L13 Blanik Mandatory Bulletin

And just in case pilots of composite aircraft feel good due to the
high fatigue life of their planes and general lack of corrosion
sensitivity, be aware that composites are very susceptible to out of
plane impact damage. A stone hitting a composite can cause (almost)
invisible delamination between layers inside the composite. The effect
is that one moment you're flying along without a care and the next
moment you have a huge hole in your wing or control surface (for
example).

German manuals tell you to look for "pressure damage"; what they mean
is to look for a small nick made by a stone hitting the composite. If
you find such a nick, a simple way to determine if there is underlying
delamination is to use a 1/8" diameter drill rod and drop its blunt
end onto the composite's surface from about an inch away. If you hear
a high pitch ring, it's OK; if you hear a dull thunk sound you've got
a delamination. You can get an idea how big it is by marking out the
extent of the area where the dull thunk sound is heard. Ultrasonic
inspection is used to search for delamination damage in military and
commerical aircraft.

-John