Issue 35

L. Songsong et alii, Frattura ed Integrità Strutturale, 35 (2016) 74-81; DOI: 10.3221/IGF-ESIS.35.09 78 seen from Fig. 6 that the dimension of the grains in the L direction is remarkably longer than that in the T and S direction; it looks like a layered up structure in L-S plane. The observed crack paths in specimens L-T-2-1 and L-T-2-2, for which the applied  max are 140 MPa and 150 MPa, respectively, are given in Fig. 7 and Fig. 8. The common type of crack branching, shown in Fig. 7(b) and Fig. 8(b), occurs at the grain boundary. For the weaker cohesion of grain boundary in this orientation, grain boundary becomes paths of easy propagation. Whereas, for the less common type of crack branching, see Fig. 7(c) and Fig. 8(c), the growth of the main crack and the surface secondary crack show transgranular characteristic, though the linkup path was along the grain boundary. Therefore, the reason for crack branching is not the weakness in grain boundaries. (a) (b) (c) Figure 6 : Grain shape observations on (a) L-T, (b) L-S, (c) T-S metallographic planes of 2324-T39 alloy. (a) (b) (c) Figure 7 : Crack growth path for L-T-2-1 specimen (CA loading,  max = 140 MPa, R = 0.06). (a) (b) (c) Figure 8 : Crack growth path for L-T-2-2 specimen (CA loading,  max = 150 MPa, R = 0.06).