Growth of surface flaws induced by the environment and machining stresses in unalloyed uranium

Computer modelling techniques are used to predict the distribution of residual stresses in a machined uranium surface. The predictions are used to address the ageing of uranium exposed to inert gas based environments in terms of microcrack initiation and subsequent propagation. Metallographic observations of microcracking are used as the basis for the initial assessment of ageing behaviour. It is proposed that the near-surface residual stress field produced by machining influences the occurrence of microcracking. It is also suggested that corrosion-induced surface features act as initiation sites for microcracks, which begin to propagate by an environmentally assisted mechanism when the surface features reach a critical depth within the residual stress field of between 5 and 10 μm. However, the majority of the microcracks appear to arrest at about 150 μm. This behaviour is discussed in terms of the predicted threshold stress intensity for crack nucleation, uranium metallurgy, and the possible effects of crack coalescence on growth.