This document discusses repair procedures for composite materials. It emphasizes that identifying the material specifications from the original part is essential before beginning a repair. The same materials and fabrication process as the original must be used for the repair. Various repair materials are discussed, including resin systems, fiber reinforcements, and core materials. Basic repair steps include inspecting the damage, removing damaged material, preparing the repair area, completing the composite repair, and inspecting the final repair. Paint removal methods like hand sanding and various blasting techniques are also outlined.

1. Composite Repair
SIVASANKAR G A
Assistant Professor
Department of Aeronautical
KIT Kalaignar Karunanidhi Institute of Technology

2. Component Identification
• Composite components should be fully identified before a repair is performed. Such
details as material specifications, number of plies and ply orientations, core ribbon
direction, ply buildups and drop-offs, and numerous other items need to be clarified
and understood before beginning a repair.
• For aircraft, this type of information is often available in the structural repair
manual (SRM) or equivalent documents. Even if an SRM or equivalent is not
available, this type of information is still needed for proper repairs.
• Determination can often be done by careful taper sanding through a small sample of
the damaged part, and reading the information directly from the composite itself.
However, a thorough understanding of composite material is mandatory for this
type of analysis.

3. Repair Materials
• A repair should use the same materials, fibre orientation,
core orientation, stacking sequence, nesting, curing
temperature and sealing as was used in the original
fabrication of the part.

4. Repair Materials
In any composite repair, the following types of repair materials need to be considered and
evaluated before the repair is begun, to ensure that the repair is structurally sound:
Matrix Resins:
• Resin systems—wet layup vs. prepreg, low temperature vs. high temperature.
• Cure cycle requirements and available equipment.
Fibres/Fabrics:
• Fibre reinforcement-type of fibre (fibreglass, aramid, carbon, etc.).
• Fibre reinforcement form (unidirectional tape, woven cloth, weave style, etc.).
Core Materials:
• Core type-Nomex R honeycomb, aluminium honeycomb, foam, balsa, etc.
• Core orientation-honeycombs have different properties in their ribbon and transverse
directions.
• Core adhesive or potting compound.

5. REPAIR OF COMPOSITE
STRUCTURES
• Repair of composite material is fairly complex. Both small
and large repairs are possible. For large repairs you will
require repair molds.
• Also use of wet lay-up or prepreg (pre-impregnated) repair
material is possible.

6. REPAIR OF COMPOSITE STRUCTURES
Cold and Hot Cure
• Cold Cure or Room Temperature (RT) cure is done using a temperature of
minimal 20 ºC. In this way curing can take up to 7 days.
• Hot Curing is done to shorten curing time. Also materials cured at higher
temperatures are stronger than room temperature cured materials. Hot curing
can be done by means of heat blankets, autoclave and ‘hot bonders’.
• Typically pressure is required during curing to strengthen the repair. Pressure
can be applied by weight, clamps, vacuum bagging or by autoclave.

7. REPAIR OF COMPOSITE STRUCTURES
The very basic fundamentals of composite repair include the following steps:
• Inspect to asses damage.
• Remove damaged material.
• Treat contaminated material.
• Prepare repair area.
• Complete composite repair.
• Inspect repair for delaminations, inclusions, proper cure, etc.
• Restore surface finish.

8. REPAIR OF COMPOSITE STRUCTURES
Paint Removal
The first step of a repair is to remove paint and/ or outer coatings.
Chemical paint strippers must not be used, unless you are certain
they are specifically designed for composite structures. Most paint
strippers are based on methylene chloride, and will attack cured
epoxy resin (which is a common matrix resin for composites).
Paint and coatings may be removed by:
• Hand sanding.
• - Various blasting methods

9. REPAIR OF COMPOSITE STRUCTURES
Hand Sanding
Hand sanding is widely used as a paint removal method for
composites. No expensive equipment is needed, and the
sanding pressure can easily be controlled to avoid laminate
or fibre damage. Hand sanding is recommended for paint
removal in a small area and is the most common paint
removal method.

10. REPAIR OF COMPOSITE STRUCTURES
Blasting
There are many types of blasting in use today:
• Sandblasting: This type of blasting is possible with composites but must be done very
carefully, as it is highly likely that fibres on the top ply will be damaged.
• Aluminium Oxide Blasting: Similar to sandblasting, and the same precautions apply.
• Plastic Media Blasting: This method is less aggressive than sand or aluminium oxide
blasting, and is used often with composites. While effective, this technique has shown
some difficulties in the field.
• Starch Media Blasting: This method uses starch media to blast paint from parts.
Boeing, the U.S.Air Force, U.S. Army, Cessna, Raytheon, Northrop Grumman,
Lockheed Martin have approved this method for paint removal from composites. As
with plastic media, there can be a contamination problem as the starch is reused.