International Rubber Hardness Degree (IRHD)
International Rubber Hardness Degree testing will also result in a value of hardness between 0 and 100. The difference is that the value on the IRHD scale is non-linear and is related to the Young’s Modulus of the material. The IRHD scale is chosen so that a material with a Young’s Modulus of zero will be represented by 0 IHRD hardness and a material with infinite Youngs modulus will correspond to a hardness of 100.
There are three types of material hardness which can be measured:
- Normal hardness test - appropriate for rubbers within the hardness range of 30 – 95 IRHD.
- High hardness test - appropriate for rubbers within the hardness range of 85 – 100 IRHD.
- Low hardness test - appropriate for rubbers within the hardness range of 10 – 35 IRHD. Micro test test - a scaled down version of the normal test, appropriate for rubbers within the hardness range of 35 – 85 IRHD.
The International Rubber Hardness Degree is measured by indenting a spherical ball into a material sample under two different loads. The difference in the depth of indentation under both loads, D, is measured. The value for IRHD for the material can be found using the conversion table published in ISO 48: 1994.
Table 1: Conversion of Values of D to IRHD for Use in Normal Hardness Method
IRHD is a non-destructive method of testing hardness, and it will not leave an indentation in the rubber. The IRHD micro test can be carried out on materials between 1 and 5mm thick. This makes this method of testing appropriate for use on thin products such as O-rings. This is beneficial as it will give a value of hardness for the particular product and not just for the material used. It has been found that shape of the material does not greatly affect the overall hardness of the product.
Both IRHD and shore scales give measurements of hardness on a scale of 0 - 100 degrees. The difference between these two methods comes from the fact that the Shore Scales are linear, based on the deflection of the rubber, whereas IRHD is a non-linear scale related to the elastic modulus of the material. For highly elastic rubbers, Shore A and IRHD give comparable values. A comparison of hardness values for IRHD and Shore A Hardness is shown in Table 1.
Table 2: Comparison of IRDH and Shore A Hardness.