Adhesive-Free, Bonded Assembly of Epoxy Composite Structures Using Stoichiometric Offset Resins

Airframe manufacturers increasingly rely on advanced composite materials to improve efficiency and safety while reducing manufacturing costs. The challenge is that the Federal Aviation Administration (FAA) requires mechanical fasteners or comparable arrest features in adhesively bonded joints to comply with Federal Aviation Regulations (FARs). Commercial aircraft commonly contain several miles of adhesively bonded joints and therefore thousands of redundant fasteners. Large-scale composite structures are assembled in one of three ways: secondary bonding, co-bonding, and co-curing. Co-bonded and co-cured products require the use of molds to support the preform, and often an autoclave which limits the size and complexity of the composite structure that can be fabricated in a single cure process. An adhesive-free technique, Co-cure-ply, is proposed to assemble large-scale composite structures without material discontinuities and uncertainties in the structural performance of bonded joints. The Co-cure-ply technique enables thermoset epoxy resins to be cured in a two-step process. The method involves two complimentary epoxy resin formulations: one designated as hardener rich (HR) with an excess of the hardener component and one designated as epoxy rich (ER) with an excess of the epoxide monomer. The method encompasses a stoichiometric offset approach, delaying the epoxy/diamine crosslinking reaction and eliminates material discontinuities by allowing resin interdiffusion across the joining interface during the assembly of a composite structure (analogous to what occurs between plies of conventional prepreg). Previous work investigated the degree of stoichiometric offset and various cure cycles to maximize mixing at the co-cured interface. In previous experiments, the fracture toughness was measured by double cantilever beam (DCB) test with values ranging from 250 to 650 J/m² which is commensurate with aerospace structural adhesive systems. This report focuses on preparing and testing resins with greater stoichiometric offset and developing additional mechanical testing methods to measure the shear and tensile strength in addition to the mode I fracture toughness of the Co-Cure-Ply fabricated composites. Characterization methods such as infrared spectroscopy, rheology and electron microscopy, will be performed to access the interface of co-cured specimens.