Rubber Hardness

The standard test methods used for measuring the hardness of rubber and plastic are either Shore or IRHD - these are specified in BS EN ISO 868 [4] and BS 903-A26 (ISO 48) [3] respectively.

Shore Hardness

Shore hardness specifies methods for determining the hardness of materials by means of durometers of two types: type A for softer materials and type D for harder materials [4], although the ASTM standard [1] covers twelve durometer scales. The method permits measurement of the initial indentation, the indentation after a specified period of time, or both. The indentation hardness is inversely related to the penetration and is dependent on the modulus of elasticity and the viscoelastic properties of the material. The shape of the indenter, the force applied, and the test duration influence the results obtained. The Shore durometer consists of a reference presser foot, an indenter, an indicating device, and a calibrated spring that applies the force to the indenter. The difference between the type A and type D durometer is in the shape of the indenter and the calibrated spring (Table 1).

Table 1:  Shore hardness scales

Shore durometer

Indenter

Applied force, F / mN

Type A

Hardened steel rod 1,1 mm - 1,4 mm dia, with a truncated 35º cone, 0,79 mm dia.

F = 550 + 75 HA

Type D

Hardened steel rod 1,1 mm - 1,4 mm dia, with a 30º conical point, 0.1 mm radius tip

F = 445 HD

Note: It is recommended that measurements be made with the type D durometer when the value
is above 90 HA, and with a type A durometer when the value is less than 20 HD.

The units of hardness range from 0 for the full protrusion of 2.50 mm to 100 for no protrusion. The force is applied as rapidly as possible, without shock, and the hardness reading made after a duration of 15 s ± 1 s. If an instantaneous reading is specified, the scale is read within 1 s of force application. There is no simple relationship between results obtained with one type of durometer and those obtained with either another type of durometer or another instrument for measuring hardness.

The operation of a durometer is based on the penetration of an indentor into the material being tested under specific conditions.

 

The movement of the indentor is related to the durometer hardness of the material, e.g. the more the indentor moves the higher the indicated durometer value of the material.

The penetration of the indentor into the test specimen is inversely proportional to the durometer hardness value of the test specimen, e.g., the more indentor penetration into the test specimen, the lower the durometer hardness value.

 

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Durometer Instrument Function

Many hardness testing methods employ a technique that "permanently" deforms the material being tested, the depth of the indentation or other characteristics of this "static deformation" are then measured to determine a hardness value.

In durometer hardness testing, the material does not remain "permanently" deformed when an indentation is made, i.e., the material is generally highly elastic and the penetration depth of the indentor is not static, nor is the indentation in the material permanent.

To overcome these material specific characteristics, a durometer measures the relative movement of the indentor, rather than the depth of the indentor penetration, or other characteristic of a "static deformation" of the material.

Due to the potential energy (pre—load) of the calibrated spring, the elastic characteristics of the material and other factors, the determination of indentor movement may not necessarily begin at the moment of indentor to specimen surface contact and it may also change over time, thus requiring a determination of indentor movement that is also time specific.

 

This movement is translated, either electrically, mechanically, or electro—mechanically to an analog or digital scale indicating the relative durometer hardness value. The determination of the final durometer hardness value is achieved by visually reading the analog indicating hand within 1s of the "moment of cessation" of the numerical increase of the indication, which is the generally agreed upon time specific reference.