Effect of materials variables on the tear behaviour of a non-crystallising elastomer

Abstract

Crack growth rates (r) were measured in pure shear test specimens as a function of strain energy release rate (G) for a non-crystallising SBR elastomer. Measurements were made as a function of: extent of swelling with Dibutyl Adipate; carbon black content; and crosslink density. In some cases experiments were carried out over a range of temperatures. In most cases the resulting G versus r plots showed a clear transition from rough to smooth crack surface behaviour with increasing crack growth rate, with an intervening slip/stick region. In the high speed/steady tear/smooth region the value of G necessary to drive a crack at a given rate was determined largely by the magnitude of the visco-elastic losses in the crack tip region, increasing with: decreasing temperature; increasing molar mass between crosslinks; decreasing extent of swelling; and increasing carbon black content. However G was independent of specimen thickness in this region suggesting that crack tip effects were minimal. In the low speed/rough region changes in the magnitude of G with materials and temperature/rate variables could not be explained by changes in visco-elastic loss alone. Furthermore the magnitude of G increased significantly with increasing specimen thickness. This suggested that in this region cavitation ahead of the growing crack tip resulting from dilatational stresses determined the crack tip diameter, and hence the magnitude of G.