IndiaRubberDirectory.com > Rubber Engineering > Elastromer Properties
Properties - Hardness
The hardness of an elastomer is
measured based on the depth of indentation by a standard size and shape
impacting gauge. The hardness is obtained by comparing the difference between a
small initial force and a much larger final force. The International Rubber
Hardness Degrees (IRHD) scale has a range of 0 to 100, corresponding to elastic
modulus of 0 (0) and infinite (100), respectively. The measurement is made by
indenting a rigid ball into the rubber specimen.
The Shore A scale is the most prevalent in the
United States. A “frustoconical” indentor with a spring force that decreases
with increasing indentation is used. The readings range from 30 to 95 points.
Harder elastomers use a pointed conical indentor with the Shore D scale. The
results of the Shore A scale and the IRHD scale are approximately equal over the
same range of resiliency. In elastomers with unusually high rates of stress
relaxation or deformation hysteresis, the difference in dwell time in the two
readings may cause different results. Also, the results of any hardness test
depend on the elastomer thickness. Specified thickness should be used when
conducting these tests.
Due to the mechanical limits of the test
instruments, hardness measurements of elastomers are rarely expressed more
precisely than 5 points.
The surface indentation or hardness usually does
not bear any relation to the ability of an elastomeric part to function
properly. Hardness is a measure of an elastomer’s response to a small surface
stress. Stiffness and compressive modulus measure the response to large stresses
of the entire elastomeric part.
ISO 48(IRHD), ISO 7619(Shore A)
ASTM D1415(IRHD), D2240(Shore