A multi-scale three-dimensional Cellular Automata fracture model of radiolytically oxidised nuclear graphite

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.

Section: Discussionmentioning

confidence: 99%

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

Liu

Zillhardt

Earp

et al. 2020

“…The initial toughness of the irradiated graphite is lower than that of the unirradiated graphite. Previous studies have reported increased fracture resistance for irradiated graphite in the absence of oxidation [16][17][18][19], so the lower toughness in this work may be largely due to the increased porosity from oxidation, which reduces the tensile properties of graphite [9].…”

Section: Discussionmentioning

confidence: 72%

“…For the non-linear model, the elastic modulus was decreased as a function of the maximum principal strain in the element. It is physically-based [9], and was fitted to experimental data for non-irradiated Gilsocarbon graphite [41]. This non-linear model [39] assumes tensile strain causes microcracks that increase the porosity in proportion to the strain.…”

Section: Finite Element Analysismentioning

confidence: 99%

“…Lattice defects induced by the interaction of fast neutrons with the graphite crystal cause dimensional change, generate microstrain, and change the physical properties of graphite [2,6]. Oxidation may also occur in some reactor environments [7], and the increased porosity has consequences for graphite properties including the elastic modulus and strength [8,9].…”

Section: Introductionmentioning

confidence: 99%

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Assessment of the fracture toughness of neutron-irradiated nuclear graphite by 3D analysis of the crack displacement field

Jin

Wade-Zhu

Chen

et al. 2021

Digital volume correlation of in situ synchrotron X-ray computed tomographs has been used to measure the three-dimensional displacement fields around quasi-static propagating cracks in neutron irradiated and unirradiated graphite in small test specimens of the double cleavage drilled compression geometry. The crack tip location and crack opening were extracted from the displacement fields using a phase congruency edge detection method as cracks were propagated over ~5 mm. The cracks propagated in mode I, maintaining a constant crack opening angle that was ~50% smaller for the irradiated graphite. 3D finite element simulations, using the measured full field displacements as boundary conditions, obtained the critical elastic strain energy release rate for crack propagation by calculation of the domain * Author's copy of paper accepted for publication in Carbon,

“…13(l), the former is largely unaffected by neutron irradiation and radiolytic oxidation. This has been observed to much higher weight losses in both thermally and radiolytically-oxidised nuclear graphite [15,53,54]. Meanwhile, the latter is only amplified by the irradiation-induced "hardening", which both strengthens and stiffens the filler particles of the graphite, and preferential radiolytic oxidation of the binder phase [54], that simultaneously weakens the graphite matrix (irradparticle σ = 652 MPa, E = 37.5 GPa vs. irradmatrix σ = 357 MPa, E = 8.6 GPa [46]).…”

Section: Crack Propagation and Arrestmentioning

confidence: 95%

4D synchrotron X-ray microtomography of fracture in nuclear graphite after neutron irradiation and radiolytic oxidation

Wade-Zhu

Krishna

Bodey

et al. 2020

Herein, the first study is presented using 4D synchrotron X-ray microtomography to capture all stages of crack development in neutron irradiated and radiolytically oxidised nuclear graphite.Employing a novel loading setup, specimens of Gilsocarbon graphite, both unirradiated and irradiated at 301 °C to 19.7 x 1020 neutrons/cm2 (~2.6 displacements/atom (dpa)), were loaded to generate a crack.All stages of the fracture process were then captured using synchrotron X-ray imaging. Reconstructed tomographic images and 3D segmented crack volumes have been used to observe and analyse the irradiation-induced evolution of the graphite microstructure as well as corresponding changes in the crack initiation, propagation, and arrest behaviour of graphite after neutron irradiation. Close examination of the applied stress-strain curves highlights the suppression of micro-crack-based damage accumulation in irradiated graphite. Moreover, as well as the crack-bridging and deflection mechanisms characteristic of unirradiated graphite, crack arrest in the irradiated graphite is shown to be significantly influenced by crack tip blunting. This change is associated with the growth of the open pore structure of graphite, specifically the enlargement and increased frequency of macro-pores, resulting from the simultaneous radiolytic oxidation of the graphite microstructure during neutron irradiation.