Section 7.14

Material Handling

Familiarisation with sheet-metal bending and bend allowance, and the handling and inspection of composite materials.

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Main ideas

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.

Key formulas

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

Solved examples

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.

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