Properties of Materials
The terms hard, strong, stiff and tough have particular meanings when describing properties of materials.
The strength or breaking force of a material is the force required to break a particular cross sectional area of the material when a force is applied to stretch it or compress it. Materials have different strengths when being stretched or compressed. Metals are strongest when being stretched and concrete is strongest when being compressed.
Tough materials are strong but do not change shape or deform easily when stretched or compressed, and will deform, absorbing a great deal of energy before breaking. They are difficult to bend are retain most of their strength when bent. Tough materials are often long lasting. Steel is tough.
Brittle materials like glass do not deform a great deal before breaking, and does not absorb a great deal of energy. Brittle materials cannot be easily bent and break with little warning.
Malleable materials can be easily worked into new shapes, and if they can be made into wires they are said to be ductile. Many materials become more malleable and ductile as they become warmer.
Elastic materials return to their original shape when any load is released. Rubber bands are elastic. Plastic materials maintain a new shape after a load is released. Most materials are not completely plastic or elastic, or the degree to which they are one or the other varies with the force applied.
Hard materials are difficult to scratched or cut. Diamond is hard. Material which are not hard are called soft.
It is important to realise the terms above are distinct. For example, glass and steel are both hard materials, but steel is tough and glass is brittle.