TM 1-1500-204-23-11
foreign objects (such as maintenance stands, tool
Delamination of fastener holes can be caused by over
boxes, dropped tools, etc.) must have the suspected
torquing fasteners during installation and by gener-
area inspected by an appropriate method (usually
ation of an excessive amount of heat when drilling
ultrasonics). Inspect for the presence of subsurface
out damaged fasteners. Fastener holes may elongate
damage per paragraph 5-2 as well as the system
with repeated loading.
speciic technical manual. Impact related delamina-
tion damage is the most common type of damage
i. Edge Damage. Edge and corner damage to
experienced with composite materials.
panels result in edge delaminations and/or broken
off pieces sometimes requiring a rebuilding effort.
d. Dent. A hollow or low place usually caused
by impact. It will usually have deformed ibers, but
j. Heat/Burn Damage. Temperature exposure in
no broken ibers on the impact surface (see igure
excess of the part cure temperature can degrade
5-4). It is detectable by visual inspection or straight
matrix strength. This can be the result of a light-
edge comparison. If honeycomb core is present,
ning strike or ire. Epoxy matrix materials that are
crushing of the core will exist beneath the dented
exposed to temperatures above their cure temperature
composite skin. The crushed core can be hidden
can experience a marked degradation in properties
by the brittle composite materials separating in the
and show little or no visual indication of damage to the
laminate, springing back and masking the buckled
laminate. However, if the laminate is painted, discol-
core beneath. On solid laminates, the backside of
oration of the paint system provides an indicator that
the dented surface will usually show signiicantly more
laminate damage may have occurred. Extreme expo-
damage than the front side; therefore, technicians
sure, beyond 600 F for example, may result in visual
should always examine both the front and back sides.
blistering and pyrolyzation of the outer plies of the
laminate. For bonded composite assemblies, epoxy
e. Disbonds. Disbond is adhesive failure at a
adhesives degrade at lower temperatures than lami-
bondline. It occurs between a honeycomb core and
nate matrix materials. Advanced composite materials
a face sheet or between adjacent bonded structures
and bonded composite assemblies exhibiting discol-
(see igure 5-4). Skin to core disbonds and skin
ored paint, coated with soot, or that are suspected of
to substructure disbonds occur as a result of core
being exposed to excessive temperatures may have
corrosion or part exposure to temperatures at or
experienced heat damage. These advanced compos-
above the part cure temperature. They can also
ite materials and bonded composite assemblies are
be caused by impact forces if the adhesive used to
suspect. The matrix material beneath any blistered,
bond the core or substructure is more brittle than
delaminated or pyrolyzed plies is suspect as well.
the composite matrix material or due to inadequate
Any composite structure that is exposed to temper-
surface preparation or cure.
atures greater than design or maximum operating
temperature requires evaluation and disposition by
engineering to evaluate laminates, laminate to core
f. Cracks. Cracks are defects where one direction
bonds, and laminate to substructure bonds. Even
is signiicantly greater than the other. Cracks include
though no NDI indications occur, any part suspected
both cracks through the matrix only and the matrix
of heat damage may have signiicant strength reduc-
and ibers. Cracks may only extend a few plies deep
tions. Composite materials exposed to aircraft ires
and may not propagate deeper due to the different
may experience pyrolyzation of the matrix material
orientation of the plies.
on the outer plies, thus eliminating support for the
ibers, while the underlying plies experience blistering
g. Puncture. A puncture is any penetration of a
and interply delamination. Although appearing intact,
structure (see igure 5-4). Composite skin penetration
the matrix material beneath blistered, delaminated or
damage is characterized by broken ibers, matrix
pyrolyzed plies experiences a strength reduction and
cracks and delaminations. This type of damage
requires evaluation and disposition. Perform NDI on
usually results in subsurface delaminations and matrix
any area of the part subjected to temperatures exceed-
cracks larger than that apparent visually. Mapping of
ing the service temperature of the part material.
these areas is required as described in paragraph
5-6. Visually apparent penetration damage on the
k. Chemical Damage. Damage to the resin can
exit side is typically 3-5 times larger than entrance
occur due to the effects of chemical attack. Com-
side damage size.
posite materials exposed to chemical paint strippers
experience a long term degradation of matrix strength
h. Fastener Hole Damage. Gouging of counter-
with no visual indication on the part surface. Chemical
sinks and areas surrounding fastener holes caused
paint strippers are not authorized for use on composite
by fastener drivers can occur during fastener removal
materials. They require evaluation and disposition by
and installation. Repeated fastener removal and instal-
engineering. Composite materials exposed to most
lation can result in excessive wear of fastener holes.
5-12
