Experimental And Numerical Studies On Fracture Performance Of Nuclear Graphite

Nuclear graphite is often used as the main structural material of high temperature gas-cooled reactors（HTGR）.In the condition of use,graphite components will sustain thermal stress,mechanical load,and potential seismic action,these complex stresses may cause cracks within the material,this results in changes to the mechanical properties of the material,which affects the structural integrity and threatens the safety of the HTGR.Therefore,it is necessary to study the fracture mechanical properties of nuclear graphite.In order to study the fracture performance of nuclear graphite,this paper begins with a three-point bending test on a nuclear graphite（IG11）beam with a center-notched.Measurement of the displacement field on the surface of a beam using electronic speckle pattern interferometry,and the beam notch opening displacement（CMOD）and mid-span deflectionδwere also measured using a clip-on displacement meter and a linear displacement meter respectively.Based on the experiments,numerical simulation of the fracture behaviour of graphite beams by using extended finite elements method（XFEM）,cohesion zone model（CZM）and virtual crack closure technique（VCCT）in ABAQUS.Numerical results indicated that the P_c is sensitive to the mesh size using XFEM and CZM while it is quite stable using VCCT.For the three models,the influences of critical parameters such as maximum principal stressσ_max for the XFEM,τ^c nominal interfacial strength for the CZM and critical strain energy release rate G_Ic,G_IIc and G_IIIcfor the VCCT,on the load-displacement curves were evaluated.Furthermore,by comparing with the experimental results measured by ESPI technique,the reliability of the finite element model was validated and appropriate values of the parameters for fracture simulation of IG11 graphite by using the three models were proposed.The influences of mesh size and above fracture parameters on the critical crack length a_cwere analyzed.And through the strain field at the crack tip,the lengths of the crack in graphite were determined.Although all three models can be used to simulate the fracture behaviour of graphite,however,XFEM simulates the non-linear fracture behaviour of graphite beams before the peak better,so this paper will focus on the fracture behaviour of graphite using XFEM.By using XFEM numerical simulation,the length l _FPZ of the fracture process zone（FPZ）of graphite during the loading process was obtained.When the FPZ was fully developed,the l _FPZ reached its maximum,which was about 14.16mm and was about 68%of the ligament length.Variations of fracture parameters corresponding to fracture initiation and instability with a₀/D were analyzed.The results indicate that with the increase of a₀/D,the fracture initiation load P_ini and the peak load P_c decrease with the increase of a₀/D,and the ratio of P_ini/P_c remains stable at about 0.8;K_Icⁱⁿⁱ and K_Ic^un remains basically unchanged;the length of FPZ l_FPZ increases linearly before the FPZ is fully developed,and then gradually decreases in length.In addition,the variation of fracture parameters of graphite at different sizes has been investigated,the results show that the initial load,peak load and fracture toughness are the parameters associated with the size effect,while the size effect of the fracture energy is not significant.Combining experimental and numerical simulation results,the tensile softening curve（TSC）of graphite was obtained by combining the incremental displacement collocation method（IDCM）and XFEM,the TSC was approximated as bilinear and trilinear using regression analysis.The results show that the trilinear fitted curve is more suitable for describing the TSC of graphite.combination with a virtual crack model,a fracture process analysis method that takes into account the crack initiation toughness,and a relevant calculation procedure based on the equations in the paper,effectively predicts the entire process of crack fracture in graphite beams.Finally,the research contents and results of this paper are summarized,and suggestion for further research are put forward.