Rubber Stiffness Vs Durometer

The first answer is partially correct, the test is both a rubber and polymer test in fact the test really could be performed on any low tensile substance though is only a commercial application for the above.

Again as in the first answer it is a penetration test, I can see how you relate it to stiffness as it generally relates to a density test or test in respect to a substance being or becoming brittle, EG: some substances rate higher on the test are softer than a harder compound due to the density creating the increase in penetration pressure reading. the test itself is not an actual proof of either, simply a measure of an already known quantified test base.

In other words you cannot use the test on an unknown substance or new substance and get any sort of usable data in comparison to another product.

Why is it so? well a durometer has a speed over density flaw, a bowl of certain types of liquid at speed would resist a durometer where a solid piece of rubber/plastic/polymer would have less resistance. to show yourself how this is so, mix 4 cups of cornflour into a liquid paste and place in a bowl, now holding your fist vertical above it, punch the liquid with your fist, it will not enter the liquid at all, cornflour works like Kevlar as do many other substances, increases in speed causes the molecular structure to increase in density at pressure points at speed, all having varying speed levels. Thus a durometer would have a zero reading the same as if you dip your finger in the paste, yet have a complete barrier reading at high speed. Thus the reason it is only applicable to a certain range of products.

Durometer is generally measured by an indentor and is specified in units of Shore (often Shore A). Stiffness is a measure of elastic modulus, modulus of elasticity, Young's modulus, etc. Its units are pressure or PSI. Because of the way most elastomers are used, durometer is often the critical parameter but modulus can also be specified if it is critical. There are other mechanical properties related to strain rate where the strain depends on the rate the load is applied. This is important to certain non-Newtonian materials such as silly putty.