Determination of Crack-Growth Characteristics Using the Angled Testpiece

Abstract

Abstract The anticipated advantages of the angled testpiece have been borne out with some important provisos. For sufficiently thin testpieces and high strain-energy release rates, it is possible to use the angled testpiece geometry and the machine described to determine the cyclic crack-growth characteristics without directly measuring the crack length or carrying out auxiliary elasticity measurements. The modifications to the theory for thicker test pieces (for which the buckling force is significant) should allow the technique to be applied, but one of the main advantages over the other techniques (the avoidance of auxiliary elastic measurements) is then lost. It has also become apparent that working at low strain-energy release rates with a machine of this sort calls for a very stable temperature. For this reason, the machine's capability to do such measurements has not yet been assessed. There is broad agreement between results from the various techniques for both time-dependent crack growth (angled, pure-shear, and trouser testpieces of SBR gum) and cyclic crack growth (angled, pure shear, and tensile testpieces of SBR gum and NR gum). However, although the fracture mechanics approach appears to be generally of considerable value in expressing the crack-growth properties of rubbers, giving results that are essentially independent of testpiece geometry, the latter feature is not invariably found. In the present work, significant discrepancies were found for NR gum at low values of T. Such discrepancies are being investigated further.