Issue 35

L. Songsong et alii, Frattura ed Integrità Strutturale, 35 (2016) 74-81; DOI: 10.3221/IGF-ESIS.35.09 74 Focussed on Crack Paths Mechanism of crack branching in the fatigue crack growth path of 2324-T39 Aluminium alloy Lu Songsong (http://orcid.org/0000-0001-7591-434X ) Bao Rui (http://orcid.org/0000-0001-7170-082X ) Zhang Ting Fei Binjun Institute of Solid Mechanics, Beihang University (BUAA), 100191, Beijing, China rbao@buaa.edu.cn A BSTRACT . The crack growth behavior in 2324-T39 aluminium alloy was experimentally investigated. Two types of crack branching were observed. The mechanism of an uncommon crack branching which results from the link up of secondary crack with the main crack was focused on. The crack paths were observed with optical microscope and in-situ SEM. Finite element crack tip simulation was performed to investigate the relationship between plastic zone size and the location of the secondary crack. Tests and analysis results indicate that the secondary crack initiates near the plastic zone boundary and from the subsurface. The mechanism of this kind crack branching relates to the interaction of grain size and plastic zone size. K EYWORDS . Fatigue crack growth; Crack path branching; Mechanism analysis; 2324-T39 aluminium alloy. I NTRODUCTION lloy 2324-T39 plate is a higher strength version of alloy 2024-T351, and is a high-purity controlled composition varient of alloy 2024. Both strength and fracture toughness properties are improved compared to 2024 plate via employing the T39 temper which was developed through special fabricating practices [1]. Since this alloy was developed for tension-dominated, fatigue and fracture critical plate applications, most of the experimental investigations about it are most focused on the fatigue crack growth and fracture. William M. [2] and Dawicke D S [3] report the fracture test results of 2324-T39 plate with difference thicknesses. Stoychev S [4] investigates the effect of varying stress intensity factor range  K on crack growth rate. Bao R et al. [5-7] present the macroscopic crack path branching observed during the crack growth tests of 2324-T39 middle crack tension (M (T)) specimen under spectrum loading. According to achievements in fatigue theory and fracture mechanics, a through thickness middle crack in an isotropic thin plate subjected to remote tension-dominated loading should propagate perpendicular to the load direction, i.e., Mode I. This is reasonable from a physical perspective because the tensile stress component, which opens the crack, will certainly promote the conversion of cyclic plastic deformation into crack extension [8]. At the macro level, crack kinking, bifurcation and branching are influenced by micro-structural irregularities, load effects (global and local multiaxial stress state, spectrum loading), and environmental factors [9-13]. Though, the T39 temper was said to account for the fracture toughness improvement, there is no evidence found in the literature that the T39 temper will contribute to the macroscopic crack path change with respect to those of other Al-Cu-Mg alloys, such as 2024-T351 and 2524-T3. A