Abstract
Specimens of hydrogen-free niobium and specimens of niobium containing hydrogen, in solution or as hydrides, were fatigued at room temperature using a tension-tension loading cycle at a frequency of 4 Hz and two load ratios, 0.05 and 0.04. It is shown that the fatigue crack propagation threshold decreases sharply with increasing hydrogen concentration and then increases when hydride is formed. At all hydrogen concentrations, except in the vicinity of the hydrogen solubility limit, the threshold decreases when the load ratio is increased. The crack tip ductility initially decreases even more sharply to a value near unity but does not recover markedly as hydride is formed. An analysis of the test results and fractographic examination suggest that hydrogen in solid solution causes embrittlement by enhancement of localized plasticity in its interaction with dislocation. When hydride is formed this mechanism is combined with fracture of hydride along grain or subcell boundaries.