Inverse identification of damage and fracture properties in fine-grained nuclear graphite using finite element analysis

Identifying the damage and fracture properties of nuclear graphite materials and accurately simulating them are crucial when designing graphite core structures. To simulate the damage evolution and crack propagation of graphite under stress in a finite element model, compression tests on discs and three-point bending tests on center-notched beams for fine-grained graphite (CDI-1D and IG-11 graphite) were conducted. During these tests, digital image correlation and electronic speckle pattern interferometry techniques were utilized to observe the surface full-field displacements of the specimens. A segmented finite element inverse analysis method was developed to characterize the graphites damage evolution by quantifying the reduction in Youngs modulus with tensile and compressive strains in disc specimens. The fracture energy and bilinear tensile softening curve of the graphite were determined by comparing the load-displacement responses of the three-point bending tests and the finite element simulation. Finally, by combining the identified damage laws with a fracture criterion based on fracture energy, a damage-fracture model was established and used to simulate tensile tests on L-shaped specimens with different fillet radii. Simulations indicate that the damage area at the fillet expands with increasing radius, creating a blunting effect that enhances the load-bearing capacity of the specimens. This damage-fracture model can be applied to simulate graphite components in core structures.