Elastomer tests

Elastomer testing is the foundation of a reproducible material quality. Whether an O-ring, a seal or a moulded part – the properties of a component are only as reliable as the control of the compound from which it is made. All three stages of the process chain are tested: raw materials, unvulcanised rubber mix and the finished vulcanisate.

Comparable results presuppose standardised test methods and standardised specimen geometries. In Germany, the DIN regulations are binding. For international projects – particularly with North American partners – the ASTM standards are used in addition. Unless expressly stated otherwise, tests are carried out at room temperature.

For practical purposes, the following applies: the characteristic values determined on standardised specimens serve to make materials comparable. They are not readily transferable to the finished component, since component geometry, wall thickness and manufacturing process can influence the properties under real conditions.

Non-destructive and destructive test methods

Elastomer tests are divided in principle into two categories.

Non-destructive test methods are used preferentially in ongoing production – as incoming inspection for delivered materials, as in-process testing during manufacture and during acceptance inspections. They provide information about dimensions, surface, hardness and visual condition without damaging the component. The hardness is measured here, depending on the component geometry, either to Shore A or to IRHD (International Rubber Hardness Degree) – the IRHD method is specially optimised for finished components such as O-rings with small, round cross-sections.

Destructive test methods intervene more deeply in the material structure and provide the actual mechanical, technological and chemical characteristic values. They require standardised specimens that can no longer be used after the test. A distinction is made between mechanical tests (e.g. tension, compression, impact), technological tests (e.g. ageing, media resistance) and chemical analyses.

Hardness testing

The hardness describes the resistance of an elastomer to the penetration of a defined indenter. The standard method is the Shore A test: a truncated-cone needle is pressed into the rubber surface by a calibrated spring. The softer the material, the deeper the needle penetrates – and the lower the value displayed on the scale from 0 to 100 Shore A. The meaningful measuring range of the Shore A instrument lies between 10 and 90 Shore A. For harder compounds and hard rubber, the Shore D method is used, which works with a more pointed needle and a stronger spring force.

Hardness testing feeds directly into the design of sealing points: it influences contact pressure, assembly force and the behaviour under gap conditions.

Overview of important test methods

The following test methods cover the entire relevant range of elastomer properties and are each described in detail in their own glossary articles:

• Tensile test – determination of tensile strength, elongation at break and stress values at defined elongations
• Compression test – assessment of the material behaviour under compressive load
• Compression set to DIN ISO 815-1 – recovery capability after sustained compression; a central characteristic value for the long-term sealing effect
• Tear propagation resistance to DIN ISO 34-1 (formerly DIN 53507 / 53515) – resistance of a pre-cracked specimen to further tearing
• Abrasion test to DIN 53516 – wear resistance under abrasive loading
• Rebound resilience to DIN 53512 – a measure of the elastic proportion of the behaviour; high rebound resilience indicates low damping behaviour
• Dynamic fatigue test – assessment of the dynamic fatigue behaviour under cyclic loading
• Impact test – testing of the resistance to sudden loading
• Torsion – assessment of the torsional behaviour, relevant for components subjected to twisting
• Hardness testing – standardised determination of the Shore hardness, DIN ISO 48-2
• Hot-air ageing to DIN 53508 – accelerated ageing test to assess the heat resistance
• Behaviour towards liquids, vapours and gases to DIN 53521 – assessment of the chemical resistance on the basis of volume swelling and property changes after immersion in a medium