Crack initiation and propagation in pile grade A (PGA) reactor core graphite under a range of loading conditions

The pile grade A (PGA) graphite used in UK gas cooled reactors is a multiphase, polygranular, aggregate material with complex cracking behaviour. Virgin, un-irradiated graphite cylinders 12 mm in diameter and 6 mm long have been subjected to controlled cracking either by insertion of a needle into the material or by compressive loading. The resultant cracking was observed using optical and focused ion beam microscopy. Micro-cracking was confirmed to precede macro-crack formation and this mechanism is consistent with the observed non-linearity in the load–displacement curve prior to peak load. Macro-cracks followed an irregular path controlled by the direction of the applied tensile stress and the microstructure, in particular porosity and filler particles. The results are discussed with respect to the quasi-brittle fracture characteristics of such an aggregate material.