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What is IRHD hardness?

table of contents

  1. Characteristics of the test cycle with respect to Shore hardness
  2. Macro and micro IRHD: how to write the final value?
  3. IRHD and Shore A hardness: are they valid alternatives?

What is IRHD hardness?

The International Rubber Hardness Degree (IRHD) was introduced in the 1960s as a standardised method for measuring the hardness of elastomer materials, such as rubber.

The goal was to create a hardness measurement method capable of offering a more thorough and reproducible analysis with respect to Shore hardness.

Test cycle characteristics versus Shore hardness

Imagine the IRHD hardness tester as a graduated gauge which, when held perpendicular to the surface being tested, applies a specific force according to the chosen scale for a total of 5 seconds.

The hardness value is calculated on the basis of the initial and final indentation difference of the spherical indenter in a rubber sample.

Due to the shape of the indenter and its field of application – particularly soft elastomers – the test cycle leaves no permanent marks on the test sample.

Unlike the Shore hardness, which has as many as 13 different scales according to the ASTM D2240 and ISO 48-4 standard, the IRHD hardness only has 8.

If you use or have ever used Shore hardness, you will certainly have noticed something very interesting from the table.

The testing of non-planar surfaces – as it is the case with the widely known O-rings – is regulated by the ISO 48-2 standard.

This is a considerable advantage of the IRHD test over the Shore hardness test, which, on the contrary, has no such specific similar application, except with specially designed accessories capable of respecting the orthogonality of the indenter in relation to the surface of the sample being tested.


It is particularly important to bear this in mind, as it is directly related to the writing of the final IRHD value.

Macro and micro IRHD: how to write the final value?

Let us take the following value as an example and analyse it: 58 IRHD SN

IRHD and Shore A hardness: are they valid alternatives?

There are many similarities, but also many the differences, between the two.

After all, we must point out that Shore hardness is the mother of IRHD.

However, because of this relation between the two, there is a tendency to use them as if they could be directly converted into each other.

The truth, however, is that the regulations see these conversions as incorrect.

Scientific ITP-type IRHD and Shore A hardness comparison studies (inter-laboratory tests, i.e. tests carried out internally by several certified laboratories) have been carried out, which have established similarities and differences between specific scales for certain parameters such as reproducibility and precision.

During these direct comparisons, similarities emerged in terms of accuracy but not in the reproducibility of the results.

(see ISO 48-4:2018; Rubber, vulcanized or thermoplastic — Determination of hardness — Part 4: Indentation hardness by durometer method (Shore hardness) page 13 para. A.4.2)

Consequently, we can say that the two hardnesses are comparable, but not convertible.

But then, how does one chooses between IRHD and Shore A hardness? And above all, what parameters must one consider to be sure of one’s choice?

To answer this question, we have to take into consideration different aspects, such as:

  • type of polymer to be tested
  • size and thickness of the material to be tested
  • quality objectives to be met by the product in its end use

In order to choose which hardness method best meets your business needs, contact us, we will be happy to guide you and find out the ideal solution for your quality control department.


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