SATRA’s STD 645 resilience tester
A description of footwear component tests performed on this equipment.
by Peter Allen
Rubbers or other elastomers used in footwear must be suitable for the intended purpose. One of the characteristics that can be confirmed is the material’s resilience. When a material is deformed, energy is expended. The ability of a material to store the energy and then release it when the force responsible for the deformation is removed is known as the material’s ‘resilience’. A highly resilient material will return a larger amount of energy compared to a material with lower resilience. Resilience in the materials used in footwear can make an important contribution to the ability of the product to perform, for example, in the provision of shock absorbency and energy return.
One additional relationship relating to resilience, expounded in theory and observed by SATRA, relates to the slip resistance of elastomers on wet surfaces. Rubbers with low resilience are expected to give better slip resistance on wet surfaces compared to rubbers with high resilience, all factors being equal.
This article describes one of SATRA’s test devices which can be used to measure the resilience of rubbers and other elastomers. The SATRA STD 645 resilience tester (previously known as the EPH-50) facilitates a simple method of measuring resilience properties, and is commonly used in conjunction with hardness testing for basic quality control of rubbers.
ISO 4662:2009 – ‘Rubber – vulcanised or thermoplastic – determination of rebound resilience’ is an example of a typical method which can be used to assess the resilience of rubbers. Different types of resilience tester are included within this standard. SATRA STD 645 is a Schob-type pendulum (as outlined in Annex B.3 of the standard). Deformation of rubber is due to energy input, part of which is returned when the rubber returns to its original shape. The proportion of the energy that is not returned as mechanical energy is largely dissipated as heat within the rubber. The ratio of the energy returned to the energy applied is what is termed ‘resilience’. When the deformation is as a result of an indentation due to a single impact, this ratio is termed the ‘rebound resilience’.
There are a number of factors which need to be considered when obtaining a resilience reading for an elastomer. For example, resilience varies significantly with temperature, and is also affected by ‘strain distribution’, ‘strain rate’, ‘strain energy’ and ‘strain history’, all of which need to be taken into account in the development of test devices and associated test methods. Testing should be conducted at specified temperatures on conditioned specimens, and the temperature reported with the result. Removing the variation of strain distribution is achieved by defining the dimensions of the indentor, the test specimen and the method used for specimen holding. The strain rate is maintained at the same level by ensuring a consistent velocity of the indentor at impact. The strain energy is maintained at a constant level by providing an indentor with defined mass, along with a consistent velocity at impact. In order to address the issue of strain history, the results are recorded after a number of conditioning impacts have been carried out. The details of the required strain history conditioning are specified in the appropriate test method.
Using the tester
When using the SATRA STD 645 resilience tester, a circular material specimen is located against a vertical polished anvil and secured by a spring-loaded compression ring (figure 1). The impact is provided by releasing a low-bearing-friction pendulum from a horizontal position by means of an electromagnet. This pendulum is fitted with a dome-shaped indentor of known mass and shape. After impact with the test specimen, the pendulum rebounds. After it strikes the specimen, the degree of angular rebound is measured precisely and displayed digitally as a percentage of the total 90º swing from the horizontal starting position. After the pendulum reaches its maximum rebound height, it is caught by hand (figure 2) before it strikes the specimen a second time. The pendulum can then be raised to the horizontal position, where it is held by the electromagnet ready for another test impact.
The STD 645 equipment incorporates a heavy solid steel base to ensure a very high percentage of the pendulum’s energy is transmitted to the specimen, and not to the external environment. The device can also be bolted securely to a support bench for extra stability and mass.
A typical specimen size – for example, as specified in ISO 4662 – is a disc with a diameter of 29mm +/- 0.5mm and a thickness of 12.5mm +/- 0.5 mm. The SATRA STD 645 has an adjustment facility which allows the pendulum to be set relative to the thickness of the material. This is to ensure that, at the point of impact, the pendulum is vertical, and so travels through a 90º swing up to the point of impact. The device also incorporates adjustable feet and a level indicator to ensure that it is set accurately on installation. This will give the required angle of swing from horizontal to vertical. Data is also sent to a USB port, where it can be stored on a memory stick if required.
This simple-to-use device allows an assessment to be readily made of an elastomer’s rebound resilience as part of the assessment of new materials in product development, or for quality auditing production specimens or materials going into footwear production.
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This article was originally published on page 40 of the March 2014 issue of SATRA Bulletin.