4 HARDNESS

A hardness measurement is a simple way of obtaining a measure of the elastic modulus of a rubber by determining its resistance to a rigid indentor under an applied force. It is such a simple concept that in its most crude form it could be, and probably was, a matter of prodding the rubber with a finger. It is not surprising that such a simple concept requiring relatively simple apparatus was one of the first and also the most commonly used rubber tests. It is also easy to understand why much effort was put in over the years to developing and standardizing hardness test methods and instruments. This development is a fascinating subject, there having been an amazing variety of instruments, most of which are now a matter of history. For an account of these, reference should be made to the work of Soden ^[24].

The apparently simple process of indentation involves deformations in tension, shear and compression but, as in the case of a perfectly elastic rubber the moduli controlling these are closely related, it is convenient to regard hardness as depending simply on Young's modulus. Approximate relationships for various geometries of indentor have been given in Section 1 and will be further considered, where appropriate, for the standard methods in later sections.

It would be convenient to have a hardness test where a given difference in indentation always represented the same proportionate difference in modulus (i.e. P a linear function of log E). None of...