JEC COMPOSITES MAGAZINE - Issue #96 - April 2015 - 39

ponent testing for selected primary structure parts, to achieve the desired Airbus
technology readiness levels (TRL) for
production application readiness.

Discontinuous long fibre composites - a viable alternative for
complex-shape parts

Matrix polymers for DLF materials in
aerospace applications may be thermoplastics (typically PEEK or PEKK) or
thermosets (typically epoxy). Greene
Tweed's Xycomp® DLF products have
focused on the use of PEEK and PEKK
thermoplastic matrix materials due to
several important properties:
- Inherent thermoplastic matrix toughness for enhanced damage tolerance
- Extremely low moisture uptake for
minimal hot/wet effects compared to

Chopped/randomized thermoplastic
matrix DLF materials are compression
moulded in matched-metal tooling,
with the capability to produce net- or
near-net-shape components with complex geometries. Some of the primary
advantages of DLF composites include
the elimination of ply-by-ply cutting and
layup operations, almost zero material
waste, the elimination or reduction of
secondary machining operations, and
design freedom for the redesign of multiple metallic parts as single-piece DLF
components. Another important benefit
of DLF compression moulding is the
ability to form complex features such as
moulded-in holes, ribs, bosses, gussets,
flanges, thickness changes, corners, and
identification labelling with minimal
added process difficulty. In addition,
secondary metallic components such as
threaded inserts, nutplates, or bushings
can be incorporated during the moulding
process - further reducing the number
of manufacturing process steps needed
while continuing to offer the level of
weight savings expected from the use of
carbon fibre composites.

CORAC development project
for fuselage primary structure
components
The Stelia Aerospace-led CORAC project focused on two different representative parts for the demonstration of DLF
in primary structures with new design,

Fig. 2: Stelia Aerospace fuselage frame
attachment fittings

both based on Airbus A350 fuselage geometries - an aluminium fuselage upper
attachment fitting currently produced
by Stelia Aerospace, and a highly-loaded
landing gear drag stay fitting analysed
by partner Latecoere. Part performance
and failure predictability were critical for
both parts, with the project particularly
intended to advance toward a future certification by analysis approach for DLF
applications.
Stelia Aerospace's work focused on a primary structure attachment fitting used to
assemble nose fuselage frames to cockpit
structures (Figure 2).
The project included material characterization of Greene Tweed's Xycomp
carbon/PEEK DLF, using a test pyramid
starting with coupons and progressing
to component testing based on environmental and performance requirements
established by Stelia Aerospace for the
selected parts. Impact, bolt holes (pullout and notch effects), moisture conditioning, temperature, effects of defects,
and fatigue were key factors in the test
programme. The resulting material data
was used in combination with existing Xycomp DLF material allowables
data for presizing by Stelia Aerospace
and additional finite element analysis
support by Greene Tweed. The project
also evaluated processing effects such as
material flow and process repeatability,
non-destructive inspection techniques,
and simulation and predictive modelling
capabilities for DLF.

No96 April 2015 / jec

Feature Aeronautics

Most DLF materials are produced
by chopping carbon fibre-reinforced
prepreg unidirectional (UD) tape into
"flakes" or "chips" with predefined
dimensions. The chopped material may
subsequently be used in a bulk form
(loose chips) or in a mat form (similar to
sheet moulding compound, or SMC) for
compression moulding, depending on
the product type.
Producing DLF materials from a
chopped UD tape product form offers a
number of benefits, including:
- Capability of achieving high reinforcement fibre contents of 55%-62% (by
volume) or 63%-70% (by weight) for
optimal performance - compared to
extruded injection moulding pellets with
a practical maximum of 30%-40% fibre
weight content
- Long fibre reinforcement (13 mm or
greater fibre length in final moulded
parts)
- Material qualification and acceptance
at the UD tape lot/batch level for precise
control of resin content, fibre areal
weight, and constituent materials, with
full traceability
- Existing OEM material specifications
and in-service "pedigree" for the parent
UD tape materials, reducing adoption
risks

epoxy
- Excellent flame/smoke/toxicity performance, exceeding all FAA, EASA and
OEM requirements for interior applications
- Excellent fire performance, meeting
5-minute and 15-minute burn-through
requirements
- Excellent resistance to typical aerospace
fluids
- High resin viscosity for minimal fibre/
resin separation during processing and
uniform fibre/resin distribution throughout the moulded parts
- Ease of recycling, for environmental
benefits

composites magazine 39
JEC COMPOSITES MAGAZINE - Issue #96 - April 2015

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