In situ measurement of elastic and total strains during ambient and high temperature deformation of a polygranular graphite

Abstract

In situ neutron diffraction and synchrotron X-ray diffraction, combined with image correlation analysis of 2D optical and 3D X-ray tomography datasets, have been used to investigate the relationship between elastic lattice strain and total strain during deformation of Gilsocarbon (IM1-24) polygranular nuclear grade graphite. The specimens were flat-end Brazilian discs under diametral loading, such that a compressive-tensile biaxial stress state was developed in the central region. The X-ray study was at ambient temperature, and the neutron diffraction was conducted at temperatures from ambient to 850°C. When under compression, there is a temperature-insensitive linear relationship between the total strain and the lattice strain that is measured perpendicular to the graphite basal planes. However, when under tensile stress, the total strain and elastic strain relationship is temperature sensitive: below 600°C, the lattice tensile strain saturates with increasing total tensile strain; above 600°C, significantly higher tensile lattice strains are sustained. The saturation in tensile lattice strain is attributed to microcracking in the graphite microstructure. Improved resistance to microcracking and damage tolerance at elevated temperature explains the increase in tensile strength of polygranular graphite.

In Situ Mechanical Loading and Neutron Bragg-edge Imaging, Applied to Polygranular Graphite On IMAT@ISIS

“…6. This shows the expected bi-axial deformation and compares well to previous correlation studies performed on similar specimens [28]. These particular sets of images have also been correlated using commercial DIC software (LaVision DaVis 8.40) and a comparison that verifies the performance of the demon code is presented in the supplementary materials.…”

“…Graphite has a high neutron elastic scattering cross-section [26], a strong Bragg edge associated with its basal plane [27], making it an ideal candidate for this study. There is a lack of evidence for the mechanisms of deformation of bulk polygranular graphites such as Gilsocarbon, which exhibits a change in deformation behaviour at elevated temperatures, leading to improved toughness and tensile strength [28]. Due to its polycrystalline nature and the ABAB stacking in hexagonal graphite crystals, shear by basal dislocation glide occurs readily [29] and graphite crystals also exhibit kink band formation [30].…”

Anharmonic phonons and anomalous thermal expansion of graphite

“…Spectroscopic studies [18][19][20] of the phonon spectrum have been reported using Raman, infrared and neutron scattering techniques. In situ measurement [21]of elastic and total strains during ambient and high temperature deformation of a polygranular graphite has been reported. The interfacial thermal conductance between graphite and substrate copper has been tuned [22] via anisotropic elastic properties of graphite.…”

Microstructure characterization of IG110 and reactor pebble graphite using synchrotron micro X‐ray diffraction 2D maps

“…Interest in nuclear graphite has gained greater attention in recent years for the development of high-temperature gas-cooled reactors and the molten salt reactors [1][2][3] as graphite acts as both the moderator and reflector in these reactor designs. The behaviors of nuclear graphite under fast neutron irradiation such as the dimensional change and irradiation creep are critical to the safety and economy of these reactor designs.…”