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

IRHD EIRHD-ECT

 

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