Repairing Techniques Flashcards
(10 cards)
What are the types of structures? How are they categorized?
In aeronautical structures, primary structures are those whose failure would lead to a catastrophic structural collapse and loss of aircraft control, secondary structures are those whose failures influences and reduces functionality of the aircraft but not loss of the former, and tertiary structures are structures whose failure does not affect functionality.
What are the possible outcomes of damage evaluation?
There is negligible damage: no repair is required (except to restore appearance of the aircraft). There is minor damage: severe but repairable damage, intervention is generally contained into SRM. Major damage: damage is not repairable for structural and economic reasons, repair would not allow restoration of mechanical and functional performance or the intervention is too expensive, so replacement is preferred.
What are the requirements for repair?
Intervention procedure must be simple, low invasivity, restore level of required performance (for example in terms of stiffness, stability, aerodynamic shape, sealing), not impair other functions of the component, imply minimal weight increasing, require reduced intervention times, and be limited to the damaged area.
What are the main ways to categorize repairing philosophies, what are the main typologies?
With or without patches (riveted, bolted, or bonded), field level or depot level, metal or composite.
Repairable damage for metallic components without patches:
Scratches, dents, corrosion and nicks. If there are no dents, the abrasion of the visible damaged area is removed, and a surface treatment is administered in case of corrosion. For very small cracks, a stop hole is created by drilling at the apex of the crack. A self-expanding insert is inserted inducing compression. If there are dents, an NDI is used to quantify the damage. The geometry is then restored using filler (resin+chopped fiber).
Repairable damage for composite components without patches:
Small delamination, matrix damages (micro-cracks) can be fixed. If there are manufacturing defects as a root cause, we do not repair. To intervene one must first remove any moisture, pre-heat the laminate, inject extremely fluid compatible resin, cure at high temperature and pressure. Thermosetting resins are typically used. If there is thermoplastics, they can be repaired by melting. If there are dents, the repair procedure is the same as with metals.
Repairable damage for metallic components with patches:
If the damage is not a scratch, nick, dent or corrosion, a patch is likely the repair mechanism to be used. The intervention consists on removing the visibly damaged area (by cut out generally, or by partial abrasion in the case of corrosion), execution of a drilling pattern, surface treatment of the area surrounding the damaged area, application of sealant (for outer surfaces), patch installation, rivet installation. Patches can be shaped to couple the geometry of the component during installation (elastic deformations) or previously (permanent deformations).
Repairable damage for composite components with patches:
Damages outside of small delaminations, scratches, and matrix damages, that is, cracks and large delaminations, are fixed via patches. The intervention depends on the thickness of the composite component.
For laminates up to 2mm thick: a cut out is performed to remove the damaged area, the surrounding area is prepared via sandblasting, an external patch is applied via bonding (use of toughened adhesives is standard, requiring higher temperatures). This is a very simple technique and therefore widely used. As it restores around 70% of the original panel resistance it is often employed for on-field repairs. The adding of a patch however creates eccentric loads in the shape of bending stresses and peeling in adhesive layers. Tapering is important to minimize these.
There are two types of patches principally used: pre-preg with the same lamination sequence as the component, or pre-preg with quasi-isotropic lamination sequences. These can be cured before being positioned, and subsequently bonded, resulting in the best mechanical performance but some coupling problems; laminated on site and co-cured with adhesive sheet, resulting in lower mechanical performance but no coupling problems.
For laminates from 2mm to 8mm thick, bonded SCARF patches are used. This results in best working conditions for the adhesive and therefore uniform stress distribution (no eccentric loads), but is more difficult to perform and therefore impossible on the field. Removal of a large un-damaged area is required to make the tapering. Usually the patches share the lamination sequence of the part.
For laminated 8mm to 25mm thick bolted patches are applied. These are also used when there are zones where bonding cannot be performed (eg, zone subject to humidity) or where geometry makes it impossible to use other alternatives. This is easy to perform and therefore often performed on the field. Clamping pressure also obstructs the propagation of delamination. Patches and bolts are both made of titanium in order to avoid galvanic corrosion.
To repair on field, a vacuum bag and thermal electric blanket are employed. Infrared lamps or ovens can also be used.
See figures on doc.
What are the classes of repair for a composite sandwich plate?
Class 1: Repair of a surface damage (dents and scratches not accompanied by holes or cracks). Performed via application of un-cured resin followed by the curing
Class 2: Repair of a damage located on a single skin (possible core damage). Performed via substitution of a part of the core
Class 3: Repair of a damage located on both skins and core. Performed via removal of a disk full of sandwiches.
Class 4: Replacement of the component
Calculate the number of rivers based on the external loads: (possibly not in exam?)
N=2×(t×w×F_tu×1.15)/Pa
Where N/2 is the number of rivets in each damage side, t is the thickness, w is the reference length, Ftu the maximum ultimate stress of the material to be repaired, and Pa is the admissible shear load of the adopted rivet.
We apply this formula for both direction in the case of a rectangular cut-out.
