Practical use of the ‘equivalent’ measured stress intensity factor to control fatigue crack propagation rates in aircraft full-scale fatigue tests—First assessment of the method in testing of a pressurized aircraft fuselage

Abstract

In the case of circumferential cracks in a cylindrical fuselage, the comparison of some analysis and test results shows that the theoretical stress intensity factor is a suitable correlation parameter of fatigue crack propagation rates, both in aircraft fuselages and in plane panels. Values of the ‘equivalent’ stress intensity factor, computed by applying the Barrois-Bhandari method to slot-opening measurements performed under decreasing loading levels, agree well with the values computed from two dimensional Theory of Elasticity, using the method of finite elements. In the case of longitudinal cracks, the experimental values of the ‘equivalent’ stress intensity factor, i.e. the stress intensity factor of the infinite plane sheet containing a centre crack with the same elastic strain and stress distributions near the boundary of the plastically strained region around the crack tip, yield a good correlation of fatigue crack propagation rates of the cracked fuselage and of cracked plane structures. The values of the ‘equivalent’ stress intensity factor are lower than those of the theoretical stress intensity factor, the interest of which disappears, but are also far higher than the bidimensionally computed values, which are no longer to be considered. Some meant of safety provided to limit crack openings will make it possible, in the near future, to investigate test conditions reaching the ultimate residual static strength of cracked structures, while avoiding, however, catastrophic failures.