Stress is the physical demand that a body or object must withstand when one or more external forces are applied to it. There are five types of stress:
- Compression: This is caused by forces acting on a body that tend to flatten it or reduce its length or thickness. It happens in structures that need to support weights.
- Tension: This is caused by forces acting on a body that tend to stretch it.
- Bending: This is caused by forces acting on an element that make it curve or bend. It is done by supporting the two ends and applying force in teh centre, as shown in the figure to the right.
- Torsion: This is caused by forces acting on a body that make it twist.
- Shear: This is caused by two equal forces applied in opposite directions that act on lines of action close to each other. Shear stress will tend to cut the material if it can not twist or bend.
Rigidity: If a material is rigid, it does not change shape when force is applied to it. By contrast, if the material does change shape, we say that it is deformable.
Exercise 12. Think about the following objects and state whether they are rigid,plastic or elastic: balloon, empty drinks can, window frame, clay, catapult, door.
- Balloon: elastic
- Empty drinks can: plastic
- Window frame: rigid
- Clay: plastic
- Catapult: rigid
- Door: rigid
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