An Evaluation of Double-Torsion Testing—Analysis

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Abstract
The double-torsion test configuration has many advantages over conventional fracture mechanics configurations for the evaluation of subcritical crack growth parameters and fracture toughness. These advantages—such as crack-length independence, four-point loading, simple specimen geometry, and ease of precracking—have been responsible for the increased use of the double-torsion configuration in recent years. However, before double-torsion testing can be adopted as a standard configuration, a number of unresolved questions need to be answered about the double-torsion specimen and about the experimental techniques involved in its use. The purpose of this paper is to address some of these questions and to indicate where further effort is needed to establish the validity of double-torsion testing. Assumptions of a compliance analysis for the double-torsion configuration are examined with particular attention given to the influence on experimental results that the violation of any of these assumptions may have. An analytical expression is derived for the compliance of a finite-thickness beam in torsion and is compared to the empirical compliance. Again the emphasis is on the assumptions of the derivation and how some of these restrictions might be relaxed.