A/C Composite Repair
Advanced Comp Repair
INTRO
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!!!
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.
COMPOSITE DAMAGE
Damage must be present first in
order repair can be made to the
structure.
Damage scenario:
- Damage during manufacturing
- Damage during maintenance
COMPOSITE DAMAGE (cont.)
In manufacturing, damage may be due
to:
- Improper manufacturing process,
- Mishandling of the parts,
- Misassemble of parts &
components of aircraft.
COMPOSITE DAMAGE (cont.)
In maintenance, a/c composite
structure may experience damage
due to the:
Operating conditions,
Environmental conditions
Mishandling of the parts.
COMPOSITE DAMAGE (cont.)
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.
COMPOSITE DAMAGE (cont.)
For example, a low velocity impact, which
normally wouldnt 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.
COMPOSITE DAMAGE (cont.)
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
Example of Aircraft Damage
COMPOSITE REPAIR - REFERENCE MATERIAL
Structural Repair Manual (SRM)
Airworthiness Directives
Service Bulletin
Manufacturers.
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)
TYPICAL REPAIR FLOW SEQUENCE
DAMAGE STRUCTURE
IDENTIFY DAMAGE AREA CONFIGURATION
PREPARE DAMAGE AREA
SANDWICH STRUCTURE
LAMINATE STRUCTURE
Remove Tedlar & Sand
Taper Sand
Remove Tedlar
And Sand
Skin Repair
Damage Core
Un-damage Core
Edge Band
Repair
Skin
Repair
Replace
Core
REPLACE PLIES
REFINISHING
Repair :
Bridging
Delaminate
Void
Fracture
Repair
Core
sand smooth with 150 grit or
finer abrasive
TYPICAL HOT BOND REPAIR PROCESS FLOW
PREPREG
TOOL
TEMPERING
CLEANING
CUTTING
RELEASE AGENT
LAY-UP
BAGGING
CURING
DEBAGGING
FITTER & FINISH
EXAMPLE OF HOT BOND REPAIR
TYPICAL COLD BOND REPAIR PROCESS FLOW
RESIN
DRY FIBER
TOOL
TEMPERING
CUTTING
CLEANING
RELEASE AGENT
WEIGHING
CUTTING
LAY-UP
BAGGING
CURING
DEBAGGING
FITTER & FINISH
58
EXAMPLE OF COLD BOND PROCESS
DAMAGE CLASSIFICATIONS
Manufacturing
(1)
(2)
(3)
Acceptable
Correctable
Rejectable
Maintenance
(1)
(2)
(3)
Negligible
Repairable
Non-repairable
COMMON DAMAGE
CATEGORY
OF DAMAGE
LAMINATE
CORE
INTERFACE
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
doesnt need to replace and fit a part and can be done by using a potting
compound.
Major Surface Damage
- All major damage should be remove and repair immedietly
DAMAGE IN MAINTENANCE
COSMETIC DEFECTS
Damage that that occurs on the outer skin without interfering the first
layer of the reinforcing fibers.
DAMAGE IN MAINTENANCE
DELAMINATION
Separation of layers of material in a laminate
DAMAGE IN MAINTENANCE
DISBOND
Separation of layers between laminate and bonded material.
DAMAGE IN MAINTENANCE
IMPACT DAMAGE
High Energy Impact
Medium Energy Impact
Low Energy Impact
Impact damage occurs usually due to struck by foreign object.
DAMAGE IN MAINTENANCE
Coating
Reinforcing
Fiber
Dent
Crack
Fracture
Matrix Layer
Dent: Dislocation or Indentation that does
not penetrate the reinforcing fiber
Crack: Partial protrusion through the
layers
Fracture: Thorough protrusion until the
next surface.
DAMAGE IN MAINTENANCE
LIGHTNING STRIKE DAMAGE
Damage that due lightning.
DAMAGE IN MAINTENANCE
CRACKS
A partial separation on the composite structure that going through one or
more layers of the reinforcing fibers.
DAMAGE IN MAINTENANCE
HOLE DAMAGE
Damage to a hole that is caused by overtightening, impact
damage mislocation of the drilled hole etc.
DAMAGE IN MAINTENANCE
WATER INGRESSION DAMAGE
Moisture absorption or trapped water in the composite structure especially to
sandwich structure.
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
replace if deterioration occured
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
rapid propagation of the damage.
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.)
Restore surface finish
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
SEQUENCE OF VACCUM BAGGING MATERIAL USED FOR REPAIR
REPAIR PROCEDURE
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
smooth shape with round corners.
REPAIR PROCEDURE: REMOVAL OF COMPOSITES DAMAGES
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
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.
GENERAL: REMOVAL OF DAMAGED
AREA
Laminate Structure
Taper (scarf) sand OR
Step sanding
Using disc @ manual
sander.
Must sand 0.5 inch or
tapered by 1:30 ratio
GENERAL: REMOVAL OF DAMAGED
AREA
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
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.
INITIAL TREATMENT PRIOR TO REPAIR (CONT)
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.
INITIAL TREATMENT PRIOR TO REPAIR (CONT)
The repair facilities should be controlled
environment
The relative humidity should be 25 percent
to 60 percent and temperatures fixed at 65
to 75F.
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
Ensure the correct orientation when replacing.
TYPES OF REPAIR
Basic types of composite repair include
the following:
Cosmetic
Resin Injection
Semi-structural Plug / Patch
Structural Mechanicallyfastened Doubler
Structural Bonded External Doubler
Structural Flush Repair
COSMETIC REPAIR
Resin Injection
Semi-structural Plug / Patch
Structural Mechanicallyfastened Doubler
Structural Patch Repair
Bolted vs. Bonded Repairs
Advantages of bolting:
Doesnt require
meticulous material &
surface preparation
Easy to inspect for
quality
Easily disassembled
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
Plies Replacement on scarf or tapered cut on
sandwich structure.
EXAMPLE: SCARF CUT METHOD
Core plug - Ensure minimum
of 1/16 inch in excess of the
parts thickness due to core
sinking after adhesive melt
TYPICAL REPAIR: STEP REPAIR
The laminate is sanded down so that a flat band of each layer is exposed
producing a stepped finish.
Typical steps are 25 50 mm (0.5 + 0.5 = approx. 1.0 (25.4 mm) per layer.
Plies Replacement on step cut on sandwich structure.
0.5 inch step
Core depression
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
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
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.
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Resin Starved Areas
Proper repair method:
a) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Tacky Areas
Proper repair method:
a) None (Reject part)
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Fabric Wrinkles
Limitation:
a) Not caused by fabric overlapping and
maximum height and depth do not exceed
certain dimension
Proper repair method:
a) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Scratches
Proper repair method:
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
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Cracks
Proper repair method:
a) Minor crack Resin injection
b) Major crack Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Fracture
Proper repair method:
a) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Delamination
Proper repair method:
a) Minor delamination Resin injection
b) Major delamination Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Delamination Fabric - Core
Proper repair method:
a) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Core Depression
Proper repair method:
a) Manual sanding
b) Plaster cavity using potting compound
c) Sand excess resin
d) Apply finishing
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defects type: Crushed Core
Proper repair method:
a) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Core Displacement (edge band repair)
Proper repair method:
a) core removal and additional microballons filler
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Nodal Delamination
Proper repair method:
a) Remove core material
b) Step OR scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
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
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Pitting on Center of Cells
Proper repair method:
a) Manual sanding
b) Potting compound
Cavities located in the center of cells
appearing on the sandwich panels
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Thread Telegraphing
Proper repair method:
a) Manual sanding
b) Plaster with resin / potting
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Porosity
Proper repair method:
a) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Foreign Object Inclusion
Proper repair method:
a) Removal of fabric layers & inclusion (during lay
up)
b) Step OR Scarf cut method
EXAMPLE OF COMPOSITES DEFECT & PROPER REPAIR METHOD
Defect type: Geometric Deviation
(exceed/non exceed drawing)
Proper repair method:
a) Manual sanding & Lay-up to obtained the
proper thickness & dimensions.
b) Apply finishing
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.
INFLUENCE OF MOISTURE ON POLYMER MATRIX.
Absorption of water by resin in some
instances may change the resin properties &
causes swelling of the resin.
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.
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.
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 200C.
At 100C to 150C compressive strength is
reduced by 30% of dry CFRP (compressive
fibre reinforce plastic) and by 54% of wet
CFRP.
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.
EFFECTS OF WATER INGRESSION TO THE
HONEYCOMB CORES
The decrease in these properties is attributed
to water absorption and swelling of the core
materials.
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
Automatic Start of Pressure Ramp Down
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.
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.
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.
RAMP & SOAK CURING
STEP CURING
END
