Supplied B1 Module 07A textbook: PDF pages 275–314.
Material identity and marking out
Verify material grade, temper, thickness and grain direction before cutting. Transfer dimensions from the drawing using suitable marking tools. Deep scratches in stressed metal can initiate cracks, so do not use aggressive scribing where prohibited. Allow for bends and finishing; a flat blank cannot simply be cut to the sum of outside flange lengths. Keep metal and composite work separated as required to control contamination. Marking out establishes a controlled reference for fabrication, not permission to alter the drawing.
Why can deep scribe marks be unacceptable on stressed metal? They may act as fatigue stress concentrations.
Bend allowance and setback
The inside of a bend compresses while the outside stretches. The neutral axis is the layer whose length is used for bend allowance. A common calculation is BA = angle in radians × (inside radius + K × thickness), where K locates the neutral axis and must come from the prescribed data. For ninety degrees, inside radius 3 mm, thickness 1 mm and K = 0.33, BA is about 5.23 mm. Setback relates the tangent geometry to outside dimensions; it is not the bend allowance itself.
Which expression represents bend allowance using a K factor? Angle in radians × (inside radius + K × thickness).
Bending, forming and inspection
Use the specified bend radius, tooling and material orientation. Springback means the final angle differs from the tool's loaded angle, so the process may require a controlled allowance. Excessively tight bends can crack the outer surface; forming can also cause wrinkles, thinning and distortion. Inspect the bend and final dimensions, not only the appearance of a straight flange. Avoid unauthorised heat treatment or repeated bending to correct a part. Each additional operation can change properties and introduce damage.
What is springback? Elastic recovery after forming load is removed.
Composite structure and lay-up
A composite combines reinforcement with a matrix. Fibre direction carries much of the load, so ply orientation and stacking sequence are structural requirements. Sandwich construction adds a lightweight core between skins. Impact may create internal delamination with little visible surface damage. Protect fibres and control cutting dust using suitable equipment and PPE. Match repair materials and resin systems to the approved repair; an equivalent-looking cloth is not enough. Inspect core, skins and interfaces rather than judging only the external finish.
Why is ply orientation controlled in a composite repair? It determines directional load carrying and stiffness.
Bonding environment and cure
Bond strength depends on surface preparation, compatible adhesive, correct bond-line geometry and cure conditions. Control temperature, humidity, cleanliness, storage life, out-time and mixing where applicable. Peel ply or abrasion may be specified to create the required surface; oily fingers can ruin prepared surfaces. Vacuum bags apply consolidation pressure and remove trapped air when the procedure requires them. Monitor cure time and temperature at the required locations and retain records. A hard surface is not proof that the full laminate reached the approved cure.
What is necessary for a controlled bonded repair? Specified preparation, materials and monitored cure conditions.
Inspection of composite and bonded work
Visual inspection detects surface defects; tap testing may help locate some disbonds but is limited by geometry and structure. Ultrasonic techniques can assess internal interfaces and delamination using an approved method. Other methods may be selected for particular materials and defects. Define the area, method, reference standards and acceptance criteria before inspection. Record damage boundaries and obtain repair data appropriate to the location and load path. A satisfactory cosmetic finish does not establish an acceptable structural bond.
Why is visual inspection alone sometimes insufficient for composites? Internal delamination or disbond may not be visible.
Apply the knowledge
Work through these examples before starting the quiz. Explain why the other choices would not meet the requirement.
For a 90-degree bend with R = 3 mm, t = 1 mm and K = 0.33, what is BA approximately?
5.23 mm — Use π/2 × (3 + 0.33 × 1); degrees must be converted to radians.
A composite panel has an impact mark with little visible damage. What is appropriate?
Assess for internal damage using the specified inspection method — Impact can cause hidden delamination or core damage.
A resin exceeded its controlled out-time. What should happen?
Apply the material-control rejection or authorised disposition process — Out-time is a controlled material limit.
A repair uses the right cloth but wrong ply directions. What is the issue?
The structural load path and stiffness may be wrong — Fibre direction is a design requirement.