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Lesson 10: Repair Design and Practices of composite materials - Coggle…
Lesson 10: Repair Design and Practices of composite materials
Types of damage
Disbond
Holes and punctures
Dents and delamination
Core damage
Water ingression
Damage removal
Methods
Routing uses diamond coated cutting tools for a cleaner cut and reduce splintering
Grinding uses diamond disks to sand away material
Cut using a sharp knife
All damage material must be removed (eg. signs of corrosion, contaminated by water/fluids)
Removed in circular or oval shapes and irregular shape corners must be round off
Drying procedures
To achieve an effective repair, all affected composite materials must be dried
Drying is done with a heat blanket and vacuum bag
Moisture present in a repair would vaporize and cause disbond in the repair
Heat converts the moisture to vapor and drawn out by the vacuum
Drying temperature should range from 150°F to 170°F
Slower drying
using
lower temperature
is an
ideal
condition drying conditions while
faster drying
using
higher temperature
is
not an ideal
condition
Rapid temperature rise high drying temperature may cause delamination
Bonded repair
Scarfed repair also known as tapered repair is the most common type of bonded repair
Repair of sandwich structures
Step 2: Bond new honeycomb (core)
Step 3: Repair first side
Step 1: remove damage
Step 4: Repair second side
Repair patch is adhesively joined to the repair area
After damage removal, surrounding repair area undergoes taper sanding
Materials
Repair patch should use the same material, fiber and orientation and stacking sequence as the original design of the laminate
Curing of repair patch
Requires elevated temperature curing and may be done in an autoclave or an oven
Placement of thermocouples
Attach thermocouples to caul plates since caul plates are good conductors of heat and can help distribute the heat evenly across the repair and thermocouples. Hence thermocouples will read a more accurate reading.
Electrically insulate each thermocouple from any conductive surface
Place inside the heat blankets to avoid the cold zones thus avoiding damage to repair patch and fire hazards
Avoid unnecessary thermal insulation around thermocouples
Hotbonder
Portable repair equipment
Used to control heat blankets which are placed over repair patch in a vacuum bag to provide the heat to cure the repair patch
Cold zone of heat blankets
If thermocouples are placed inside the cold zone, the hot bonder will underestimates the temperature of the repair patch and overheat the repair patch which causes a risk of fire and damage to repair
Heat blanket needs to larger than the repair patch
Properties of repair patch
Repaired structure will be stiffer than the original
Structure flexes significantly under load, the increased stiffness of repair patch will cause failure at the edges of the repair thus repair is designed to have lower strength
Repaired structure will not be as strong as the original hence extra repair piles are added to compensate for the loss in strength
Tapered (Scarfed) Distance Per Ply
Taper sand 0.5 inch per ply of composite laminate
Tapered (Scarfed) Angle
Flatter
tapered angle,
larger
contact area -> repair strength
increases
Steeper
tapered angle,
smaller
contact area -> repair strength
decreases
Scarf angle = scarf distance/thickness of laminate
Suitable for thin laminate
Tapered (Scarfed) Diameter
(Scarfed distance x 2) + hole diameter
Bolted repair
Advantages
No risk of heat damage
Easier to perform and suitable for field repair
Less surface preparation required
Easier to inspect to check for quality
Disavantages
Increased weight to structure
External surface is not smooth and hence not aerodynamic
Drilling result in greater possibility of delamination in composites
Stress concentration due to drilled holes
Involves mechanically fastening repair patches onto repair area
Load is transferred through the fasteners to the patches
Titanium patches are used as it matches the strength and stiffness of carbon fiber laminate and prevent galvanic corrosion
Sealants are applied to prevent moisture ingression
Suitable for thicker laminates as thin laminates have low bearing area to tolerate the load transfer from the fastener resulting in bearing failure of repair joint
Thicker
laminates -> Bolted joint strength and repair strength
increases
Design
Having adequate edge distance and bolt pitch may prevent shear out failure
Bolts in repair may fail via
fastener shear
and
bending failure
Prevent bending failure by using large bolts compared to the thickness of the joint [d/t >1]
Prevent fastener shear failure by selecting correct bolt diameter and material
Damage detection
Ultrasonic inspection
Thermography
Tap testing
Goal of repair
Stiffness
Stiffness of repair material should match closely to parent material
Prevents excessive deflection and unnecessary load redistribution
Weight
Repairs often add additional weight
May affect balancing of the component
Strength
Repaired structure should take the same load as original structure
Aerodynamic smoothness
Require smooth external surfaces to minimize drag
Structure Repair Manual (SRM)
Repair should use the same materials, fiber orientation, stacking sequence and processing methods and conditions that was used in the original manufacturing of the part
