January/February 2023 - 18

A IRFR AMES & COMPOSI TES
Common Composite Damage
Where a composite is located on an aircraft is going
to dictate what kind of damage it's most likely to be
exposed to. But that said, with the overwhelming
majority of composite material being located in the
fuselage and structural components of an aircraft,
it's impact damage that makes up the majority of
damage a technician is likely to see. This is often a
result of a collision.
" I'd say the main one is often impact damage,
which we affectionately call hangar rash, or ground
impact. So, something where either the landing gear
did not come out and you bang the airplane onto the
runway or even impacts with some of the vehicles
that are around the hangar. That happens very, very
commonly, " said Thouin.
Dorworth said that the kind of material damaged
also makes a difference.
" We classify damage, but to what kind of strucAutomated
Scarf Machine - Used to inspect, remove damage, taper scarf, provide
bond prep, send ply cut pattern to cutting table, and inspect final repair.
ture? If the structure is made with a solid laminate
or a monocoque laminate, meaning that it's all one
thick laminate like it is in the 787 fuselage. If it's a big
thick laminate that can carry a lot of load at a much
lighter prospect than say a frame metal equivalent,
the type of damage that we see in those structures
is usually high energy, wide area, impact damage.
" When you have big composites like that, it's
much like the difference between an aluminum
aircraft where... Like if it's an aluminum can, if you
bump into it, you're likely to leave a big dent. With
composites, it's like a big water bottle where you
squeeze it and it bounces back. So the damage might
be done, but the ability to see it may not be (there).
So it would be a barely visible impact damage. It
falls into that category, " Dorworth said.
Barely visible damage is one of the challenging
aspects of identifying a composite in need of repair.
Often, a material can show little to no signs of having
been damaged. For example, Dorworth says
lighting strikes can leave only a small trace behind.
" Lightning strikes can go internal in some of these
Repairs are vacuumbagged
and cured
using a heat blanket
and hot bonder.
" Ceramics can take a lot more heat than plastics,
basically, " added Dorworth. " So when you're
trying to put something into an engine and hold
tight tolerances like you are with fan blades and
such, where you want the most efficient thrust,
the thing that you want to have is a material that
doesn't have a very high thermal expansion coefficient.
It can handle the temperatures and it can
also survive that environment. "
But the guarantee that comes with more composite-based
aircraft is more composite damage.
18 JANUARY/FEBRUARY 2023
panels and all you see is a small little hole on the
outside. So, where the energy comes from one end to
the other, it could be running just inside the panel
along the core line or something like that and come
out somewhere else. And the damage is internal
to that panel rather than external. So, it's hard to
detect, " he said.
Debonding is another risk with composite materials
that Thouin says happens from time to time.
" With a composite to composite structure,
a composite metal structure, a composite to
AIRCRAFT MAINTENANCE TECHNOLOGY
January/February 2023

Table of Contents for the Digital Edition of January/February 2023
