Development of a neutron diffraction-based strain measurement capability for triaxial loading conditions

Understanding incipient failure conditions associated with the hydraulically- induced fracture of rock is critical to the realization of Enhanced Geothermal Systems. The knowledge of critical stresses in particular defines the basis for rock failure and fracture initiation in all models of fracture propagation. The complex composition of rock requires extensive sample testing to produce a large number of stress states in order to comprehensively define the rock failure envelope. The typical tests performed do not directly measure stresses or strains within the materials and generally measure either bulk deformations or deformations of exposed surfaces. This paper describes the development of a neutron-diffraction based strain measurement technique that will permit the mapping of the lattice strains and macroscopic stresses within a geological material in a triaxial stress condition. It is believed that this capability will provide a powerful tool that facilitates the investigation of a number of rock mechanics topics including the development of improved rock failure theories, the study of unusual rock deformation modes and the primary application interest of the authors, improved rock hydraulic fracture models.