Fracture behaviour of an anisotropic polygranular graphite (PGA)

This paper reports a numerical and experimental study on the mechanical and fracture properties of an anisotropic polygranular graphite: extruded nuclear grade Pile Grade A (PGA). The dynamic elastic moduli and Poisson's ratios were measured by the ultrasonic method and the fracture toughness was investigated using compact tension (CT) specimens: both as a function of orientation relative to the material grain direction. Digital image correlation was applied to monitor the full displacement field of the surface of the CT specimens, allowing the propagating crack and the fracture process zone around the tip of the crack to be observed and quantified. Rising fracture resistance (R-curve) behaviour is observed before unstable fracture, commensurate with the observed fracture process zone. We show that to accurately obtain the fracture resistance, the material anisotropy must be properly accounted for. This is demonstrated by comparing the experimental data for specimen compliance with finite element simulations for isotropic and orthotropic material properties. Digital image correlation validated the crack dimensions that were obtained from specimen stiffness, which was dominated by the lower of the anisotropic elastic moduli for all orientations. The use of a numerical method to evaluate fracture resistance, based on the incremental work done with crack extension (after Turner and Kolednik) is shown to be not reliable when the crack extension increments are insufficiently small relative to the gradient of the fracture resistance curve. There is a significant difference between the fracture properties of PGA with respect to the extrusion direction; cracks propagating across the extrusion direction (i.e. against the grain direction) have a fracture resistance that is approximately 50% higher than that for cracks propagating in a plane containing the extrusion direction.