Study of stress relaxation and strain recovery in elastomeric compounds used in pipe seals

Elastomer compounds have been used in pipe seals for many decades, with their main problem, the stress relaxation due to various mechanisms. In these applications, a service life of more than fifty years is expected- and obtained. The purpose of the programme was to establish mathematical models to enable longevity of seals to be predicted from laboratory measurements. It seemed that stress relaxation in compression, together with strain recovery measurements, would provide a basis for such an investigation. Accordingly, such tests were carried out with up to 23 rubber compounds, at temperatures of 5°, 23°, 40°, 70°, 100° and and 125° C, with environments of air, mains water and town gas. The work outlined above is the main thrust of the programme, the data obtained being the raw material for the development and assessment of mathematical models associated with the viscoelasticity of the various rubbers. This work has been supported by supplementary investigations, including: effect of loading rate step changes in temperature inclusion (or not) of mechanical working of the specimen before testing use of solid phase lubricant changing the dimensions of the specimen comparison of air environment with water immersion, and town gas comparison of air environment with vacuum and nitrogen An important facet of the work has been the comparison of tensile behaviour with the bulk of the work carried out in uniaxial compression. A comparatively novel property has been introduced in this work: strain recovery after compression. This has proved to be a property related to 'permanent-set', yet more amenable to interpretation and correlation, recovery data are now available for most of the conditions in the stress relaxation programme whilst additional results have been obtained from compression set equipments. The other objective of this programme is to investigate short comings of BS 2494-1986, the current rubber seal specification, and to suggest new techniques such as strain recovery. Finally, the behaviour of seals in simulated service has been investigated to compare with the data generated in the main programme.