Abstract
KEYWORDS
Elastomers, life prediction, energetic approach method, strain-life, crack growth.
ABSTRACT
Elastomers are vital building blocks in a wide range of products, from medical devices to car braking systems. Failure to perform can put lives at risk, it is therefore important to look at ways of predicting the behaviour of moulded rubber components to make them safer, and to reduce design and manufacturing costs.
One critical elastomer application is in brake-booster diaphragms for automotive braking systems. In order to ensure the safety and reliability of braking systems, the functional life of this rubber brake component has been studied, analysing key factors affecting functional life by means of computer simulations and physical testing of materials and components.
In general, the fatigue resistance of elastomers depends on the composition of the material, the processing conditions and the operating environment. Rubber brake diaphragms are exposed to huge temperature variations between –40ºC and 120ºC. The chemical structure of the material changes also with time in the environment so ageing may also need to be considered. The work presented in this paper aims to show the application of the so-called energetic approach, which is based on a combination of fracture mechanics and strain-life curves, to the results obtained from simulations of finite element models on a brake booster diaphragm under service loads. The basis of the fracture mechanics approach is the use of the strain energy release rate, or tearing energy, as the parameter governing the crack growth behaviour, while the strain-life approach relates the fatigue life to the strain state in the material under specific conditions. This approach aims to take into account the influence of the temperature and the degradation caused by a controlled material ageing, so that a relationship between the durability of the component, in terms of life cycles to failure, and finite element results on the component, in this case the maximum principal strain level, can be established. The results obtained from this approach are compared with experimental results from durability testing on real components to assess the reliability of the method as well as its limitations.
This work was carried out as part of the “FULPEC“ Project, G5RD-CT-1999-00165, funded by the European Community under the “Competitive and Sustainable Growth Programme” (1999-2002), as well as as part of the “PVEC” Project, DPI2001-2406, funded by the Technology and Science Ministery of Spain under the “Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica 2004-2007”.