Rock deformation tests to large shear strains in torsion

Experiments performed to high strains in simple shear are necessary to resolve many important problems in rock deformation, including the simulation of non-coaxial deformation, the exploration of the influence of strain on rheology, and the development of mineral segregation, dynamic recrystallisation, and other aspects of microstructure. Using a new torsion system we are able to simulate both the high strains and the simple shear style commonly observed in naturally deformed rocks, and to make accurate measurements of shear stress, strain and strain rate at high temperatures and pressures. The torsion system, which is described briefly, is based on a standard gas-medium high-pressure high-temperature triaxial deformation machine to which has been added an additional module allowing rotary shear of a cylindrical specimen. In this paper, we discuss procedures for carrying out torsion tests and we analyse the mechanics of torsional deformation. We show how to extract information about the rheological parameters from the torque-twist data, and how to deal with the problems posed by the variation of strain within a cylindrical specimen. Procedures for microstructural studies are also set out. Finally, some results on calcite rocks are given to illustrate the application of torsion tests. (C) 2000 Elsevier Science Ltd. All rights reserved.