Fracture behavior of nuclear graphite under three-point bending tests

Abstract

Three-point bending tests are performed on center-notched graphite beams to study the fracture properties of IG11 graphite. Electronic speckle pattern interferometry (ESPI) is used to measure the full-field deformation of the graphite beams. A clip gauge is used to measure the crack mouth opening displacement (CMOD) of the beams, which validates the ESPI measurements. In addition, finite element (FE) analysis is performed by using the ABAQUS code coupled with the extended FE method to simulate the growth of cracks in graphite. A combined stress-energy based fracture criterion, specifically, damage for traction separation laws based on maximum principal stress damage initiation and energy based damage evolution with linear softening is applied. The numerical results are compared with the experimental results which show exceptional agreement, thus confirming the accuracy of the FE model. The fracture toughness KIc, fracture energy GF, peak load Pmax and critical displacements at the peak load of each specimen are determined. From the displacement contours measured by ESPI, the positions of the neutral axis and crack opening displacement (COD) profiles at different loading levels are estimated. From the phase maps, the changes in the fracture process zone (FPZ) are analyzed and the size of the FPZ is determined. The strain contours at different loadings indicate that the material near the crack tip experiences biaxial tensile stress.