Investigation on fatigue crack propagation behaviors in the thickness direction of 2519A aluminum alloy thick plate

• The delamination crack in the thickness direction of 2519A Al alloy was investigated. • The bridging effect of the second phase makes delamination crack easy to occur. • Crack tends to expand along grain boundary due to the influence of grain orientation. 2519A aluminum alloy thick plate is a promising structural material in the military industries, owing to its low density, high tensile strength and excellent ballistic performance. However, the delamination crack in the thickness direction (ND) of this alloy restricts its further application in light armors. The fracture mechanism of the alloy under the thickness direction fatigue load (ND fatigue load) remained unclear. In this work, the influence of microstructures on the fatigue crack propagation (FCP) behavior in the thickness direction of the alloy was investigated. Compared with the alloy subjected to rolling direction fatigue load (RD fatigue load), the alloy under the ND fatigue load is more susceptible to fracture with weak FCP resistance due to the effect of the coarse second phase. The FCP mode of the alloy under the ND fatigue load is mainly intergranular crack propagation. The fracture mechanism of the alloy under ND fatigue load mainly has three aspects. Firstly, due to the bridging effect of the second phase, the FCP of the alloy is accelerated under the ND fatigue load, and the delamination cracks occur easily. Secondly, the crack tends to expand along grain boundaries (GBs) due to the large percentage of GB when the alloy is subjected to ND fatigue load. Thirdly, the crack is inclined to propagate grains with a small twist angle of GB and a high Schmid factor. Moreover, when the orientation factor difference between two grains is large, the crack is more likely to extend along the GB of the grains.