Co-infused and secondary bonded composite stiffened panels in compression: numerical and experimental strength assessment combined with NDI and guided waves based SHM

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. Today, composites primary structures that work mostly under compressive loads are designed following the no-buckling criteria up to Ultimate Load. Such a design approach leads to stiffer and heavier structures if compared to letting the compressed skins 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 to correctly predict the nonlinear mechanisms of load absorption beyond skin buckling onset. Furthermore state-of-the art Non-Destructive- Techniques and Structural Health Monitoring Systems can be employed for a continuous monitoring of the joints health status. In this context, the joining technique of the stringers to the skin has a particular importance; indeed, although different joining processes barely influence the linear behavior of a stiffened plate until its first instability load, they are responsible for relevant differences in the ultimate failure load. This paper presents numerical and experimental activities carried out to study the behavior of compressed stiffened plates obtained by different manufacturing processes as well as monitoring techniques of the health status of the panels by classical NDT and guided waves based SHM systems. The numerical problem has 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.