Aircraft composite repair

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A/C Composite Repair

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Composites repair methods differs base on initial

design requirements by specific aircraft

manufacturer.

2 types of repair procedures often done to

composites parts on aircraft are temporary repair and permanent repairs

Temporary repairs are performed for such

requirements as a onetime flight or base on flight hours.

Most repairs are intended to be permanent!!!

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THE PURPOSE OF REPAIR



To

reduce

the number of

unserviceable or damaged parts due to

the mishandling or improper

manufacturing process.



To reduce cost ($), in manufacturing

new parts

or buying new parts.



To

maintain

the parts in good

condition.

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COMPOSITE DAMAGE



Damage must be present first in

order repair can be made to the

structure.



Damage scenario:

- Damage during

manufacturing

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• In manufacturing, damage may be due

to:

- Improper manufacturing process,

- Mishandling of the parts,

- Misassemble of parts &

components of aircraft.

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

In maintenance, a/c composite

structure may experience damage

due to the:

 Operating conditions,

 Environmental conditions

 Mishandling of the parts.

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

Hidden damage is critical and

difficult to be seen.



The extend of the damage must be

evaluated to determine its:-

 Type, depth and location

•

Hidden damage issues are also

including defects during

manufacturing.

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•

For example, a low velocity impact, which

normally

wouldn’t cause much damage

may cause a sandwich structure to

disbond

between the skin and core due

to poor adhesion during manufacture.

•

If this disbond is the only damage, there

may be

no visible trace of it from the

surface.

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Unexpected damage sources;

Example, an aircraft vertical tail part

may be designed to withstand

hailstone impact but not able to resist

damage from being dropped during

shipping or removal for inspection

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COMPOSITE REPAIR - REFERENCE MATERIAL

 Structural Repair Manual (SRM)  Airworthiness Directives

 Service Bulletin  Manufacturers.

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COMPOSITES REPAIR

 The PERFECT repair is to replace the damaged part with a new one.

 If replacement is not possible, then the ideal repair is to match all original design

parameters exactly (e.g. materials, fiber orientation, curing temperature, etc.).

 However, the goal remains to return the

structure, as much as possible, to its original strength, stiffness, shape and surface finish, etc.(e.g atleast 70% of the structural strength remains)

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IDENTIFY DAMAGE AREA CONFIGURATION

DAMAGE STRUCTURE

PREPARE DAMAGE AREA

LAMINATE STRUCTURE SANDWICH STRUCTURE

Repair : Bridging Delaminate Void

Fracture

Remove Tedlar & Sand

Un-damage Core Damage Core

Edge Band

Repair Repair Skin Replace Core Repair Core

Taper Sand Remove Tedlar

And Sand Skin Repair

REPLACE PLIES _{sand smooth with 150 grit or}

finer abrasive

REFINISHING

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PREPREG T O O L

TEMPERING CLEANING CUTTING RELEASE AGENT

LAY-UP

BAGGING

CURING

DEBAGGING

FITTER & FINISH

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RESIN T O O L

TEMPERING CLEANING WEIGHING RELEASE AGENT

LAY-UP

BAGGING

CURING

DEBAGGING

FITTER & FINISH

DRY FIBER

CUTTING

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DAMAGE CLASSIFICATIONS

Maintenance

(1) Negligible (2) Repairable (3) Non-repairable

Manufacturing

(1) Acceptable (2) Correctable (3) Rejectable

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COMMON DAMAGE

CATEGORY OF DAMAGE

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Minor Surface Damage

-The most common types of minor damage to the composites surface are abrasions, scratches, scars, and minor dents.

-These minor surface damages require no repair other than the replacement of the original protective coating to prevent corrosion (water inclusion) provided no fiber breaks, holes, or cracks exist.

- Damage which are characterized by a depth typically less than 1/16" (2 mm), where the damage does not extend into the primary reinforcement. - Minor repair is any repairs that not consider to be a major repair which is

doesn’t need to replace and fit a part and can be done by using a potting compound.

Major Surface Damage

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DAMAGE IN

MAINTENANCE

COSMETIC DEFECTS

Damage that that occurs on the outer skin without interfering the first layer of the reinforcing fibers.

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DAMAGE IN

MAINTENANCE

DELAMINATION

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DAMAGE IN

MAINTENANCE

Separation of layers between laminate and bonded material.

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DAMAGE IN

MAINTENANCE

IMPACT DAMAGE

High Energy Impact Medium Energy Impact Low Energy Impact

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DAMAGE IN

MAINTENANCE

Coating

Reinforcing Fiber

Matrix Layer

Dent _Crack _Fracture

• Dent: Dislocation or Indentation that does

not penetrate the reinforcing fiber

• Crack: Partial protrusion through the

layers

• Fracture: Thorough protrusion until the

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DAMAGE IN

MAINTENANCE

LIGHTNING STRIKE DAMAGE

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CRACKS

A partial separation on the composite structure that going through one or more layers of the reinforcing fibers.

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HOLE DAMAGE

Damage to a hole that is caused by overtightening, impact damage mislocation of the drilled hole etc.

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WATER INGRESSION DAMAGE

Moisture absorption or trapped water in the composite structure especially to sandwich structure.

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REPAIR PROCESS DIAGRAM: DEFINITION



Interim Repair (existed for short time)

 Repair that stay on the aircraft if do not find

any sign of deterioration during regular supplemental inspection.

 Must be inspected at specific interval and

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REPAIR PROCESS DIAGRAM: DEFINITION

 ‘Time limited’

 Temporary repair.

 Must be replaced with proper permanent repair

after a specified time

 Defined by flight hours, flight cycle or inspection.

 Usually in the form of a patch.

 Use when a composite repair is urgently needed for

components in use for A/C.

 If damage left unrepaired they may lead to further

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BASIC REPAIR PROCESS

The very basic fundamentals of composite repair include the following steps:

• Inspect to assess damage (extent and degree)

• Remove damaged material

• Treat contaminated material

• Prepare repair area (Material & surface for repair)

 _{Complete composite repair}

• Inspect repair for quality assurance (e.g.

delaminations, inclusions, proper cure, etc.)

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ASSESSMENT OF DAMAGE



Manual Ref.

 Structural Repair Manual (SRM)

 Jet liner @ Commercial

 Service Manual

 Small aircraft

 Using NDT available procedure e.g. u/sonic

 Refer to data on

 Specific allowable damage  Repair limitation

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REPAIR PROCEDURE: REMOVAL OF COMPOSITES DAMAGES



Masked off the damage part with an

adequate area.(masking tape)



Trim out / Sand the damaged part to a

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Remove damage in circular or oval shapes, and do not use sharp corners. If an irregular shape must be used, then round off each corner to as large radius as practical.

DIAMETER TO BE INCIRCLED

= D + 30N (mm) = D + 1N (inches)

Where:

D – Approximate diameter of the damaged area. N – Number of involved layer

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REMOVEL OF DAMAGE:ROUTING @ GRINDING

• Routing or Grinding method

Routing Tool Grinding Tool

– Is use to remove damages for solid laminate and thick solid laminate with damage only on surface plies

- Also use for removel of damage for sandwich structure with damage into the core.

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GENERAL: REMOVAL OF DAMAGED

AREA



Laminate Structure

 Taper (scarf) sand OR

Step sanding

 Using disc @ manual

sander.

 Must sand 0.5 inch or

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

Sandwich Structure

 Taper (scarf) sand

OR Step sanding on the laminate

 Full core removal if

thickness less than 1 inch. Partial remove is allowed if the

thickness is more than 1 inch

GENERAL: REMOVAL OF DAMAGED

AREA

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INITIAL TREATMENT

PRIOR TO REPAIR

 Dust must be removed by using vacuum.

 _{Wipe out the removed area with solvent}

 Composite materials must be dried before an effective repair can be achieved.

 _{Cured resin as well as fibers will}_absorb

moisture from the environment, and

honeycomb cores can hold large quantities of fluid.

 If performing a repair using high-temperature curing resin or prepreg, all moisture must be removed to prevent steam from forming and disbonding the repair.

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E nvironment is one of the major factors that influence the repair quality.

The presence of moisture is critical to bonded repairs.

Epoxy resins can absorb 1.5 to 2 times their weight in moisture, thereby reducing the

ability of the resins to support the fibers.

Environment dirt and dust can seriously affect bonded repairs.

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The repair facilities should be controlled

environment

The relative humidity should be

25 percent

to 60 percent

and temperatures fixed at

65°

to 75°F.

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MATERIAL PREPARATION

 Material preparation:

 Determine correct material by SRM

 Check shelf life limits

 Identify proper curing system

 Determine correct mixing resin and weight

 Fabricate core plug

 _{Ensure correct}_{ribbon direction}_{before installation}  _{Trim the core 0.5” smaller from the cut out.}

 Preparation for fabric Kitting

 _{Ensure the plies are proportionate with the removed fabric}

with extra plies

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TYPES OF REPAIR

Basic types of composite repair include

the following:

 Cosmetic

 Resin Injection

 Semi-structural Plug / Patch

 Structural Mechanically–fastened Doubler

 Structural Bonded External Doubler

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‘Bolted’ vs. ‘Bonded’ Repairs

Advantages of bolting:

•

Doesn’t require

meticulous material &

surface preparation

•

Easy to inspect for

quality

•

Easily disassembled

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TYPICAL METHOD:

SCARF REPAIR

• Typical scarf distances are from 20 to 120 times the thickness of the laminate being scarfed.

• Ratio of scarfing as per example 15:1. (length: thickness)

Whereby ‘1’ indicates thickness and ’15’ is the length of damaged area to be scarfed

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Plies Replacement on scarf or tapered cut on sandwich structure.

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Core plug - Ensure minimum of 1/16 inch in excess of the part’s thickness – due to core sinking after adhesive melt

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TYPICAL REPAIR:

STEP REPAIR

• The laminate is sanded down so that a flat band of each layer is exposed producing a stepped finish.

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Plies Replacement on step cut on sandwich structure.

Core depression

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EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD

Defect type: Blisters or Air-Pockets

Limitation:

a) maximum dimension does not exceed 6 mm, b) the number of blisters shall not be more than

4 blisters per square meter

Proper repair method: a) Resin injection

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Defect type: Resin Rich Area

Proper repair method:

a) Manual sanding with grit 240 or finer Scothbrite or grit 150 or finer sandpaper until totally removed without damaging the surface fibers. b) Clean with filtered low pressure air and vacuum the area.

c) Cover sanded area with water-proofing resin and cure.

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Defect type: Resin Starved Areas

a) Step OR Scarf cut method

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Defect type: Tacky Areas

Proper repair method: a) None (Reject part)

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Defect type: Fabric Wrinkles

Limitation:

a) Not caused by fabric overlapping and maximum height and depth do not exceed certain dimension

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Defect type: Scratches

a) Manual sanding the area with abrasive paper 180 grit or grit 240 Scotchbrite, or finer.

b) Clean with filtered low pressure air and vacuum the area

c) Plaster with resin / potting compound. d) Cure according type of resin used. e) Remove excess resin with wet sand paper, grit 220 or finer.

f) Apply finishing

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Defect type: Cracks

a) Minor crack – Resin injection

b) Major crack – Step OR Scarf cut method

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Defect type: Fracture

a) Step OR Scarf cut method

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Defect type: Delamination

a) Minor delamination – Resin injection

b) Major delamination – Step OR Scarf cut method

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Defect type: Delamination Fabric - Core

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Defect type: Core Depression

Proper repair method: a) Manual sanding

b) Plaster cavity using potting compound

c) Sand excess resin d) Apply finishing

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Defects type: Crushed Core

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Defect type: Core Displacement (edge band repair)

a) core removal and additional microballons filler

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Defect type: Nodal Delamination

Proper repair method: a) Remove core material b) Step OR scarf cut method

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Defect type: Bridging

Proper repair method: a) Resin injection

– not severe and not causing any inteference b) Step OR Scarf cut method

– too severe damage

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Defect type: Pitting on Center of Cells

b) Potting compound

EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD

Cavities located in the center of cells appearing on the sandwich panels

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Defect type: Thread Telegraphing

b) Plaster with resin / potting

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Defect type: Porosity

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Defect type: Foreign Object Inclusion

a) Removal of fabric layers & inclusion (during lay up)

b) Step OR Scarf cut method

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Defect type: Geometric Deviation

(exceed/non exceed drawing)

a) Manual sanding & Lay-up to obtained the proper thickness & dimensions.

b) Apply finishing

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DEFECTS DUE TO MOISTURE ENTRAPPED

 Polymer matrix composites absorb moisture when exposed to humid air.

 Moisture concentration increases initially with time and finally approaches the saturation

point (equilibrium).

 The exact rate of moisture depends on void content, fiber and resin type, fiber orientation, temperature, applied stress level, presence of micro cracks, and etc.

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INFLUENCE OF MOISTURE ON POLYMER MATRIX.

 Absorption of water by resin in some

instances may change the resin properties & causes swelling of the resin.

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INFLUENCE OF MOISTURE ON FIBERS.

 Water at the glass fibre interface lowers its surface energy which in turn can promote crack growth.

 Aramid fibers can absorb considerable quantities of water resulting in swelling.

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LOW TEMPERATURE EFFECTS



Temperature

below zero celcius

can

cause the

water to freeze

. Due to this

microcracks

can occur.



Low temperature can also

effect the

stiffness and strength

of the

matrix

system.

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EFFECT OF HIGH TEMPERATURE ON COMPOSITES

 It is found that bond strength of composites get reduced by about 80-90%, and tensile strength by 20% at 200°C.

 At 100°C to 150°C compressive strength is reduced by 30% of dry CFRP (compressive

fibre reinforce plastic) and by 54% of wet CFRP.

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EFFECTS OF WATER INGRESSION TO THE MECHANICAL PROPERTIES



In Aramid cores of composite sandwich

parts, the moisture or fluid can

seriously

degrade

mechanical properties such as

stiffness and shear strength.

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EFFECTS OF WATER INGRESSION TO THE HONEYCOMB CORES

 The decrease in these properties is attributed to water absorption and swelling of the core materials.

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Autoclave / Manufacturing

The Autoclave Process

The heat treatment process is similar in vertical and horizontal autoclaves:

• Load autoclave with product • Close and lock door

• Raise to soak temperature (ramp) • Treat product for a set time (soak) • Relieve pressure

• Unload autoclave

Raise to Soak Temperature

This period occurs after all interlocks are in place (doors closed and locked etc.). Digital inputs, used in conjunction with logic equations can be used to verify that interlocks are in place before the profile is started. The user-programmed set point profile takes the temperature from ambient to the desired soak value.

The Soak Period

This period is essential in order to sterilize and treat the product correctly. Time schedules are determined by the product being treated and the end properties required.

Pressure Relief Period

The autoclave pressure, attained as a result of heat treatment at high temperature in a closed vessel, must be relieved before the product can be removed safely.

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Using Autoclave for Repairs

Autoclave processing is used for the repair of high quality structural components.

The autoclave technique requires a similar vacuum bag 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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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.

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