The determination of the elastic properties of an anisotropic polycrystalline graphite using neutron diffraction and ultrasonic measurements

The elastic properties of an extruded graphite (GR-280) used in nuclear industry have been examined. The lattice preferred orientation was determined by time-of-flight neutron diffraction that revealed weak texture with a texture index of less than 1.2. The bulk elastic properties of polycrystalline graphite with such a texture have been calculated using various averaging methods and compared with the properties obtained from the measurements of the longitudinal sound velocities, performed using special equipment at different hydrostatic pressures up to 150 MPa. The static elastic modulus of the GR-280 graphite as well as the diffraction elastic modulus was measured in situ by high resolution neutron diffraction by observing the shift of the (0 0 2) Bragg reflection under uniaxial loads up to 20 MPa. The static elastic moduli of two pyrolytic graphites have also been measured for comparison. It was shown that the anisotropy of the elastic properties of reactor graphite GR-280 is due to the crystallographic texture formed during the extrusion process, but the internal pores and microcracks are not closed even at a pressure of 150 MPa and they greatly influence the exact values of the bulk elastic moduli of graphite.