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

H0 H1

• 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.