SAMPE Journal - May/June 2017 - 8

Feature Article
Experimental
Preform Fabrication:
A) For initial mechanical evaluation
for open-hole and un-notched
compression and short beam shear
strength, carbon-fiber preforms were
constructed from 8 plies of T650-35
fabric with UC309 epoxy sizing in
a quasi-isotropic layup and stitched
with E-glass fiberglass thread at 1.24
stitches/cm2 (8 stitches/in2) density
to create a preform with a thickness
of 3 mm, and then were cut into 61
cm x 61 cm (24 in x 24 in) panel for
infusion.
B) For compression-after-impact
and thermal-cycling evaluation,
two stacks of T650-35 preforms
as described above were stitched
together into a 6-mm-thick preform
at 6.2 stitches/cm2 (40 stitched/in2)
penetration, and then cut into 56
cm x 56 cm (22 in x 22 in) panels for
infusion.

Table 1. Tg of RTM370/T650-35 stitched composites.

Resin Film Infusion (RFI)
RTM370 Resin powder was
degassed under vacuum at 288°C,
and made into resin plaques to be
placed on top of the carbon-fabric
preforms and then vacuum bagged.
The bagged panels were put into
an autoclave, then heated to 348°C
(660°F) at ~3°C/min, and cured for
2 h. Some of the resulting panels
were subjected to post-cure at 343°C
(650°F) for 8 h to raise the Tg and
achieve better mechanical strength
for comparison purposes.

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Results and Discussion
Characterization of 3-mm RTM370
Thin Stitched Composites
The glass-transition temperature
(Tg) of the RTM370 composite as
cured at 348°C (660°F) for 2 h is
304°C. However, additional postcuring at 357°C for 8 h and 16 h
advanced the Tgs to 318°C and
344°C, respectively (Table 1). A cure
temperature of 371°C for 2 h would
be sufficient to provide higher Tg and
afford better properties, but there is
always a danger of delamination
in polyimide processing by RFI at
this temperature, based on Boeing's
previous experience with similar
polyimide resins.
An RTM370 stitched, carbon-fiber
composite panel after fabrication
(Figure 1) was inspected using
5
MHz
through-transmission
ultrasonic C-scan and shown to be of
high quality displaying no noticeable
defects in the laminates (Figure 2).
The void content of the laminates was
~0.1% as measured by acid digestion.
The analysis also indicated a resin
content of 32.48% and a fiber volume
fraction of 56.75%.
The quality of the laminates was
also inspected by photomicrographs
in cross-section at 50x magnifications
to identify any microcracks, porosity
or other internal anomalies. The
specimens were cut and polished
from various portions of the laminate
to check consistency throughout
the panel. The laminates showed
evidence of high quality with no
porosity or delamination throughout
the cross section, as indicated by the
photomicrographs with no post-cure
(Figure 4) and after post-cure (Figure
5).

Designed to Perform. Built to last.

8

SAMPE Journal, Volume 53, No. 3, May/June 2017
http://www.mokon.com/composites
Table of Contents for the Digital Edition of SAMPE Journal - May/June 2017
