Principal scientific officer, Metallurgy Division, Ministry of Defence (Procurement Executive), Atomic Weapons Research Establishment, Aldermaston, Berkshire
Multiple-beam interferometry has been used to study the size and shape of plastic zones both at the tip of subcritical cracks and at the edges of running cracks in rolled ingot and in hot pressed, fully dense and porous beryllium. It is shown that from the zone shape an effective yield stress for the material can be derived and that this stress corresponds to somewhere between the 0.2% proof stress and the microyield stress of the material as measured under uniaxial tension. The relevance of this effective yield stress to practical fracture toughness testing is discussed and it is shown that from the points of view of both overall crack tip plastic zone shape and of the effects of plasticity on elastic stress redistribution, the 0.2% proof stress is probably the best choice.
Paper ID: JTE10142J