The double torsion loading configuration for fracture propagation: an improved methodology for the load-relaxation at constant displacement

For most materials the dynamics of subcritical crack propagation during str ess-corrosion can be described uniquely by a relationship between the mode- I stress intensity factor KI and the crack velocity v that generally has th e form of a power law. In last 30 years, the double-torsion load-relaxation test has shown to be the most reliable method for measuring such a relatio n. The standard analysis, developed by Evens (J Mater Sci 1972,7.1137-46), is based on an analytical approxatimation that fails to accurately describe the specimen compliance outside a narrow region in the center of the speci men. This paper deals with the implications on data inversion of the exhaus tive three-dimensional finite-element analysis recently performed by Ciccot ti (J Am Ceram Soc 2000, in press) on double-torsion specimens. The results are presented in terms of corrective coefficients to the classical analyti cal approximation. A full methodology is developed for the numerical implem entation of such corrections. By numerically simulating some relaxation tes ts, the classical analysis based on the analytical approximation is shown t o generally underestimate the stress-corrosion index up to 30% even if the most conservative operational constraints are satisfied. On the contrary, t he operational constraints can be comfortably relaxed as a consequence of t he capability of correcting the finite size effects in relation to the diff erent experimental parameters. (C) 2000 Elsevier Science Ltd. All rights re served.