Rock deformation tests to large shear strains in torsion

Experiments performed to high strains in simple shear are necessary to reso lve many important problems in rock deformation, including the simulation o f non-coaxial deformation, the exploration of the influence of strain on rh eology, and the development of mineral segregation, dynamic recrystallisati on, 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. Th e torsion system, which is described briefly, is based on a standard gas-me dium high-pressure high-temperature triaxial deformation machine to which h as been added an additional module allowing rotary shear of a cylindrical s pecimen. In this paper, we discuss procedures for carrying out torsion tests and we analyse the mechanics of torsional deformation. We show how to extract info rmation about the rheological parameters from the torque-twist data, and ho w to deal with the problems posed by the variation of strain within a cylin drical specimen. Procedures for microstructural studies are also set out. F inally, some results on calcite rocks are given to illustrate the applicati on of torsion tests. (C) 2000 Elsevier Science Ltd. All rights reserved.