Stretching is a sheet metal forming operation in which the metal is pulled in tension to increase its length and surface area.
The sheet is stretched beyond its elastic limit to obtain a desired shape.
It is commonly used in automobile body panels and aircraft skin parts.
In this article:
Stretching in Sheet Metal
What is Stretching?
Stretching is a sheet metal forming operation in which the metal sheet is stretched and plastically deformed over a die by applying tensile force.
👉 Thickness reduces slightly
👉 No material is removed
👉 Deformation occurs due to tensile stress
Stretching is mainly used to produce large curved surfaces.
Working Principle
- The sheet is clamped firmly at both ends.
- Tensile force is applied.
- The sheet is pulled over a die block.
- Plastic deformation occurs.
- The sheet takes the shape of the die.
How Stretching Works
- Tensile stress exceeds yield strength.
- Material elongates permanently.
- Thickness decreases slightly.
- Surface becomes smooth and curved.
Unlike bending:
✔ Entire surface is under tension
✔ No compression on inner side
Important Terms
Stretch Ratio
Ratio of final length to original length.
Reduction in Thickness
Occurs due to elongation.
Formability
Ability of sheet to undergo stretching without cracking.
Stretching Force
Force depends on:
Types of Stretch Forming
1. Stretch Forming
Sheet is stretched over form block.
2. Stretch Draw Forming
Combination of stretching and drawing.
3. Rotary Stretching
Used for cylindrical shapes.
Machines Used
- Stretch forming machine
- Hydraulic press
- Tension-controlled forming systems
Applications
- Aircraft wing skins
- Automobile roof panels
- Large curved panels
- Architectural panels
- Aerospace components
Advantages
✔ Produces smooth curved surfaces
✔ Good surface finish
✔ Accurate shape
✔ Suitable for large panels
Disadvantages
✖ Thickness reduction
✖ Risk of tearing
✖ Requires high force
✖ Expensive equipment
Stretching vs Bending
| Feature | Stretching | Bending |
|---|---|---|
| Stress Type | Tension only | Tension + Compression |
| Thickness | Reduces | Nearly constant |
| Shape | Large smooth curves | Sharp bends possible |
Summary:
- Stretching is a tensile forming process.
- Thickness decreases during operation.
- Used mainly in aerospace industries.
- Entire sheet is under tensile stress.
- No material removal.
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