Evaluations of J-integral of nuclear graphite combining experimental and numerical methods

Three-point bending tests were performed on centrally notched graphite beams to study the fracture properties of nuclear graphite. Electronic speckle pattern interferometry (ESPI) was used to measure the full-field deformation of graphite beams. In order to understand crack growth in graphite, numerical simulations were performed using a combination of ABAQUS software and the extended finite element method (XFEM). The numerical results were compared with the experimental results and good agreement was observed, confirming the validity of the numerical models. For evaluation of the J-integral, single specimen method is recommended; however, its geometric factor is uncertain and not easily determined. Thus, the load-displacement curves of beams with similar geometry and different notch depths were computed using the numerical simulations and a formula was proposed to obtain the geometric factor. Based on the ESPI results, the J-integral for crack extension was obtained by numerical computations using the measured full-field displacements as boundary conditions. Furthermore, the fracture resistance (R-curves) of the graphite was evaluated experimentally and compared with that calculated by way of the empirical formulas. The results indicated that energy dissipation rose rapidly to approximately 137 N/m before the peak load, and then increased gradually until reaching a plateau of approximately 209 N/m, when the fracture process zone was fully developed. Finally, the fracture parameters, including the critical crack opening displacements and J-integral at the fracture initiation of the graphite beams, were evaluated.