The International Hardness test is based on measurement of the penetration of a rigid ball into the rubber specimen under specified conditions. The measured penetration is converted into IRHD, the scale of degrees being so chosen that 0 represents a material having an elastic modulus of zero, and 100 represents a material of infinite elastic modulus.

The scale also fulfills the following conditions over most of the normal range of hardness: one IRHD range represents approximately the same proportionate difference in Young's modulus, and for rubber vulcanizates in the usual range of resilience, readings in IRHD are comparable with those given by a Type A durometer (Test Method D2240) when testing standard specimens.

The term “usual range of resilience” is used to exclude those compounds that have unusually high rates of stress relaxation or deformational hysteresis. For such compounds, differences in the dwell time in the two hardness tests (Test Methods D2240 and D1415) result in differences in hardness values. Readings may not be comparable when testing curved or irregularly shaped test specimens.

For substantially elastic isotropic materials like well-vulcanized natural rubbers, the hardness in IRHD bears a known relation to Young's modulus, although for markedly plastic or anisotropic rubbers the relationship will be less precisely known.

The relation between the difference of penetration and the hardness expressed in IRHD is based on the following:

The relation between penetration and Young's modulus for a perfectly elastic isotropic material:

Use of a probit (integrated normal error) curve to relate log₁₀ M and hardness in IRHD, as shown in Fig. 1. This curve is defined as follows:

The value of log₁₀ M corresponding to the midpoint of the curve is equal to 0.364, that is, M = 2.31 MPa or 335 psi.

The maximum slope is equal to 57 IRHD per unit increase in log₁₀ M.

FIG. 1 Point Curve to Relate Log₁₀ M and the Hardness in IRHD

1.1 This test method covers a procedure for measuring the hardness of vulcanized or thermoplastic rubber. The hardness is obtained by the difference in penetration depth of a specified dimension ball under two conditions of contact with the rubber: (1) with a small initial force and (2) with a much larger final force. The differential penetration is taken at a specified time and converted to a hardness scale value.

1.2 This test method is technically similar to ISO 48.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.