Accelerating the fatigue life characterization of composite sailplane structures - a novel approach

Abstract

Current accepted methods of composite sailplane life certification are based on full scale structural testing of the aircraft’s wing. Two accepted test procedures exist, namely a static test to an elevated load level and a fatigue test simulating actual flight loads. Both these methods have their limitations. The static test procedure has a 12000-flight hour limitation on the certifiable life and the fatigue test procedure is very time consuming. The service life requirement has steadily crept past the ability of the static test method and the only alternative is thus through expensive fatigue testing. Attempts in reducing the cost have been proposed from various angles, namely load spectrum truncation, empirical life predictions, load spectrum alteration and accelerated methods for fatigue curves determination. Here an empirical method is proposed and evaluated. Based on the Palmgren-Miner assumption, it is proposed that the relationship in damage accumulation between a chosen constant amplitude fatigue test sequence and the representative flight load spectrum is linear. Consequently, this relationship can be determined with minimal empirical work and can be used to replace the time-consuming flight load spectrum with a more severe and less time-consuming constant amplitude loading sequence. This hypothesis was evaluated on composite specimens, resembling representative structures at increased load levels. It was shown that a second order fit is more appropriate. Consequently, more empirical data is required to establish the relationship than was initially expected. Though not linear, equivalence between the two load-sequences can still be established and thus used to reduce the full-scale testing time. It is further proposed to conduct research at represented load levels, since it might reduce the slope of the second order fit in such a way that a linear approximation might be adequate.