Stiffness should not be confused with strength. Stiffness is measured using Young’s modulus, whereas strength comes from the maximum stress that the material can endure before failure. For example, steel is stiff and strong but a wafer biscuit is stiff and weak. In a similar vein, a polymer rope and plasticine are both flexible, but the rope is strong and the plasticine is weak.

Young’s modulus is a measure of the stiffness or rigidity of a material. It provides a direct indication of the extent of the distortion that can be expected for a given load. For example, one would think that steel and aluminium were both rigid materials, but Young’s modulus shows that steel (E=2×1011Nm−2) is three times less yielding than aluminium (E=7x1010Nm−2). A building whose framework was made from aluminium would bend three times farther than an identical building that used steel! Without this quantity, comparing the rigidity of any two materials is difficult.

Behaviour of materialsDetermining a value for Young’s modulus provides only a part of apicture about the behaviour of a material under different loads. The stated values for Young’s modulus are only relevant where the material is behaving elastically. For a more complete picture, the complete stress–strain graph is necessary.

Past the elastic limit, where the material deformed, the value ofYoung’s modulus becomes very small. This is because the bonds within the material itself are being broken and reformed, so the internal structure of the material is unable to provide much support. In this region the material is acting more like a viscous liquid, and the atoms are moving rather like cards sliding over each other. This is what happens when a metal is being drawn into a wire. For similar reasons it is worth noting that the stiffness of a material (as measured by Young’s modulus) will become smaller as the temperature rises. At the melting point, a material has no rigidity whatsoever.