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Repair Design and Practices - Coggle Diagram
Repair Design and Practices
Damage Removal
top skin and core damage
Results from greater impact
may produce disbonding in bottom skin
top skin damage only
Caused by low energy impacts : only top skin removed
damage through laminate
case of puncture, both top and bottom skins will require removal
removed in circular or oval shape
vacuum and downdraft table is used to extract dust
Drying
typically done with heat blanket and vacuum bag.
Heat converts moisture to vapor and is drawn out by vacuum
can cause disbond in the repair if it requires high temperature curing as moisture present would vaporise.
Temperature
should not be too high (150oF - 170oF)
slower drying & lower temperature (ideal drying condition)
rapid temperature rise & high temperature (not ideal) - delamination
bolted repair
involves
mechanically
fastening repair patches onto repair area
Load transferred through fasteners to patches
Advantages
Easier to perform and suitable for field repair
less surface preparation
no risk of heat damage
easier to inspect
Disadvantages
stress concentration due to drilled holes
delamination
increased weight to structure
external surface is not smooth and not aerodynamic
Titanium patches are preferred as it can match the strength and stiffness
materials for patches and fasteners prevent galvanic corrosion
bolted repair is more suitable for thick laminates
thin laminates have low bearing area to tolerate the load transfer
Design
(d/t>1)
larger bolts are less likely to bend
Edge distance normal to direction of loading - 2.5D
Edge distance parallel - 3.0D
Bolt pitch (distance between bolts) normal - 5.0D
Bolt pitch parallel - 4.0D
Bonded repair
adhesive joint is formed between repair patch and the prepared repair area
scarfed repair (tapered repair)
Repair patch is made by replacing each ply of laminate that has been removed
surrounding repair area undergoes taper sanding
load transferred directly through edges of repair plies
improve aerodynamics
Joint becomes stronger with increased
contact area
of bonding
Flatter tapered angle results in
larger contact area
and repair strength increases
scarf angle of 20:1 is expected to give a greater scarf repair strength than 12:1
Curing of repair patch
hot bonders are used to control heat blankets which are placed over repair patch in a vacuum bag to provide heat to cure.
hot bonders require the use of thermocouples to measure laminate temperature and control in a very precise manner
