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