Issue34

T.-T.-G. Vo et alii, Frattura ed Integrità Strutturale, 34 (2015) 237-245; DOI: 10.3221/IGF-ESIS.34.25 241 brick is slotted and externally loaded in order to replicate the internal stress state of graphite bricks after years of service and exposition to oxidation and irradiation. The crack path is two-dimensional and planar. Figure 3 : Geometry of the graphite bricks. Section (a) , lateral view (b) , and representation of the methane holes (c) . Figure 4 : Crack propagation methods tested for AGR graphite bricks: X-FEM, CZM, Damage modelling and Configurational Forces. From the results obtained in the benchmark study, the most promising one developed in Code_Aster was the use of the X-FEM. In this case, the crack paths were similar to the experimental ones. Incremental crack growth was based on energetic criteria relevant to quasi brittle materials: the direction of propagation along the crack front was based on the maximum hoop stress criterion, and the length of propagation for each step of calculation depends on the value of the strain energy release rate along the crack front. The crack paths are presented in Fig. 5 for different loading scenarios using X-FEM, and compared with their experimental counterparts. Planar crack propagation in ageing AGR graphite brick The behaviour of these ageing graphite bricks has been studied for many years. Fast neutron irradiation causes graphite to shrink initially before expanding again to exceed its original volume. The attenuation in dose between the bore and the outside causes differential rates of graphite shrinkage that generate internal stresses that vary through life. Fast neutron irradiation also causes the elastic modulus of the material to increase significantly early on, while oxidation causes  a)  b) c)