Repairing techniques Flashcards
What are primary, secondary and tertiary aeronautical structures?
1) Primary structures
Structures whose failure leads to a catastrophic structural collapse and loss of aircraft control
2) Secondary structures
Structures whose failure influences and reduces the functionality of the aircraft but does not lead to loss of the same
3) Tertiary structures
Structures whose failure does not affect the functionality of the aircraft
Repairing procedures differ for?
Repairing procedures differ for
- Type of damage (eg corrosion, scratch, crack, indentation)
- Material to be repaired (eg metallic or composites)
- Shape of the component to be repaired
- Accessibility of the damaged area
- Functional characteristics and the performance required to the components
- Place of repair (on-field or field-epot levels)
How can damage be evaluates?
1) Negligible damage: no repair is required, meaning that the mechanical properties haven’t changed (with the exception of interventions to restore the original appearance of the aircraft)
2) Minor damage: severe but repairable damage. The intervention procedures are generally contained into the Structural Repair Manual (SRM)
3) Major damage: damage is not repairable for structural and economic reasons. Its repair doesnt allow to restore the mechanical and function performance of the component or such intervention is too expensive so that replacement of the component is preferred.
What are the repair requirements?
- Intervention procedure has to be simple to be applied
(need to repair a compoonent on field, hardly accessible, without devoted tools and materials) - Repair has to be low invasive
- Repair has to restore the level of required performance in terms of resistance, stiffness, stability, aerodynamic shape, sealing
- Repair must not impair other functions of the component
- Repair must require reduced intervention times in order to minimize aircraft downtime
- Repair must be limited to the damaged area
What are the repairing philosophies typologies
1) Without patches and with patches (riveted or bolted; bonded)
2) Field level (level of personnel specialization; availability of specific equipment and systems) and depot level (level of complexity of procedures; temporary nature of the repair)
3) Metal and Composite
The patch is made of the same material of the part to be repaired to avoid or limit internal stress or galvanical corrosion
How can the repair without patches be done for metallic components? What is the Intervention procedure for no dents, small cracks and dents?
- For a metallic component, the repairable damages are
scratches, nicks, dents, corrosion.
The intervention procedure (no dents) includes:
- Removal by abrasion of the visible damages area
- Surface treatment with primer (corrosion)
For very small crack:
- Drilling at apex of the crack (stop-hole)
- Placement of a self-expanding insert inducing compression state around the hole (temporary solution)
The intervention solution for dents includes:
- NDI in order to quantify the damage
- Restoring of the geometry by using filler (resin + chopped fibres).
How can the repair without patches be done for composite components? What is the Intervention procedure?
- For a composite compoent the repairable damages are small delaminations, matrix damages (micro-cracks) caused by wear but NO MANUFACTURING DEFECTS
The intervention procedure includes:
- Removal of any moisture
- Pre-heating of the laminate
- Injection of compatible resin, extremley fluid
- Curing at high temperatures and pressure
This is typically used for thermosetting resins ( resins with low viscosity). If it’s a thermoplastic, repair happens by melting of themroplastics. In case of dents, same procedure as metals.
How can the repair with patches be done for metallic components? What is the Intervention procedure?
- For a metallic component the repairable damage are those who are outside the limits of repairability without patches, like scratches, nicks, dents, corrosion and cracks (basically all damages)
The intervention procedure includes:
- Removing of visible damages area either by partial abrasion (for corrosion) or by cutout
- Execution of drilling pattern
- Surface treatment of the area surounding the damaged area
- Application of sealant (if outside) for external components
- Patch installation
- Rivets installation
Pathces can be shaped to couple the geometry of the component during installation (elastic deformations) or previously (plastic deformation)
How can the repair with patches be done for composite components? What is the Intervention procedure?
- For composite components, the repairable damages are those who are outside the limits of repairability without patches (small delaminations, scratches, damage matrix), cracks and large delaminations
- The intervention procedure differed depending on the thickness of the component
What is the thickness range for the different intervention procedures?
1) Laminates up to 2mm thick
2) Laminates 2-8mm thick
3) Laminates from 8-25mm thick
How can repair with patches be done on laminates up to 2mm thick?
- Removing the damaged area by using tools for composites (cut-out)
- Surface preparation of the area surrounding the damaged area (sandblasting, peel-ply is used for the patch)
- Application of external patch by bonding (use of toughned adhesives, where high curing temperature is required)
- Simple technique widely uses
- Used for on-field repairs, restores 70% of components resistance
What are the type of patches and its application procedure for composite materials?
Type of patches
1) Pre-preg with the same lamination seuence
2) Pre-preg with quasi-isotropic lamination sequence (often fabrics)
Application procedure
1) Laminated and cured before its positioning and then bonded. This has the best mechanical performance but has coupling problems, so it’s a temporary and on field application.
2) Laminated on site and co-cured with adhesive sheet. This has lower mechanical problems but no coupling problems. It is made of cured sub-laminates interposed with adhesive sheet
What can the application of patches lead to?
1) Birth of eccentric loads, such as bending stresses and peeling in adhesive layers
2) tapering is important to minimize peeling and shear stresses
How can repair with patches be done on laminates up to 2-8mm thick?
For laminates from 2-8m thick SCARF patches are used. Different type of patches induce different stress distributions along the joining layer. The scard patch induces a constant stress distribution, which means there are no eccentric loads.
- Best working condition for the adhesive, because of uniform distribution of stress
- No eccentric loads
- Difficult to be done. It isnt possible to apply scarf patches on th efield
- Removal of a large un-damage area to make tapering is required
- Patches with same sequence lamination of the part is usually used.
How can repair with patches be done on laminates up to 8-25mm thick?
For laminates from 8-25mm thich, bolted patches are applied.
- It is used where the complex shape of the component makes impossible other solutions
- It is used when bonding cannot be performed (eg due to humidity)
The advantages are:
- Easy to be done and is often performed on field
- Clamping pressure obstructs the propagation of any delamination
- Patches and bolds are made of titanium in order to avoid galvanic corrosion
For the on-field repair:
A vacuum bag and thermal electric blanket is used
- The heating blankets can be placed inside the vacuum bag and embedded into an elastometric sheet
- Infrared lamps
- Ovens
