Evaluation of Skin-core Adhesion Bond of Out-of-autoclave Honeycomb Sandwich Structures

Composite sandwich structures offer several advantages
over conventional structural materials such as lightweight, high
bending and torsional stiffness, superior thermal insulation and
excellent acoustic damping. One failure mechanism in a composite
sandwich structure is the debonding of the composite facesheets from the
core structure. A well-formed adhesive fillet at the interface of the
honeycomb core cell walls and the laminate is a significant factor in
preventing bond failure. In the present work, honeycomb composite
sandwich panels are manufactured using a low-cost
vacuum-bag-pressure-only out-of-autoclave manufacturing process.
CYCOM®5320 out-of autoclave prepreg is used for the facesheet laminates
and FM® 300-2U film adhesive is used for the facesheet-to-core bond. The
manufactured composite sandwich panels are of aerospace quality with a
facesheet laminate void content of around 1%. In this study, adhesive
fillet formation and adhesive mechanical strength are evaluated as a
function of several different sandwich construction design variables.
Both aluminum and aramid Nomex® honeycomb core materials are considered
to study the effect of core cell size and core material. The effect of
film adhesive thickness is studied. A process for reticulation of the
adhesive is applied and studied. A quantitative investigation of the
adhesive fillet geometry is carried out for all the panels. Manufactured
panels are evaluated for flatwise tensile strength in accordance with
test method ASTM C297. Optimized combinations of core material, core
density, cell size and adhesive thickness are identified. Results show
that the reticulation process improves fillet formation and increases
flatwise tensile properties.

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