TM 1-1500-204-23-11
2 to 4 inches larger than the proposed
heat cure (paragraph 6-19) or by drying the
repair area in all directions.
part per paragraph 6-6e.
(2) Blistering of laminate plies can occur if the
2
Continue wiping until the entire repair
vapor pressure of the moisture in the lam-
area has been cleaned. The area is
inate exceeds the strength of the matrix
clean when the dry wiping cloth con-
material. The potential for blistering can
tains no visible residue after use.
be reduced by drying the repair area as
described in paragraph 6-6e.
Pay special attention to fasteners,
3
crevices, and seams where contam-
(3) Skin to core disbonds and honeycomb core
inants may be entrapped. These
node bond failures can occur if the pres-
areas may have to be lushed with
sure in the core (due to heating air and
solvents. Solvent resistant brushes
trapped moisture in the core cells) exceeds
may be used as an aid for crevice
the strength of the skin to honeycomb core
and fastener cleaning.
bond and/or the strength of the core node
bond. The potential for node bond failure
6-6. DRYING PROCESS. This paragraph is used in
and skin to core disbonds can be reduced
conjunction with paragraph 6-18, Vacuum Bagging,
by reducing the repair cure temperature.
and paragraph 6-19, Curing. For information on
Laminate drying as described in paragraph
the equipment cited in the following paragraphs, see
6-6e is still required prior to performing the
cure cycle to reduce bondline porosity and
the potential for laminate blistering. If liquid
a. Composite Moisture Absorption. Composite
water is present in the honeycomb sandwich
skins and sandwich structure inherently absorb water
assembly (as detected by x-ray), use one of
and moisture when exposed to the environment for
the water removal procedures described in
extended periods of time (several months). This
water and moisture intrusion may not be obvious by
visual inspection of the structure. The structure may
c. Field Concerns. In the ield environment, repair
look dry, but it is not. Some composite materials,
time and equipment may be limited. The parts must
including resin, iber, and core, have inherent water
be dried as much as possible, and this can be
absorption characteristics. This is reversible and
a lengthy process. To mitigate the effects of any
time/ temperature/humidity dependent. Painted lam-
moisture not removed during the drying process,
inates have higher moisture content than unpainted
cure temperatures of 210 F or below should be
laminates as the paint tends to inhibit the moisture
used if authorized by the system speciic technical
egress from the laminate. Sandwich or honeycomb
manual or engineering authority. Curing at less than
panels are particularly prone to water absorption and
the optimum temperature, even when authorized,
retention due to the increased air volume in these
will prevent adhesives from reaching their ultimate
panels. Water intrusion in honeycomb rotor blades
properties; however, loss of adhesive strength is
is one potential cause of not being able to track
more desirable than inducing damage from water
and balance the blades. Cracked coatings, impact
vapor pressure during the cure cycle.
damage, and loose fasteners all provide paths for
moisture intrusion. Moisture present in honeycomb
d. Drying Veriication. To ensure the repair area
sandwich assemblies may not be detectable by x-ray.
is dry, a desiccant can be installed into the vacuum
bag to monitor the moisture removal. A moisture
b. Types of Moisture Damage. It is extremely
indicator is used to determine if a part is dry following
important to ensure water and moisture are removed
a drying cycle; it uses a color changing desiccant to
prior to performing any bonding, especially one incor-
indicate the presence of moisture. See paragraph 3-5j
porating a heated curing process. Standing water,
for details on the moisture indicator. The veriication
water vapor, and absorbed moisture in laminates
process involves inserting a tube, which will be used
and honeycomb can produce the following damaging
to hold the color changing desiccant, in line between
effects:
the vacuum bag and the vacuum pump. The part is
heated in an oven (if possible) or in an envelope bag
(1) Moisture in laminate skins absorbs into the
with heat blankets. The temperature is held at the
adhesive bondline during the cure process
drying temperature for several hours. At the end of
and results in bondline voids. This can be
the drying cycle, a fresh supply of desiccant is placed
alleviated when curing two part adhesive
in the tube. If the desiccant picks up moisture over
systems by allowing the adhesive to gel
the inal hour (which will be indicated by a change in
at room temperature prior to beginning the
color of the desiccant), the part is not dry; continue
6-12
