Issue 19

P. K. Pradhan et alii, Frattura ed Integrità Strutturale, 19 (2012) 51-60; DOI: 10.3221/IGF-ESIS.19.05 58 through the machined holes. The material in between the holes have deform substantially. This leads to the shear band formation connecting the machined holes along an easy path, resulting in the final fracture. However, it is observed that the crack propagation is not exactly along the middle of the shear band (i.e. not along the shortest path between the holes). Microscopic study of fracture path In order to identify the exact crack path and the mechanism of failure of the material containing the machined holes, the specimens were not allowed to fail completely. The region ahead of the growing crack was sectioned, and thinned by the surface grinder. These were mounted in thermosetting resin, polished, etched and then observed under the microscope to investigate the exact crack path. The microphotograph of one such sample is shown in Fig. 12. In microscopic level, propagation of crack is not truly linear, it is zig-zag. As discussed by Stec and Faleskog [16], due to differences in orientation of cleavage planes of two neighbouring grains, crack changes its direction when it advances from grain to grain. Grain deformation at regions away from the holes is not evident. However, extensive elongation of grains indicating large plastic deformation is evident at regions adjacent to the holes. The grain elongation is maximum at the regions of propagating of crack surfaces (Fig. 13). Due to these large elongation of the grains, compatibility between adjacent grains are lost at grain boundary regions resulting in the formation of sharp defects/cracks at these regions. Submicroscopic cracks were found nucleating at grains boundary regions only (a few isolated voids were observed inside the grains). The voids at regions of grain boundaries are large in number (Fig. 14). The main crack propagated through regions of high void population leading to a zigzag pattern when observed at a microscopic level. Voids nucleated ahead of the growing crack coalesce before they joined the main crack (Fig. 15 and Fig. 16). Stress intensity factor near the crack tip plays an important role for void growth and coalescences. This coalescence of the sharp micro-cracks ahead of the main crack leads to very high stress concentration facilitating the fracture process. In some cases, instead of single crack propagation, two or more cracks may be formed simultaneously and later they joined with each other by joining intermediate voids as shown in Fig. 17. Figure 12 : Fracture path under Microscope. Figure 13 : Grain in front of crack tip. Figure 14: Voids present at grain boundary. Figure 15 : Void coalescence in front of crack.