Fatigue analysis of folded composite structures

Abstract

The use of composite materials and morphing structures are becoming increasingly popular in future aircraft and airframe designs. New production processes, based on Kirigami techniques, are evaluated to manufacture folded material sandwich cores that constitute potential candidates for these types of cellular structures. The research project is aimed at designing and validating a suitable test rig to characterize the fatigue life of the hinge sections (folds) that exist in such structures. The design of the test was based on the Double Cantilever Beam ASTM standard procedure and specimens were manufactured with PEEK as the base structural composite, provided it represents the only thermoplastic polymer certified for aeronautical structural applications. Using a universal testing machine, two different configurations were tested and the output was processed by means of a Matlab routine. Relevant parameters, such as the moment against the rotation of the fold and the loss factor of the fold under cyclic loading, were computed. It has been shown that the material suffers no decay during the first 1000 loading cycles. Based on the results obtained, it has been possible to prove the validity of this procedure to start an initial estimation of the service life of folded composite cores. Improvements have been pointed out to aid future research in the field; these include performing accelerated tests or designing devices that will broaden the testing capabilities of the equipment used. Although further work needs to be performed in order to fully characterize creases in folded cores, this project provides a suitable baseline for upcoming research.