Design and Progressive Failure Analysis of 3D-Reinforced Composite Force Introduction Loops

Abstract

Composites structures are increasingly being used in today’s aircrafts. Therefore advanced load introduction concepts for composite structures are becoming more important than ever. This paper concentrates on composite force introduction loops that are used at highly loaded attachments with small design space. These attachments have high strength at pure tension loads, but show poor performance at side (bending) loads. A damage analysis of different force introduction loops is performed to determine the cause for the poor performance at bending loads. Based on the results of these numerical investigations, 3Dreinforcements are applied at the critical area at the force introduction loops. Experimental tests show that first ply failure can significantly be increased for side loads. Furthermore the 3D-reinforcements hinder crack propagation after first ply failure, which leads to a major residual strength increase. Additional experimental tests were performed to determine the effect of 3D-reinforcements on the fatigue strength of force introduction loops. It is shown, that the fatigue strength for bending loads can significantly be increased. A slight load increase is also observed for tension loads. The tests are validated with progressive failure analysis investigation of the 3D-reinforced force introduction loop. These non linear numerical investigations show good compliance with the experimental results. In summery, it is shown that 3D-reinforcement of force introduction loops significantly increases the performance for bending loads, with an additional slight improvement for pure tension loads.