Mechanical Properties Investigation by Uniaxial Load Conditions on Porous Sandstone Samle Using Neutron TOF Diffraction Method

Abstract

The knowledge of the stress and strain state in rock sample is of importance in the understanding of the rock properties for the interpretation of geodynamic processes and for geotechnical applications such as mining and tunneling. Recently, the topic of induced seismicity has become a major issue in geoenergy production (gas production, geothermal). Thus, the role of tectonic stress for the initiation of rock failure is of high scientific but also economic interest. The investigation of deformation with conventional lab scale experiments allows the determination of macro-strain in the cm- to m-range. On the nano to micro-scale, the application of diffraction methods offers the investigation of the strain which is localized in the crystal lattice, the so-called intra-crystalline strain. This localization of strain in a sample with dimensions in cm-range is achieved by strain scanning. Because of the low absorption properties of neutrons in matter, like metals and minerals, neutron diffraction is an excellent method for the investigation of strain in bulk samples, especially in multiphase samples, like rocks. The neutron time-of-flight strain diffractometer EPSILON operated at the pulsed neutron source IBR-2M at the JINR Dubna is designed for the investigation of residual and applied intra-crystalline strain of bulk samples. Because of the large wavelength-range (up to \({{\lambda \;}} = {\;}7.8{\;\mbox{\AA}}\)) diffraction pattern with d spacings up to \(d = 5.6~\,{\mbox{\AA}}\) can be investigated. Using the long flight path of about 107.03 m a good spectral resolution is achieved. That allows the investigation of multiphase rocks containing minerals with lower crystal symmetry. The diffractometer is equipped with an uniaxial applied stress device, allowing load states up to 100 kN, e.g., 150 MPa with sample diameters of 30 and 60 mm length. The investigations concentrated on 3 locations within the sample: in the centre, at the rim and at a location in-between (half-radius). To avoid immediate rock failure a pre-experiment to determine the failure stress has been performed. Sample OU-2 (sandstone) has been scanned for 4 load steps (0, 11.65, 23.5, 35.21 MPa axial load). For each load level, the intra-crystalline strain has been determined in the direction of \({{\sigma }_{1}}\) and \({{\sigma }_{3}}\). A newly developed analysis tool which uses all diffraction peaks for detection of the lattice deformations has been applied to determine the in situ modulus of elasticity and the Poisson´s ratio at different positions (rim, half-radius, center) within the sample.