Hardness Testing of Vulcanized Rubber. VIII. Hardness Tests with a Conical Indentor

The relation between load and depth of indentation for a conical indentor has been investigated. The theoretical relationship applicable to perfectly elastic materials (indentation proportional to square root of load) is found to hold for all the rubbers examined except one very hard sample, whose comparative inelasticity causes some deviation from this relationship. This result further confirms the correctness of the theoretical investigation on indentation hardness testing of rubber by indicating that the forces required to produce indentations of similar shape, but different sizes, are proportional to the squares of corresponding linear dimensions of the indentations. With a conical indentor under a constant load, the depth of indentation is approximately inversely proportional to the square root of the elastic modulus of the rubber at small elongations. A conical indentor is preferable to a ball indentor in that it shows more nearly uniform sensitivity to small differences in modulus (i.e., true hardness) at all parts of the hardness scale. It has previously been shown that the ball is superior to a flat-ended plunger in this respect. To approach still nearer to uniform sensitivity would presumably require a pointed indentor having a profile with concave sides. This would be difficult to make accurately, and very likely to damage on account of its sharp point; this might, indeed, be a disadvantage of the conical form also.