JEC COMPOSITES MAGAZINE - Issue #80 - April 2013 - 37

Fig. 8: Hat-section stringer with padups

length stringer thickness, reduces weight and
cost and increases local strength for stringerstiffened structures. For example, Figure 10
shows that for a 508-mm-long C-channel
stringer with a 12-ply base stringer thickness,
the strength can be increased 2.8 times by
adding local padups or by increasing the full
length thickness to the same total thickness
as the base thickness + padup thickness.
By using local padups, rather than a global
increase of the full length thickness, a weight
savings of 351g - 191g = 160g, or 46%, is
achieved. Therefore, specific application to
aircraft load floors for strength and weight
savings is possible in volume fabrication of
stamp-formed thermoplastic composite
stringers. n

Fig. 6: Stamp-forming process steps

Fig. 7: Sub-element test setup

Test results
Figure 9 is the load-displacement curve
from the test. Loads reported were per
stringer for the double stringer test specimen
configuration. The yield point for the
specimen is noted on the graph. Yield is
defined as the point at which the load-displacement curve goes non-linear, indicating
that something has failed. In actuality, the
stringers continued to carry increasing load
up to the point of ultimate failure where the

Conclusions
Continuous-fibre-reinforced
thermoplastic composite stringer
structures are viable candidates for
applications requiring low weight,
high strength, and increased durability. Stamp-formed stringers can be
fabricated to include local reinforcements at areas of high stress concentration due to support conditions.
Using local reinforcement padups,
rather than increasing the global full

Fig. 10: Comparative weight for padup vs. no-padup stringer

No80 April 2013 /

Feature Aeronautics

stability in the test specimen as it is loaded
in compression. The test is designed to load
one end of the test specimen, concentrating
the load into the underlying support frame.
The layup of the facesheets is as indicated,
with the 0° direction going into the page,
that is, along the stringer axial direction.
The non-padup test specimen had a layup
of [90/90/0/90/45/-45]s. The padup
test specimen had a base stringer layup of
[90/0/90/45/-45]s and a 16-ply padup
layup of [90/0]8s. A controlled load was
introduced into the specimen using an
Instron 4400R with a 9-ton load cell.

load-displacement curve
takes a significant and
immediate drop and is
usually accompanied
by a significant acoustic
event. The yield failure
load was 3.2 kN for the
non-padup stringer
and 9.1 kN for the
stringer with padups. A
significant increase in
strength capability of
280% was realized with
More information: www.fiberforge.com
the use of local
padups. Note
that the base ply count of the
stringer for the two test specimens
is different, that is, 12 ply versus
10 ply. This is inconsequential in
terms of radius crush strength as the
strength is solely a function of the
local total layup thickness, which is
base layup + padup thickness. Base
ply count, which is the common
layup throughout the stringer, is
mainly responsible for carrying the
non-concentrated stringer bending Fig. 9: Sub-element load-displacement curve
loads.

jec composites magazine 37

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JEC COMPOSITES MAGAZINE - Issue #80 - April 2013

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