CO-INFUSED AND SECONDARY BONDED COMPOSITE STIFFENED PANELS LOADED IN COMPRESSION: NUMERICAL ANALYSES AND EXPERIMENTAL TESTS IN LINEAR AND POST-BUCKLING REGIMES

Adhesive junctions or co-infusion of skin and stiffeners represent efficient manufacturing processes for aircrafts composites stiffened panels leading to weight saving, although they have not been widely adopted yet due to certification issues and the lack of well-established design tools and procedures. Airworthiness requirements for composite structures pose major challenges to the certification of adhesively bonded or co-infused stiffened structures. FAA Advisory Circular 20-107B prescribes the methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane. Most of today design approaches lead to stiffer and heavier structures if compared to letting the compressed skins to work in post-buckling until failure. In order to exploit the full structural potentiality of this type of structures under compressive loads new design approaches, mostly based on Finite Element Modelling, have to be developed and validated with experimental results. In this context, the joining technique of the stringers to the skin has a particular importance, barely influencing the linear behavior of a stiffened plate until its first instability load, but being responsible for relevant differences in the ultimate failure load. Within this paper compressed stiffened plates obtained by different manufacturing processes have been modelled with approaches of increasing complexity, from “classical” FE models to predict the first buckling load, to post-buckling analyses up to more refined techniques including the behavior of the skin-stiffener interface. These latter, based on the use of cohesive elements, allow to account for interface properties due to different manufacturing processes. A critical analysis of the numerical and corresponding experimental results as well as a comparison with the expected nominal structural performances will be presented.