COMPOSITE REPAIR

INTRODUCTION
Composites are used in a wide range of applications in
aerospace, marine, automotive, surface transport and sports
equipment markets. Damage to composite components is
not always visible to the naked eye and the extent of
damage is best determined for structural components by
suitable Non Destructive Test (NDT) methods.

Sandwich Structures
Thin, high strength skins are separated by, and bonded to,
lightweight honeycomb cores; the thicker the core the stiffer
the panel with minimal weight increase.

Alternatively the damaged areas can be located by simply

tapping the composite surface and listening to the sound.
The damaged areas give a dull response to the tapping, and
the boundary between the good and damaged composite
can easily be mapped to identify the area for repair.
Awareness of and inspection for composite damage should
be included in the regular maintenance schedules for
composite structures. Particular attention would be made to
areas which are more prone to damage.

Laminate Structures
Laminate structures are assembled so that the fibre orientation
provides most of the desired mechanical properties and the
matrix largely determines the environmental performance.

Repairs to aircraft structures are controlled and should be

carried out according to the Aircraft Structural Repair
Manual (SRM). For other applications the repaired
components would normally be expected to meet the
original specification and mechanical performance
requirements.
This guide aims to give a general approach to composite
repair in all applications and will examine both sandwich and
laminate structures.

COMPOSITE REPAIR

REPAIR FLOW CHART

The flow chart gives the key stages for composite repair.

Excessive

Damage
assessment

Scrap

Complex repair

Repair
type

Easy repair

Submit / Check scheme with manufacturer

/ OEM

Temporary
repair

Improvise and return to repair workshop

Approved temporary repair

Permanent composite repair according to approved guidelines

Quality Check / NDT

Return to service

KEY STAGES OF COMPOSITE REPAIR

Damage assessment

Repair type

Temporary repair

Permanen
t repair

Quality
Check

Some damage to composites is obvious and easily assessed but in some cases the damage
may first appear quite small, although the real damage is very much greater. Impact damage
to a fibre can appear as a small dent on the reinforced composite surface but the underlying
damage can be much more extensive.
The decision to repair or scrap is determined by considering the extent of repair needed to
replace the original structural performance of the composite. Other considerations are the
repair costs, the position and accessibility of the damage and the availability of suitable repair
materials.

Easy repairs are usually small or do not effect the structural integrity of the component.
These repairs are made by following the simple guidelines indicated for laminate or
sandwich panels (see repair sections).
Complex repairs are needed when the damage is extensive and needs to replace the
structural performance of the component. The best choice of materials would be to use the
original fibres, fabrics and matrix resin. Any alternative would need careful consideration of the
service environment of the repaired composite, i.e. hot, wet and mechanical performance.
The proposed repair scheme should meet all the original design requirements for the structure.

When a composite repair is needed for components in use. Some repairs need the specialist
equipment of the workshop and some form of improvised repair is needed to return the
component to a suitable repair workshop. A temporary repair, usually in the form of a patch,
can be fixed to the component. Usually a 'belt and braces' approach is taken to ensure safety
until the component can be repaired at a later date.

The approved general guidelines for laminate and sandwich repairs should be followed.
These repair operations should be carried out in controlled workshop areas to ensure high
quality repairs. Good housekeeping and attention to repair detail will ensure success.

For comprehensive inspection of repaired parts a number of Non Destructive Tests (NDT)
can be used. The inspector should examine the quality of the repaired area and particular
attention should be given to the interface between the original part and the repaired area.
Usual inspection methods use some form of ultrasonic test equipment where the reflection of
the ultrasonic sound waves detect and identify any damaged areas or faults.

COMPOSITE REPAIR

Typical Damage
Most damage to fibre reinforced composites is a result of low velocity and sometimes high velocity impact. In metals the energy
is dissipated through elastic and plastic deformations and still retains a good deal of structural integrity. Whilst in fibre reinforced
material the damage is usually more extensive than that seen on the surface.

Delamination following impact on a monolithic

laminate

Dents in Sandwich Structure

Point of impact

Underlying damage can extend to a much greater extent in

laminate structures.

Puncture Damage in a Sandwich Structure

Laminate Splitting
The damage does not extend through the full length of the
part. The effects on the mechanical performance depend on
the length of split relative to the component thickness.

Both skins may be damaged.

Bolt Hole Damage

The damage could be elongation of the hole causing laminate
splitting, or damage to the upper plies.

Heat Damage
A local fracture with separation of surface plies. Its effect on
the mechanical performance depends on the thickness of the
part.

REPAIR OPTIONS
When a composite structure sustains damage in service one of
three levels of repair must be employed.

Cosmetic repair
In this case inspection has determined that the damage has
not affected the structural integrity of the component. A
cosmetic repair is carried out to protect and decorate the
surface. This will not involve the use of reinforcing materials.

Temporary or interim repairs

It is often the case in service, that small areas of damage
are detected which in themselves do not threaten the
integrity or mechanical properties of the component as a
whole. However if left unrepaired they may lead to further
rapid propagation of the damage through moisture ingress
and fatigue.
Simple patch type repairs can be carried out, with the
minimum of preparation, to protect the component until it can
be taken out of service for a proper structural repair.
Temporary repairs should be subject to regular inspection.

Structural repair
If the damage has weakened the structure through fibre
fracture, delamination or disbonding the repair will involve
replacement of the damaged fibre reinforcement, and core in
sandwich structures, to restore the original mechanical
properties. Since a bonded-on repair constitutes a discontinuity
of the original plies, and therefore a stress raiser, structural
repair schemes normally require extra plies to be provided in
the repair area.
If the damaged area is very small it can be questionable
whether a structural repair, requiring removal of a substantial
amount of the structure in damage removal and preparation, is
preferable to a cosmetic repair.

COMPOSITE REPAIR

LAMINATES AHD SANDWICH PANELS

The main purpose of a structural repair is to fully support
applied loads and transmit applied stresses across the
repaired area. To do this the repair materials must overlap,
and be adequately bonded to the plies of the original
laminate.
There are three basic approaches to this.

1. Patch repair
In this case the thickness of the original laminate is made up
with filler plies and the repair materials are bonded to the
surface of the laminate.
Advantages
Quick and simple to do
Requires minimum preparation

Disadvantages
A repaired laminate is thicker and heavier than the
original
Very careful surface preparation is needed for
good adhesion

2. Taper sanded or scarf repair

In this case an area around the hole is sanded to expose a
section of each ply in the laminate. Sometimes one filler ply is
added to produce a flatter surface. Taper is usually in the
region of 30-60:1
Advantages
Repair is only marginally thicker than the original
Each repair ply overlaps the ply that it is repairing giving
a straighter, stronger load path
Good bonds can be achieved on the freshly
exposed surfaces
Disadvantages
Time consuming
High skill needed and difficult to achieve

3. Step sanded repair

The laminate is sanded down so that a flat band of each
layer is exposed, producing a stepped finish. Typical steps
are 25- 50mm per layer.
Advantages
Same as taper sanded repair
Disadvantages
Extremely difficult to do

TYPICAL LAMINATE REPAIRS

Note: If the component has been in service it must be dried to
remove any moisture to obtain the best repair.

1. Patch
repair

Extra ply

Repair plies
Film adhesive

Filler plies

2. Taper sanded repair

(Scarf repair)

Backup ply

Extra ply

Repair plies Film adhesive

Filler ply

3. Step sanded
repair

Backup ply

Extra ply

Repair plies
Film adhesive

Filler ply

Backup ply

COMPOSITE REPAIR

TYPICAL SAHDWICH PANEL REPAIRS

Note: If the component has been in service it must be dried to
remove any moisture to obtain the best repair.
Extra ply
Repair plies
Film adhesive

Filler plies

1(a) Patch repair

Replacement core

Core splice adhesive

Extra ply
Repair plies
Film adhesive

1(b) Alternative
patch repair

Replacement core

Core splice adhesive

Extra ply

Filler ply
(optional)

Repair plies
Film adhesive

2 Taper sanded repair

(scarf repair)

Replacement core

Core splice adhesive

Extra ply

Filler ply
(optional)

3. Step sanded repair

Repair plies
Film adhesive

Replacement core

Core splice adhesive

REPAIR SEQUENCE FOR DOUBLE SIDED REPAIR

1. Puncture damage

4. Bond new honeycomb

Release film
(peel ply)

Packing piece

Temporary
mould

Core splice adhesive

2. Remove damage
5. Repair first side
typically 12.5mm

Repair plies
Extra ply

3. Taper sand

Temporary
mould

6. Repair second side

Filler ply
(opt)

Packing piece

COMPOSITE REPAIR

REPAIR USING PRE-CURED DOUBLER

Some composite repairs can be achieved by the use of precured doublers.
A pre-cured doubler is a sheet of composite material made
from layers of fibre reinforced epoxy which has been cured
in using heat and pressure.

Pre-cured doublers can be bonded on to the component

using wet lay-up resins or adhesive films. However one
major drawback of this method has been entrapment of air
under the doubler which leads to a weak repair.
The repair can be improved by using a layer of scrim cloth
under the adhesive film to allow the air to escape.

A patch is cut from the pre-cured material and bonded on to

the surface of the component.
The edges of the doubler are chamfered to increase the
peel off strength.

EQUIPMENT AHD AHCILLARIES FOR REPAIRS

Typical Layup and Equipment for one side access repair
When access is possible from both sides then a complete envelope bag is recommended.

Compressed air

Vacuum line to repair patch

Power supply
Thermocouples
Repair patch

Hot bond unit

Output to heater mat

Vacuum bag
Heater mat

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EQUIPMENT AHD AHCILLARIES FOR REPAIRS

Hot Bonders
Heater Blankets
Bagging Materials
- vacuum bags
- sealant tape
- release film
- breather
- vacuum take off connector

To vacuum pump
Vacuum bag
Breather fabric Heater blanket Release film Peel ply
Repair patch
Prepreg
Film adhesive

Seal

Damaged composite

Fig. 1 Detail of vacuum bag lay-up

All the components of a vacuum bag lay-up are shown in

the diagram above. This lay-up is ideal for high quality
components, however alternative lay-ups are possible to
suit the materials and application.

Consumables for Vacuum Bag Processing

Peel ply (optional)
Allows free passage of volatiles and excess matrix during
the cure. Can be removed easily after cure to provide a
bondable or paintable surface.
Release film
This prevents further flow of matrix and can be slightly
porous (with pin pricks) to allow the passage of only air and
volatiles into the breather layer above.

Breather fabric
Provides the means to apply the vacuum and assists
removal of air and volatiles from the whole assembly.
Thicker breathers are needed when high autoclave
pressures are used.
Vacuum bag/sealant tape
Provides a sealed bag to allow removal of air to form the
vacuum bag.
Note: It is recommended that new consumables are
used each time to ensure the manutacture ot good
quality composites.

COMPOSITE REPAIR

REPAIR PROCESS
Vacuum bag or autoclave - which process?
Vacuum bag and autoclave processing are the two main methods for the repair of components from prepreg. The processing
method is determined by the quality, cost and type of component being manufactured.

Component

Processing costs

Processing method

Quality

ection thickness

Equipment cost

Cure cycle time

Vacuum bag

Good

Thin

Moderate

Short

Autoclave

Excellent

Thick

High

Long

Vacuum bag processing is suited to components with thin sections and large sandwich structures. The vacuum bag technique
involves the placing and sealing of a flexible bag over a composite lay-up (fig. 1) and evacuating all the air from under the bag
(fig. 2).

The removal of air forces the bag down onto the lay-up with consolidation pressure of 1 atmosphere (1 bar). The completed
assembly, with vacuum still applied, is placed inside an oven with good air circulation, and the composite is produced after a
relatively short cycle cure.

Autoclave processing is used for the repair of high quality structural components. The autoclave technique requires a similar
vacuum bag (fig.1) but the oven is replaced by an autoclave. The autoclave is a pressure vessel which provides the curing
conditions for the composite where the application of vacuum, pressure, heat up rate and cure temperature are controlled. High
processing pressures allow the moulding of thicker sections of complex shapes. Honeycomb sandwich structures can also be
made to a high standard. Long cure cycles are required because the large autoclave mass takes a long time to heat up and cool
down.
Sometimes slow heat up rates are required to guarantee even temperature distribution on the tooling and composite components.

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