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 stress-corrosion can be described uniquely by a relationship between the mode-I stress intensity factor KI and the crack velocity v that generally has the 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 relation. 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 specimen. This paper deals with the implications on data inversion of the exhaustive three-dimensional finite-element analysis recently performed by Ciccotti (J Am Ceram Soc 2000, in press) on double-torsion specimens. The results are presented in terms of corrective coefficients to the classical analytical approximation. A full methodology is developed for the numerical implementation of such corrections. By numerically simulating some relaxation tests, the classical analysis based on the analytical approximation is shown to generally underestimate the stress-corrosion index up to 30% even if the most conservative operational constraints are satisfied. On the contrary, the operational constraints can be comfortably relaxed as a consequence of the capability of correcting the finite size effects in relation to the different experimental parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.