Elasticity

A quantity that measures an object or substance’s resistance to being deformed elastically

Elasticity is a property possessed by many solid materials, but the class of materials known as rubbers or elastomers possess a different property called high elasticity.This differs from conventional Hookean elasticity in that it exhibits much higher values of elongation at break, up to 1000% or more.

This huge value for elongation at break means that many of the other properties measured in the accepted manner for metals show interesting variations.

Tensile strength, for example, although still calculated in the usual way using the initial cross sectional area, will give values much lower than the true stress in the material because the actual cross section at break will be approximately one tenth of the original value. Another important point is that, over the elongation at break range, the slope of the line varies substantially, giving typically an S-shaped curve, so that the normal engineering term of modulus becomes meaningless.

Rather confusingly a description of the relative stiffness of the rubbery materials, frequently used, is also called ‘modulus’. This is simply a measure of the load at a given elongation, e.g. ‘modulus at 300%’ is simply a stress value for the material at an elongation of 300% (i.e. at four times the original length). Although 300% is a commonly used value, a range of extensions is used to give ‘modulus’ values relevant to different applications. In the context of rubbers and elastomers modulus refers to this rubber property.