Mixed mode fracture analysis of multiple cracks in flat and curved stiffened panels of aircraft fuselage structures

In real time, aircraft panels are mainly subjected to fatigue loading induced by the pressurization cycle. When two cracks approach one another, their stress fields influence each other and produce enhancing or shielding effect depending on the position of the cracks. In the present work, an attempt has been made to determine the effect of plate curvature on mixed mode stress intensity factor (SIF) of multiple cracks in a riveted joint using numerical and experimental methods. Diametrically opposite surface cracks of various crack depth ratios [(a / t); 'a' crack length and 't' thickness of the plate] were considered for a typical longitudinal splice joint. 'Frictional contact interaction' was defined between rivet hole, rivet head-sheet, between sheet surfaces and stringer to sheet interfaces in order to simulate the service conditions. At lower crack depths, [] higher SIF was observed for biaxial loading compared to uniaxial loading. Marginal influence of loading condition was observed for deep cracks []. SIF values of curved panels were observed to be higher than that of flat panel irrespective of the crack depths considered the present work. This is mainly due to 'flattening' of curved plate when loading. It was also noticed that, the presence of stiffener reduces the in plane (mode-II) and out of plane (mode-III) shear fracture considerably for curved panel as it exerts compressive force on the joint. To validate the numerical observations, experiments were conducted on a Al 2024-T3 specimens to determine the fatigue crack growth rate and it showed a good correlation with numerical methods. The experimental results also indicated a higher crack growth rate for curved panel compared to flat panel which is mainly due to additional bending stress at the rivet hole region caused by eccentric loading.