Crack propagation on helicopter panel: Experimental test and analysis

Aerospace structures need excellent structural efficiency and damage tolerant behaviour to avoid critical failure in pres- ence of small defects and repeated small loads typical of contingent loads. With the aim of improving the performance of the structures, new materials have been developed. Such materials, as the Al–lithium alloy, are designed with the purpose of optimizing stress/strain vs. weight. Considering the distinct advantage, but also the disadvantage, of this innovative material, it is important to verify the damage tolerant behaviour of the component and this is the aim of the present work. With this aim, some tests have been performed on full scale panel specimens representative of a rear helicopter frame. Dedicated test equipment has been designed and built in order to apply the effective service loads on artificial damaged panels. During the tests, the propagation of the crack, started from an artificial damage, has been monitored until the pro- gressive failure of the panel reaches one or more stringers. Moreover, each specimen has been instrumented with several strain gauges to obtain a strain map during the crack propagation. The crack parameters and the strains recorded during the propagation have been compared with the results from a detailed FE model and an analytical model, with a good cor- relation. In addition, a detailed FE submodel of the bolted joint (stringer and skin in the crack propagation path zone) has been constructed to obtain the crack parameter of a particular panel specimen whose test has also been carried out in the stringer bolted joint.