A measure of the indentation resistance of elastomeric or rubber materials based on the depth of penetration of a ball indentor. An initial contact force is applied to an indentor and the penetration is set to zero. The force is increased to a specified total load and the depth of penetration is measured. The IRHD value is related to the depth of indentor penetration. For highly elastic materials, IRHD and Shore A are comparable. An IRHD value of 0 represents a material with an Young's Modulus of zero and an IRHD value of 100 represents a material with an infinite Young's Modulus. The relationship between the depth of penetration (hardness) and the IRHD number is based on: F/E=0.0038r0.65*D1.35

where:

F is the indentation force in Newtons,

E is the Young's Modulus in MPa,

r is the indentor ball radius in mm, and

D is the depth of penetration in mm.

**Test** |
**Ball Diameter (mm)** |
**Contact Force**
(N) |
**Total Force**
(N) |
**Thickness**
(mm) |
**IRHD Range** |

N (Normal) |
2.5 |
0.3 |
5.7 |
4 |
30 to 95 |

H (High-Hardness) |
1.0 |
0.3 |
5.7 |
4 |
85 to 100 |

L (Low-Hardness) |
5.0 |
0.3 |
5.7 |
6 |
10 to 35 |

M (Micro) |
0.395 |
0.0083 |
0.153 |
4 |
30 to 95 |