Timber
Design & Technology: AQA GCSE, topic 3.2.2
Torsion and Shear Force
Materials can be manipulated to resist and work with forces and stresses.
Torsion with woods
- Wood has very good torsional strength but there are often issues with ensuring that the wood does not warp under moisture and differing conditions.
- Some scaffolding boards have metal brackets that hold and prevent the wood timber from warping and increasing its torsional strength.
- It goes without saying that torsional strength is key when being used for scaffolding
Hardwoods and softwoods
- Hardwood materials that are dense and have close grains that make them less likely to warp compared to softwoods.
- However, softwoods are used with metal brackets as they are more sustainable and they are more economical.
Shear force with woods
- Wood, especially close grain hardwoods, resist shear force.
- Some softwoods are weaker but generally speaking, can withstand shear forces just as well.
Affecting shear force
- Knots and looser grains reduce the strength of wood which directly affects its ability to resist shear force.
- If moisture is increased you will weaken the ability for the timber to withstand shear forces.
- If plywood is not glued and laminated properly, its ability to withstand sheer force will be reduced greatly.
Tension, Compression and Bending
Materials can be manipulated to resist and work with forces and stresses.
Tension
- The tensile strength of wood is very high due to its grain and structure.
- Moisture, over time, can affect the tensile strength.
- Wood is a good choice for when tensile strength is needed, but wood is 30% stronger in compression than tension.
Compression
- Wood is excellent for compression.
- Moisture content can severely affect its ability to withstand compression which results in an ineffective piece of timber/wood.
- Wood, when working in the direction of the grain, can withstand compression well. However, if pushed too far, hardwood will fail.
Bending
- Bending, as a force, can be desirable in certain situations.
- Often Aeroply is used (as it is a thin laminated sheet) with water and heat. Bending can be used to show off how curves can be used with wood.
Kerfing
- Bending can also be achieved through kerfing.
- Kerfing is creating half slots through the use of a saw. Glue is applied once the wood is bent.
Test yourself on Timber
Why is the tensile strength of wood very high?