Material Handling
Sheet metal marking out, bend-allowance calculation, forming and inspection; composite and non-metallic bonding practices, environmental conditions and inspection.
Material handling, summary notes
- Covers forming sheet metal and the handling of composite and non-metallic materials.
- Sheet-metal work: marking out, cutting, and bending to a specified radius. Bending a sheet stretches the outside and compresses the inside; the NEUTRAL AXIS (about 0.4-0.45 of the thickness from the inside) neither stretches nor compresses.
- The BEND ALLOWANCE is the length of material consumed in the bend, measured along the neutral axis. It must be added to the flat pattern so the finished part comes out to size, too little or too much and the flanges are wrong.
- Minimum bend radius must be respected: bending a sheet tighter than its minimum radius cracks the outer surface, especially across the grain. Bend lines are best placed across (not along) the grain direction.
- Composite handling: fibre-reinforced materials (carbon, glass, aramid) with a resin matrix, cured under controlled temperature, pressure and time. Pre-preg has a limited out-of-freezer life and must be logged.
- Composite inspection: delamination and disbond (tap test or ultrasonic), impact damage (which may be barely visible on the surface but extensive within), moisture ingress and heat/lightning damage.
- Correct storage and environmental control, cleanliness, humidity and temperature, are essential to a sound composite repair; contamination of the bonding surface is a common cause of disbond.
- ⚠ Exam trap: bend allowance is measured along the neutral axis (~0.4t from the inside), not along the outside or inside surface.
- Neutral axis factor K
- ≈ 0.4 - 0.45 of the thickness from the inside of the bend
- Bend allowance (90°)
- BA ≈ (π ÷ 2) × (r + K·t)
- Grain direction
- bend across the grain to avoid cracking
A 90° bend is made in 2 mm sheet over a 3 mm radius, K = 0.4. Estimate the bend allowance.
BA ≈ (π ÷ 2) × (r + K·t) = 1.571 × (3 + 0.4 × 2) = 1.571 × 3.8 ≈ 6.0 mm. That length is added to the flat pattern so the finished flanges come out to size.
Why can composite impact damage be more serious than it looks?
A low-energy impact can leave only a small surface mark while causing extensive internal delamination, 'barely visible impact damage'. The strength loss is far greater than the surface suggests, which is why tap testing or ultrasonic inspection is used rather than relying on visual assessment alone.
Material handling concept map
Material Handling
Material handling quiz
Material Handling, quiz
1. The neutral axis of a bend lies approximately:
0.4t from the inside of the bendAt the outer surfaceAt the inner surface2. Bend allowance is measured along the:
Neutral axisOutside surfaceInside surface3. Bending a sheet tighter than its minimum bend radius will:
Crack the outer surfaceStrengthen the bendHave no effect4. To reduce the risk of cracking, a bend line is best placed:
Across the grainAlong the grainAt 45° to the grain always5. Barely visible impact damage in composites is significant because:
Internal delamination may be extensiveIt is only cosmeticIt heals in service6. Pre-preg composite material has:
A limited out-of-freezer lifeAn unlimited shelf lifeNo storage requirement7. Composite delamination is detected by:
Tap test or ultrasonic inspectionA magnetMeasuring resistance8. A common cause of a failed composite bonded repair is:
Contamination of the bonding surfaceExcess fibre strengthToo much inspection