Mixed-mode fatigue crack growth behaviour in aluminium alloy

Fatigue crack propagation tests in compact mixed-mode specimens were carried out for several stress intensity ratios of mode I and mode 11, KI/II, in AlMgSi 1 -T6 aluminium alloy with 3 mm thickness. The tests were performed in a standard servo-hydraulic machine. A linkage system was developed in order to permit the variation of the K-I/K-II ratio by changing the loading angle. Crack closure loads were obtained through the compliance technique. A finite element analysis was also done in order to obtain the K-I and K-II values for the different loading angles. Crack closure increases under mixed-mode loading conditions in comparison to mode-l loading due the friction between the crack tip surfaces. Moreover, the crack closure level increases with the K-I/K-II ratio decrease. Correlations of the equivalent values of the effective stress intensity factor with the crack growth rates are also performed. Finally, an elastic-plastic finite element analysis was performed to obtain the plastic zones sizes and shapes and model the effect of mixed-mode loading on crack closure. (c) 2005 Elsevier Ltd. All rights reserved.