Lattice strain measurement in rock sample by neutron diffraction technique

Strain gauge and mechanical extensometers are commonly used to measure strain in rock samples. In recent years, diffraction techniques with X-rays and neutrons for investigating strain in engineering materials have been developed. Strain measurements using diffraction technique are based on Bragg's law. Lattice spacing changes with strain, which induces peak shift of Bragg peak. Strain value can be estimated from this peak shift value. Strain measurements using the world's highest intensity neutron beam can be performed at Japan Proton Accelerator Research Complex (J-PARC) constructed at Tokai in Ibaraki. Neutron powder diffractometer dedicating to investigate strain state in engineering materials has been constructed at BL19 in J-PARC, which is named as The Engineering Materials Diffractometer 'TAKUMI'. In order to examine applicability of the diffractometer to rock materials, in situ neutron diffraction experiments on rock samples under uni-axial compression have been performed. Higher resolution strain data has been obtained in shorter time compared to other diffractometers. In addition, neutron diffraction peaks of not only major component, such as quartz, but also minor components, such as feldspars, could be observed. Anisotropy of strain with respect to the quartz crystal orientation and discrepancy between macroscopic strain (measured by strain gauge) and lattice strain (measured by neutron diffraction) were also recognized. Change in peak width with respect to stress magnitude showed a different behavior depending on rock type. Strain measurements using neutron diffraction technique give us new insight in rock deformation which cannot be obtained by common technique. (author)