propagation plane (specifically, to the active sliding plane in the adjacent grain). The active sliding plane is dictated by Schmid’s factor. This leads to non-symmetric cracks that propagate at dissimilar rates by effect of successive acceleration and deceleration [6-8]. Fig. 9 provides a detailed depiction of Fig. 7f. The deformation field shown is that occurring immediately before plastic collapse. This example is one of “microstructurally long” cracks, where the specimen microstructure has no influence. Thus, cracks grow symmetrically on both sides of the notch, in a plane at 90º with the loading axis. Visual comparison of Figs 8 and 9 exposes the influence of microstructure on the way cracks and their associated plastic zones propagate. Thus, the crack of Fig. 9, corresponding to the specimen with the smaller grain size, exhibited a typical propagation pattern (viz., horizontal growth from the notch equator). On the other hand, growth in the crack of Fig. 8 was strongly dependent on microstructure. As can be seen, there was little symmetry. Thus, immediately before plastic collapse, the crack on the left and its plastic zone stopped at the grain boundary. Also, the new plastic zone did not start at the next grain, but rather at one below that was probably more favorably arranged with respect to the loading direction. Finally, let us highlight the sturdiness of the microscopes, which were used without problem to acquire images over more than 20 tests. Also, they are very inexpensive to replace (less than 50 € each) if they fail.