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(1) DESCRIPTION OF MISHAP PIECES. Paragraph
3.5a gave general information for the types of material in
a composite system and the various finished forms a part
may have. This will help with the identification of the
undamaged pieces. However, information is needed to
determine what damaged debris looks like. Damaged
debris is categorized in the following way (largest to smallest
in size):
(a) FRAGMENTS. Whole laminate pieces, bulk composite
debris is termed fragments. Impact fragments can
be found within the emergency response cordon, in the
impact crater or in the debris field some distance from
the crater. Because of their weight, fragments will not
travel far from their initial contact with the ground. (PHOTOS
18 and 19)
(b) STRIPS. Single laminate layers are called strips.
Damage can cause the layers to separate. Strips produced
from physical damage will have resin attached to
the fibers except at the fractured ends and will be found
close to the originating composite part. If a separation
was caused by fire damage, some resin or char material
will be holding the fibers together. Fire generated strips
will be found both within and right outside the combustion
zone. (PHOTO 20)
(c) FIBER BUNDLES. Bundles are broken fiber/matrix
pieces created only by physical damage. When a composite
breaks, the fiber and matrix cracks creating broken
fiber/matrix sections. It could crack within a layer or between
layers. The fibers are held together by the matrix
creating a bundle of fibers. Fiber bundles are found on
1 Composites are a “system” of two or more unlike materials, paragraph
3.7a.
3-22
TO 00-105E-9
3-23
and near the fracture surfaces (on the debris surface and
within the laminate layers). If the fracture damage was
severe, bundles will be dispersed all around the immediate
area. (PHOTOS 21 and 22)
(d) CLUSTER. A cluster contains hundreds or thousands
of long continuous length fibers generated from a
fire exposed unidirectional tape or filament wound layer.
Clusters differ from the strip by the amount of time spent
in the fire. Clusters will have very little resin or resin char
holding the fibers together. The fibers will be free to move.
Carbon fiber clusters are dark wooly-looking mass that
resembles a clump of hair. If produced, clusters can be
found dispersed around the site and outside of the combustion
zone or attached to strips. Clusters will not remain
airborne. (PHOTO 23)
(e) DUST. Damage creates composite dust. The dust
is shattered or crushed resin and fiber fragments. Fire
damage creates resin char, degraded fiber dust and fuel
soot. Microscopic dimensions will vary. Resin char and
soot particles will be spherical in size. The dust generated
from crushed resin or fiber and burnt fibers will have
irregular shapes. The more severe the damage the
greater the dust generation will be. Dust will be found on
and near the damaged surfaces.
(f) SINGLE FIBER. Physical damage causes fiber sections
to pull out of the matrix. Single fibers are fibers that
are small enough to become airborne. Depending on the
size, airborne fibers may not be visible. Single fibers produced
from physical damage are not a source of freefloating
fibers at a mishap site. A carbon fiber unidirectional
tape laminate when engulfed in a JP-8 fire is the
major source for free-floating fibers that can linger right
at the burnt debris for a period of time. (PHOTO 24)
(2) PHYSICAL DAMAGE. Composites do not break like
metal when stressed. At failure the system will develop
matrix and fiber fracture lines. How and where a composite
material will fracture depends on the direction of
the load, fiber orientation in each layer, and the type of
material within the system. Damage can occur either
primarily within the matrix or can affect both the fiber and
matrix. Aircraft mishaps will affect both. The severity of
the damage (the amount and length of fracture or crack
lines) will determine what type of debris and how much is
generated. Severe impact damage crushes and shatters
the material into pieces of varying sizes.
(a) SOLID LAMINATE. When damaged the matrix
forms crack lines between the layers and between the
fibers within a layer (PHOTO 25). A severe matrix fracture
between layers will cause a complete separation of
layers known as delamination. Delamination exposes
individual layers within the laminate stack. Identification
of the comsite system can be attempted because the
type of prepreg tape, fiber direction and core material is
exposed. Under severe load the fiber bends, kinks, buckles,
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