Journal Papers Published by the Rubber Research Group

"Contributions of time dependent and cyclic crack growth to the crack growth behaviour of non strain-crystallising elastomers"

Author(s): J.J.C. Busfield, K. Tsunoda, C.K.L. Davies and A.G. Thomas

Journal: Rubber Chemistry and Technology, Vol. 75, p. 643-656

Engineering components are observed to fail more rapidly under cyclic loading than under static loading. This reflects features of the underlying crack growth behaviour. This behaviour is characterised by the relation between the tearing energy, T, and the crack growth per cycle, dc/dn. The increment of crack growth during each cycle is shown here to result from the sum of time dependent and cyclic crack growth components. The time dependent component represents the crack growth behaviour that would be present in a conventional constant T crack growth test. Under repeated stressing additional crack growth, termed the cyclic crack growth component, occurs. For a non-crystallising elastomer, significant effects of frequency have been found on the cyclic crack growth behaviour, reflecting the presence of this cyclic element of crack growth. The cyclic crack growth behaviour over a wide range of frequencies was investigated for unfilled and swollen SBR materials. The time dependent crack growth component was calculated from constant T crack growth tests and the cyclic contribution derived from comparison with the observed cyclic growth. It is shown that decreasing the frequency or increasing the maximum tearing energy during a cycle results in the cyclic crack growth behaviour being dominated by time dependent crack growth. Conversely at high frequency and at low tearing energy, cyclic crack growth is dominated by the cyclic crack growth component. A large effect of frequency on cyclic crack growth behaviour was observed for highly swollen SBR. The cyclic crack growth behaviour was dominated by the time dependent crack growth component over the entire range of tearing energy and/or crack growth rate. The origin of the cyclic component may be the formation/melting of quasi crystals at the crack tip, which is absent at fast crack growth rates in the unswollen SBR and is absent at all rates in the swollen SBR.