Issue 35

L. Songsong et alii, Frattura ed Integrità Strutturale, 35 (2016) 74-81; DOI: 10.3221/IGF-ESIS.35.09 75 In the microscopic perspective, a series of investigations on crack branching, e.g. in-situ SEM tests of a nickel-based single crystal in Ref [14] and EBSD tests on crack propagation path of 7050 alloy in Ref [15], were carried out, which suggest that the crystallographic orientations and geometry of local microstructure, particularly the tilt and twist angle , differences of slip planes or crack planes, together with cooperation and competition between movement of slip systems and loading stress influence the crack propagation behavior. Patton’s research indicated that crack bifurcation may be caused by plastic anisotropy and pre-existing defects in front of crack tip [16]. Zhiyi Liu’s research indicated that the extending potential decides the further extending of these bifurcated cracks [17]. The potential of the propagation concerns the tilt angle, twist angle and Schmid factor differences between two neighboring grains. These factors, of commonsense, could result in kinking, bifurcation or branching in fatigue crack path, the appearance of which could be captured as soon as it arises [18]. However, Ref. [6] reports that the crack branching observed in alloy 2324-T39 results from the linkup of secondary crack with the lead crack, the process of which is quite different from those common procedures found in the literatures, e.g. Ref. [14-18]. Ref. [7] presents the effect of overloads and  K level on crack path change in alloy 2324-T39. For better understanding the mechanism of the uncommon macroscopic crack branching in 2324-T39 alloy, optical metallography analysis, in-situ SEM crack growth tests and numerical simulations on crack tip stress field were carried out in this paper, which will reveal the differences in macroscopic crack morphologies and metallographic fracture characteristics of the two types of crack branching, as well as the effect of the less common crack branching on crack growth rate and some relationship between the appearance of the less common crack branching with the crack tip plastic zone size. E XPERIMENTS hree kinds of crack growth tests were carried out to investigate the crack growth behavior in 2324-T39, i.e. tests with M(T) specimen of 4.5mm in thickness under spectrum loading (stated as test type I), tests with relatively small specimens of 1.5mm in thickness with single edge notch under constant amplitude loading(stated as test type II), and in-situ scanning electron microscope (SEM) crack growth tests under constant amplitude loading(stated as test type III). The details of the tests type I, such as specimen configuration, loading sequence, test procedures, etc., have been reported in Ref. [6, 7]. The aims of designing the other two kinds of tests are 1) to eliminate the potential effects of specimen thickness and loading sequence on crack branching; 2) to rule out the effect of crack configuration, i.e. middle crack and edge crack, on crack branching; 3) to observe the position where the secondary crack appears and the linkup via in-situ SEM test. The small specimens with single edge notch used in test type II and III were cut from the aforementioned M(T) specimen. The configurations of the specimens are designed to fit the loading facilities attached to test machines, which are given in Fig. 1(a) for test type II and Fig. 1(b) for test type III. All the specimens are in L-T orientation as demonstrated in Fig. 1(c). The thickness of the specimen for test type II is 1.5mm, while it is 0.7mm for test type III. (a) (b) (c) Figure 1 : Specimen dimensions and orientation (Unit: mm). T