Issue 35

L. Songsong et alii, Frattura ed Integrità Strutturale, 35 (2016) 74-81; DOI: 10.3221/IGF-ESIS.35.09 77 The surface secondary crack cannot be observed immediately after it initiates even in the in-situ tests. Once it appears on the specimen surfaces, it is already in a certain length as shown in Fig. 4(b). The crack growth of the secondary crack was transgranular, while the link up path was long the grain boundary. The omen of the appearance of surface secondary crack is the slowing down in the growth rate of the previously dominated crack, as shown in Fig. 5. It can be seen from Fig. 5 that there is a remarkable crack growth rate drop at about 3467 loading cycles, however, there was no abnormal observed around the crack tip in the picture corresponding to 5969 cycles in Fig. 4(a). Compare Fig. 4(a) and (b), it can be found that there is little growth of previously dominated crack during the 2598 cycles. There is a sudden increase in crack length in Fig. 5(a) at about 8567 cycles that is because of the appearance of the surface secondary crack of about 150  m in Fig. 4(b). From these clues, it can be deduce that the secondary crack initiates from subsurface at about 3467 cycles, which dissipates the energy and in consequence decreases the growth rate of the main crack. After the link up of secondary crack and the main crack, the crack growth rate recovers. (a) (b) (c) Figure 3 : Appearance of secondary surface cracks and cracks linking up under spectrum loading [6]. (a) (b) Figure 4 : In-situ SEM pictures on crack path of L-T-3-1 specimen (a) at 5969 cycles and (b) at 8567 cycles under constant amplitude loading. (a) (b) (c) Figure 5 : Crack growth data for L-T-3-1 specimen, (a) crack length a vs. loading cycles N , (b) crack growth rate d a /d N vs. loading cycles N and (c) crack growth rate d a /d N vs. stress intensity factor range  K To investigate the difference in mechanism of the uncommon kind of crack branching from the common ones, optical microscope observations on the grain shape and crack growth path were carried out on the single edge notch specimen after the crack growth test. Metallographic samples were prepared etching with Keller (1mL HF, 1.5mL HCl, 2.5mL HNO 3 , 95mL H 2 O) for 3 minutes after the conventional mechanical polishing to observe the grain boundary. Fig. 6(a), (b) and (c) demonstrate the microstructure on L-T, L-S and T-S metallographic planes, respectively, where S is the short transverse direction. It can be