SAMPE Journal - May/June 2016 - 44

Feature Article
Processing and Performance of Out-of-Autoclave
Bismaleimide Composite Sandwich Structures
S. Anandan, G. S. Dhaliwal and K. Chandrashekhara
Department of Mechanical and Aerospace Engineering
Missouri University of Science and Technology, Rolla, MO
T. R. Berkel
Boeing Research & Technology, St. Louis, MO

D. Pfitzinger
GKN Aerospace, St. Louis, MO

Abstract
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.
In the aerospace industry, sandwich composites are commonly manufactured using the autoclave process which
is associated with high operating cost. Out-of-autoclave (OOA) manufacturing has been shown to be capable of
producing low cost and high performance composites. Unlike the autoclave process, OOA processing avoids the
issue of core-crushing due to high pressure. However, Bismaleimide (BMI) prepregs require high cure and postcure temperatures which can lead to high internal core pressures, core-to-facesheet disbonding and voids. In the
current work, OOA sandwich composite panels are manufactured using aluminum honeycomb core, BMI adhesive
film and carbon/BMI prepregs. Two vacuum levels were used during OOA processing, full vacuum (100 kPa)
and partial vacuum (80 kPa). Adhesive bond quality was evaluated using flatwise tensile and fracture toughness
tests. Mechanical performance was evaluated using edgewise compression. It was observed that vacuum level
variation during processing had no significant effect on mechanical properties of manufactured laminates. Tests
are performed at room temperature and 177°C (350°F). All manufactured laminates exhibited room temperature
flatwise tensile strengths comparable to those of aerospace grade epoxy adhesives. Sandwich mechanical
properties reduced when test temperature was increased.
Introduction
High performance composite materials are
conventionally processed in autoclaves which are
associated with high capital requirements and operating
costs. Part size is also limited by the autoclave chamber
volume. Out-of autoclave (OOA) processing has emerged
as an alternative to autoclave processing1,2. OOA involves
curing at atmospheric pressures which obviates the
need for an autoclave and provides greater design
flexibility for production of large and complex shaped
composite components. It also reduces capital and
operating costs. In an OOA prepreg process, removal of
entrapped air from the prepreg layup is critical which
can otherwise lead to voids which degrade mechanical
properties of the composite. Sandwich components can
be co-cured, a process where adhesive and the prepreg
are cured simultaneously, resulting in time and cost
reduction. Manufacturing co-cured sandwich panels
out-of-autoclave, eliminates core crushing which can
occur at autoclave pressures3 and enables the use of
lighter cores.
In a composite sandwich structure, the facesheet
absorbs in-plane loads while the core is resistant to
shear loading. The composite facesheet is bonded
to the core using a film adhesive. The adhesive bond
44

between core and facesheet is of great importance
in the design of sandwich structures. The relation
between the adhesive fillet and sandwich properties
was described by Grimes4. The mechanical
performance of a sandwich composite depends on
the quality of the core-to-facesheet adhesive bond. In
the case of honeycomb cores, the cell walls provide a
relatively small area for bonding. Structural strength
depends on the presence of a well-formed fillet at the
interface of the core cell wall and facesheet. Larger
adhesive fillets absorb a higher amount of energy
upon fracture, due to which they have high fracture
toughness5. Grove et al.6 showed that the higher
debonding energy was obtained with larger, regularshaped adhesive fillets between the honeycomb cell
walls and the skin. Rion et al.7 showed the failure
occurs in the adhesive meniscus when low weight
adhesive is used. Butukuri et al.8 evaluated the effect of
the reticulation technique on properties of sandwich
constructions. Core reticulation resulted in improved
fillet geometry and flatwise tensile strength. Hou et
al.9 reported that thermal pre-treatments can reduce
adhesive foaming in OOA cured composite sandwich
structures. Nagarajan et al.10 studied the effect of
processing variables on OOA sandwich panels with
SAMPE Journal, Volume 52, No. 3, May/June 2016

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