Review and evaluation of the double-torsion technique for fracture toughness and fatigue testing of brittle materials

Abstract

This paper reviews the double-torsion (DT) test as an experimental technique for the measurement of fracture toughness and slow-crack-growth behavior in brittle materials based on the authors' experiences and an evaluation of current literature. The DT technique has numerous advantages due primarily to the fact that the stress intensity is independent of crack length, at least for the central half of the specimen. Although the technique was first proposed about 20 years ago, and has been used extensively since then, there are a number of important unresolved questions concerning the methodology. To date there has been no standardization of test procedure or specimen geometry. A review of specimen geometries in use indicates that the proportions that are most commonly employed (based on the literature and experience) are width:W, length: 3W, thickness:W/6–W/15. Grooves on both the top and the bottom surfaces have been used to guide the crack, however it has been found that ungrooved, but very accurately aligned, specimens give the best results. Theoretical thickness-correction factors which account for both relatively thick specimens (with respect to width) and the effect of the size of the loading points, have been considered. The effect of crack-front profile on measured values of crack velocity and stress intensity is contentious. Althogh the stress intensity,K, varies along the crack front, the front merely translates axially. For the presentation of crackvelocity stress-intensity (V-K) data, the consensus seems to be that the only rational velocity to use is that based on the crack's intersection with the tensile surface. Despite some of the shortcomings mentioned above, the DT technique is widely accepted and gaining in popularity. It is particularly useful under cyclic fatigue conditions for investigating the effect of a change in a single parameter on crack-growth rate, using the very effective and elegant ‘changeover’ method.