Technical data

13773-001
51-10
Page 1
ASSESSING COMPOSITE DAMAGE
1. DESCRIPTION
Most damage to a composite structure will be visually detectable. An impact is the most common cause of
damage and will usually leave visual evidence. Other types of damage are more difficult to detect but can
be detected by simple methods. If the exterior surface is damaged, always assume that the underlying
structure may also be damaged.
2. MAINTENANCE PRACTICES
A. Determining Extent of Damage
There are three basic methods to determine the extent of composite damage: visual, coin tap, and
exploration.
(1) Visual
The visual method can be used when the suspect area is clearly visible. Damage to the outer
surface of the aircraft will usually crack the paint. Paint is generally more brittle than the compos-
ite and will crack before the laminate is damaged. However, this does not help the technician to
determine the extent of the damage, only that damage has occurred. When a crack in the paint
is found, further investigation is required. Paint cracks on fairings can often occur due to the flex-
ing at these intersections.
Dimples, dents or creases are also a sign of damage. Dimple and dent damage is similar in
appearance to hail damage on a metal surface. Again, this does not reveal how extensive the
damage is, only that it occurred. If tears or broken fibers are visible, there is no question the part
is damaged and must be repaired.
Separation between plies of a laminate (delamination) or between the laminate and the core or
between two bonded laminates (disbond), is more difficult to detect. This type of damage may
evidence itself in the form of a raised area or puckering outward of the skin. It is sometimes pos-
sible to feel this type of damage by pressing on the area. A disbond or delamination may feel
soft and movement between the separated layers may be detected.
If possible, the backside of the suspected area should be examined. Use of a borescope, if avail-
able, is highly recommended to assess internal damage. The interior surfaces are usually not
painted and damage to glass-fabric structures will show up as a white area. The white color indi-
cates separation of fabric from resin, which changes the way light refracts in the laminate.
(2) Coin Tap
Coin tap is just what the name implies, tapping with a coin, or similar object. By tapping at a con-
sistent rate and energy, it is possible to audibly detect discontinuities in the underlying surface.
The coin tap method is useful for detecting delamination and disbonds. Areas of disbond or
delamination will sound flat or hollow, undamaged areas should sound sharp and clear. The coin
tap method helps to assess damage in hard to see areas and when disbond or delamination is
suspected; the coin tap method should be used in conjunction with the exploration method. All
suspect areas, including obvious damage, should be checked with the Coin Tap method. This
method will help determine the extent of damage and whether or not the aircraft can be field
repaired.
The coin tap method is effective if used properly. It is important that the area being investigated
is similar to a standard, or undamaged area, to which it is compared. For instance, if the sus-
pected area lies directly over a rib, or is sandwich construction, the reference area should also
be this type of construction.
(3) Exploration
30 Nov 2000