Fillet curvature • Fillet curvature radius is invariant under pressure Transport phenomena • Increase of core pressure → Growth of the flow of matter → Displacement of skin resin and migration of adhesive resin → Thinner adhesive layer Impact of processing parameters on adhesive fillet size and shape of aerospace honeycomb composite sandwich panels R.Trigueira, Prof. P. Hubert, Prof. V. Michaud McGill University - Structure and Composite Materials Laboratory Objectives • Out of Autoclave (OoA) Vacuum Bag Only (VBO) co-cure of honeycomb sandwich panels • Investigation on the effect of core pressure during processing on the interface between the core and skins of co-cured honeycomb sandwich panels Skins Core Honeycomb sandwich with cyanate ester/carbon fibres skins Set-up • Controlled core pressure (80kPa, 30kPa and full vacuum (FV)) • Placed in an oven for curing • Recording of bag and core pressure, as well as the flow Sandwich panel Adhesive A • Unsupported epoxy film, 100gsm • Designed for VBO co-cure at 121°C • Minimum viscosity : 30 Pa∙s Materials Adhesive B • Unsupported toughened epoxy film, textured, 100gsm • Designed for a cure at 150°C under 300kPa • Minimum viscosity : 380 Pa∙s Adhesive A, 80kPa Adhesive A, 30kPa Adhesive A, FV Presence of defects due to the lack of resin in the skins Dry-spots Fillet height H 0 H 1 • Both adhesives exhibit the same behaviour : the lower the pressure the higher the fillets • Interquartile range stays stable → Distribution of fillet heights is almost not impacted by the core pressure. Adhesive A , 30kPa Adhesive A R Adhesive A, 30kPa Fillet contact angle • Contact angle undergoes little to no variation with the pressure. θ Adhesive A, 30kPa Adhesive A, FV Scattering of the data can be due to cell wall curvature, excrescent para-aramid fibres, toughening particles (Adhesive B), motion of the skin (different advancing and receding angles). Film thickness • As the flow between the core and the bag increases (80 and 30kPa), the film thickness lowers because of the transport phenomena. Adhesive A, FV Scattering of the data : • Film thinner near the fillet : flow of matter not sufficient to refill this region • Inhomogeneous topology of the skin • Locally increased migration of adhesive resin Adhesive A, horizontal line : initial film thickness (84 μm) Conclusions • For both adhesives, the lower the pressure, the larger the fillets. • Transport phenomena are dominating the fillet formation for the 80 and 30kPa conditions, decreasing the fillet quality. • Core pressure seems to have little to no influence on the mechanisms driving the fillet formation itself. • Adhesive viscosity is a key factor in the fillet formation as it impacts the flow of matter. Adhesive A References • T. Centea, L. K. Grunenfelder, and S. R. Nutt. A review of out-of-autoclave prepregs : Material properties, process phenomena, and manufacturing considerations. Composites Part A, 70:132-154, 2015. • S. Sequeira Tavares, N. Caillet-Bois, V. Michaud, and J. A E Månson. Vacuum- bag processing of sandwich structures: Role of honeycomb pressure level on skin- core adhesion and skin quality. Composites Science and Technology, 70(5):797- 803, 2010. • J. Rion, Y. Leterrier, and J. A. E. Månson. Prediction of the adhesive fillet size for skin to honeycomb core bonding in ultra-light sandwich structures. Composites Part A: Applied Science and Manufacturing, 39(9):1547-1555, 2008. • T. Centea, D. Zebrine, M. Anders, C. Elkin, and S. R. Nutt. Manufacturing of Honeycomb Core Sandwich Structures: Film Adhesive Behavior Versus Cure Pressure and Temperature. Proceedings of the Composites and Advanced Materials Expo, 2016.