Aircraft composite repair

8/10/2019 Aircraft composite repair

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A/C Composite Repair

Advanced Comp Repair


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Composites repair methods differs base on initialdesign requirementsby specific aircraftmanufacturer.

2 types of repair procedures often done tocomposites parts on aircraft are temporaryrepairand permanentrepairs

Temporary repairs are performed for suchrequirements as a onetime flight or base on flighthours.

Most repairs are intended to be permanent!!!

INTRO


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THE PURPOSE OF REPAIR

To reduce the number ofunserviceable or damaged parts due tothe mishandling or improper

manufacturing process.

To reduce cost ($), in manufacturingnew partsor buying new parts.

To maintainthe parts in goodcondition.


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COMPOSITE DAMAGE

Damage must be present first inorder repair can be made to thestructure.

Damage scenario:

- Damage during manufacturing

- Damage during maintenance


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In manufacturing, damage may be dueto:

- Improper manufacturing process,- Mishandlingof the parts,- Misassembleof parts &

components of aircraft.

COMPOSITE DAMAGE (cont.)


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In maintenance, a/c compositestructure may experience damage

due to the: Operatingconditions,

Environmentalconditions

Mishandlingof the parts.



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Hidden damage is critical anddifficult to be seen.

The extend of the damage must beevaluated to determine its:-

Type, depth and location

Hidden damage issues are alsoincluding defects duringmanufacturing.



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For example, a low velocity impact, whichnormally wouldnt cause much damagemay cause a sandwich structure to

disbondbetween the skin and core dueto poor adhesion during manufacture.

If this disbond is the only damage, there

may be no visible trace of it from thesurface.



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Unexpected damage sources;

Example, an aircraft vertical tail part

may be designed to withstandhailstone impact but not able to resistdamage from being dropped during

shipping or removal for inspection



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Example of Aircraft Damage


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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 thedamaged part with a new one.

If replacement is not possible, then the ideal

repairis to match all original designparameters exactly (e.g. materials, fiberorientation, curing temperature, etc.).

However, the goal remains to return the

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


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IDENTIFY DAMAGE AREA CONFIGURATION

DAMAGE STRUCTURE

PREPARE DAMAGE AREA

LAMINATE STRUCTURE SANDWICH STRUCTURE

Repair :B r i d g i n g

D e l am i n a t e

V o i d

F r a c t u r e

Remove Tedlar & Sand

Un-damage Core Damage Core

Edge Band

Repair

SkinRepair

ReplaceCore

RepairCore

Taper Sand

Remove TedlarAnd Sand

Skin Repair

REPLACE PLIESsand smooth with 150 grit or

finer abrasive

REFINISHING

TYPICAL REPAIR FLOW SEQUENCE


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PREPREG T O O L

TEMPERING CLEANING

CUTTING RELEASE AGENT

LAY-UP

BAGGING

CURING

DEBAGGING

FITTER & FINISH

TYPICAL HOT BOND REPAIRPROCESS FLOW


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EXAMPLE OF HOT BOND REPAIR


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RESIN T O O L

TEMPERING CLEANING

WEIGHING RELEASE AGENT

LAY-UP

BAGGING

CURING

DEBAGGING

FITTER & FINISH

DRY FIBER

CUTTING

CUTTING

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TYPICAL COLD BOND REPAIRPROCESS FLOW


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EXAMPLE OF COLD BOND PROCESS


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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

CATEGORYOF DAMAGE

LAMINATE CORE INTERFACE


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Minor Surface Damage-The most common types of minor damage to the composites surfaceare abrasions, scratches, scars, and minor dents.

-These minor surface damages require no repair other than thereplacement of the original protective coating to prevent corrosion (water

inclusion) provided no fiber breaks, holes, or cracks exist.

- Damage which are characterized by a depthtypically less than 1/16" (2mm), where the damage does not extend into the primary reinforcement.

- Minor repair is any repairs that not consider to be a major repair which isdoesnt need to replace and fit a partand can be done by using a pottingcompound.

Major Surface Damage

- All major damage should be remove and repairimmedietly


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DAMAGE IN MAINTENANCE

COSMETIC DEFECTS

Damage that that occurs on the outer skinwithout interfering the firstlayer of the reinforcing fibers.


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DELAMINATION

Separation of layersof material in a laminate


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Separation of layers between laminate and bondedmaterial.

DISBOND


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IMPACT DAMAGE

HighEnergy Impact MediumEnergy Impact LowEnergy Impact

Impact damage occurs usually due to struck by foreign object.


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Coating

ReinforcingFiber

Matrix LayerDent Crack Fracture

Dent:Dislocation or Indentation that doesnot penetrate the reinforcing fiber

Crack:Partial protrusion through thelayers Fracture:Thorough protrusion until the

next surface.


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LIGHTNING STRIKE DAMAGE

Damage that due lightning.


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CRACKS

A partial separationon the composite structure that going through one ormore layersof the reinforcing fibers.



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HOLE DAMAGE

Damage to a holethat is caused by overtightening, impactdamage mislocation of the drilled hole etc.



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WATER INGRESSION DAMAGE

Moisture absorption or trapped waterin the composite structure especially tosandwich structure.



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REPAIR PROCESS DIAGRAM: DEFINITION

Interim Repair (existed for short time)

Repair that stay on the aircraft if do not findany sign of deterioration during regular

supplemental inspection. Must be inspected at specific interval and

replace if deterioration occured


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REPAIR PROCESS DIAGRAM: DEFINITION

Time limited Temporary repair.

Must be replaced with proper permanent repairafter 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 forcomponents in use for A/C.

If damage left unrepaired they may lead to further

rapid propagation of the damage.


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BASIC REPAIR PROCESS

The very basic fundamentals of compositerepair include the following steps:

Inspectto assess damage (extent and degree) Removedamaged material Treat contaminated material Preparerepair area (Material & surface for

repair) Complete composite repair Inspectrepair for quality assurance (e.g.

delaminations, inclusions, proper cure, etc.) Restore surface finish


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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

Repair data


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SEQUENCE OF VACCUM BAGGING MATERIAL USED FOR REPAIR


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REPAIR PROCEDURE


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REPAIR PROCEDURE: REMOVAL OF COMPOSITES DAMAGES

Masked off the damage part with anadequate area.(masking tape)

Trim out / Sand the damaged part to asmooth shape with round corners.


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Remove damage in circular or oval shapes, and do not use sharpcorners. If an irregular shape must be used, then round off eachcorner 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

REPAIR PROCEDURE: REMOVAL OF COMPOSITES DAMAGES


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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 solidlaminatewith damage only on surface plies- Also use for removel of damage for sandwich structure with damageinto the core.

GENERAL REMOVAL OF DAMAGED


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GENERAL: REMOVAL OF DAMAGEDAREA

Laminate Structure

Taper (scarf) sand ORStepsanding

Using disc @ manualsander.

Must sand 0.5 inch or

tapered by 1:30 ratio

GENERAL REMOVAL OF DAMAGED


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Sandwich Structure Taper(scarf) sand

OR Step sanding on

the laminate Full core removal if

thickness less than 1inch. Partial remove

is allowed if thethickness is morethan 1 inch

GENERAL: REMOVAL OF DAMAGEDAREA


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INITIAL TREATMENTPRIOR TO REPAIR

Dust must be removed by using vacuum. Wipe out the removed area with solvent Composite materials must be driedbefore an

effective repair can be achieved. Cured resin as well as fibers will absorb

moisturefrom the environment, andhoneycomb cores can hold large quantities offluid.

If performing a repair using high-temperaturecuring resin or prepreg, all moisture must beremoved to prevent steam from forming anddisbonding the repair.


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E nvironmentis one of the major factors thatinfluence the repair quality.

The presence of moisture is critical to

bonded repairs.Epoxy resins can absorb 1.5 to 2 times theirweight in moisture, thereby reducing theability of the resins to support the fibers.

Environment dirt and dustcan seriouslyaffect bonded repairs.

INITIAL TREATMENT PRIOR TO REPAIR (CONT)


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The repair facilities should be controlledenvironment

The relative humidity should be 25 percentto 60 percentand temperatures fixed at 65

to 75F.

INITIAL TREATMENT PRIOR TO REPAIR (CONT)


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MATERIAL PREPARATION

Material preparation: Determine correct material by SRM Check shelf lifelimits Identify proper curingsystem Determine correct mixingresin and weight

Fabricate coreplug Ensure correct ribbon directionbefore 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 Ensure the correct orientationwhen replacing.


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TYPES OF REPAIR

Basic types of composite repair includethe following:

Cosmetic

Resin Injection Semi-structural Plug / Patch

Structural Mechanicallyfastened Doubler

Structural Bonded External Doubler Structural Flush Repair


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COSMETIC REPAIR


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Resin Injection


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Semi-structural Plug / Patch


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Structural Mechanicallyfastened Doubler


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Structural Patch Repair


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Bolted vs. Bonded Repairs

Advantages of bolting:

Doesnt requiremeticulous 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 thelaminate being scarfed.

Ratio of scarfing as per example 15:1. (length: thickness)Whereby1 indicates thickness and 15 is the length of damaged area to bescarfed


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Plies Replacement on scarf or tapered cut onsandwich structure.


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Core plug - Ensure minimum

of 1/16 inch in excess of the

parts thickness due to core

sinking after adhesive melt

EXAMPLE: SCARF CUT METHOD


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TYPICAL REPAIR: STEP REPAIR

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

Typical steps are 25 50 mm (0.5 + 0.5= approx. 1.0 (25.4 mm) per layer.


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Plies Replacement on step cut on sandwich structure.

Core depression

0.5 inch step


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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 sandingwith grit 240 orfiner Scothbrite or grit 150 or finersandpaper until totally removedwithout damaging the surface fibers.

b) Clean with filtered low pressure airand vacuum the area.

c) Cover sanded area with water-proofingresin 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


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 sandingthe 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





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Defect type: Delamination


a) Minordelamination Resin injection

b) Majordelamination Step OR Scarf cut method



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Defect type: Delamination Fabric - Core

Proper repair method:a) Step OR Scarf cut method



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Defect type: Core Depression


a) Manual sanding

b) Plaster cavity using potting compound

c) Sand excess resind) Apply finishing



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Defects type: Crushed Core

Proper repair method:a) Step OR Scarf cut method



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Defect type: Core Displacement (edge band repair)

Proper repair method:a) core removal and additional microballons filler



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Defect type: Nodal DelaminationProper repair method:

a) Remove core material

b) Step OR scarf cut method



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Defect type: Bridging


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


a) Manual sanding

b) Potting compound


Cavities located in the center of cellsappearing on the sandwich panels


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Defect type: Thread Telegraphing


a) Manual sanding

b) Plaster with resin / potting



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Defect type: Porosity





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Defect type: Foreign Object Inclusion

Proper repair method:a) Removalof 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-upto obtained the

proper thickness & dimensions.b) Apply finishing


DEFECTS DUE TO MOISTURE ENTRAPPED


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DEFECTS DUE TO MOISTURE ENTRAPPED

Polymer matrix composites absorb moisturewhen exposed to humid air.

Moisture concentration increases initially withtime 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 ofmicro cracks, and etc.


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INFLUENCE OF MOISTURE ON POLYMER MATRIX.

Absorption of water by resin in someinstances may change the resin properties &causes swelling of the resin.

INFLUENCE OF MOISTURE ON FIBERS


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INFLUENCE OF MOISTURE ON FIBERS.

Water at the glass fibre interface lowers itssurface energywhich in turn can promotecrack growth.

Aramid fibers can absorb considerablequantities of water resulting in swelling.


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LOW TEMPERATURE EFFECTS

Temperature below zero celciuscancause the water to freeze. Due to thismicrocrackscan occur.

Low temperature can also effect thestiffness and strengthof the matrixsystem.


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EFFECT OF HIGH TEMPERATURE ON COMPOSITES

It is found that bond strengthof compositesget reduced by about 80-90%,and tensilestrengthby 20%at 200C.

At 100C to 150Ccompressive strengthisreduced by 30%of dry CFRP(compressivefibre reinforce plastic) and by 54%of wetCFRP.

EFFECTS OF WATER INGRESSION TO THE


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EFFECTS OF WATER INGRESSION TO THEMECHANICAL PROPERTIES

In Aramid cores of composite sandwichparts, the moisture or fluid can seriouslydegrademechanical properties such as

stiffness and shear strength.

EFFECTS OF WATER INGRESSION TO THE


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HONEYCOMB CORES

The decreasein these properties is attributedto water absorption and swellingof the core

materials.

Autoclave / Manufacturing


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Autoclave / Manufacturing

The Autoclave Process

The heat treatment process is similar in vertical and horizontalautoclaves: Load autoclave with product Close and lock door Raise to soak temperature (ramp) Treat product for a set t ime (soak) Relieve pressure Unload autoclave

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

The Soak PeriodThis period is essential in order to sterilize and treat the product correctly. Timeschedules are determined by the product being treated and the end properties

required.

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

Automatic Start of Pressure Ramp Down


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Using Autoclave for Repairs


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Using Autoclave for Repairs

Autoclave processing is used for the repair of high quality structuralcomponents.

The autoclave technique requires a similar vacuum bag but the oven isreplaced by an autoclave.

The autoclave is a pressure vesselwhich provides the curing conditions forthe composite where the application of vacuum, pressure, heat up rate andcure temperature are controlled. High processing pressures allow themoulding of thicker sectionsof complex shapes. Honeycomb sandwich

structures can also be made to a high standard. Long cure cyclesarerequired because the large autoclave mass takes a long time to heat up andcool down.Sometimes slow heat up ratesare required to guarantee eventemperature distributionon thetoolingand compositecomponents.


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Vacuum bag or autoclave - which process?

Vacuum bag and autoclave processing are the two main methods for therepair of components from prepreg.

The processing method is determined by the quality, cost ($)and type ofcomponentbeing manufactured.

RAMP & SOAK CURING


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RAMP & SOAK CURING

STEP CURING


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STEP CURING


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END

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Aircraft composite repair