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It is well documented that for the effective modelling of the static and dynamic behaviour of elastomeric components using the finite element (FE) method it is essential to obtain material parameters that are derived from a range of physical tests. For a conventional linear solid, uniaxial testing suffices to fully characterise the material, but for rubber it is necessary to test in at least two deformation modes. Also, the determination of fatigue lives of rubber components is still in its infancy and a reliable, repeatable fatigue test for rubber will have important consequences for component design, functionality, maintenance and cost.
Equi-biaxial displacement has significance in FE simulations since some phenomenological models are based on even-powered strain invariants. For multi-axial deformations, which are most common for rubber components, the second strain invariant (I2) has a pronounced effect on the predicted stress-strain relationships. Also, the initiation of failures and crack propagation are very different in uniaxial and equi-biaxial load cases, so there is a proven need to obtain reliable dynamic equi-biaxial stress-strain data.
Previously, the source of equi-biaxial dynamic data for rubber compounds was mainly derived from tests performed using stretch frames. These tests have numerous deficiencies that are overcome by using the bubble inflation method. Similarly, the high levels of scatter normally associated with fatigue testing of rubber are significantly ameliorated by employing this technique.
An overview of fatigue life theories for elastomers is given in this text and the use of crack propagation approaches and Wöhler (S/N) curves is discussed. Dynamic physical tests and analyses of ethylene propylene diene monomer (EPDM) disc samples are described and insights into the consequences of pre-stressing samples and swelling phenomenon are also offered. The prevalence of large flaws and their influence is briefly discussed and the benefits of applying bubble inflation to characterising a range of viscoelastic phenomena in rubber are highlighted.
Jerrams, S., Murphy, N., Hanley, J.: The Significance of Equi-biaxial Bubble Inflation in Determining Elastomeric Fatigue Properties. Published in the book 'Elastomers' edited by Anna Boczkowska (ISBN 979-953-307-1019-5), InTech.