In-situ scanning electron microscopy of fatigue crack behaviour in ductile materials

The activity of naturally grown short cracks in nickel was studied by in-situ measurements of the crack opening displacement in a scanning electron microscope. Plastic crack tip blunting was observed at macrocracks in single crystals as well as at naturally grown microstructurally short cracks in plastic strain control at loading amplitudes below cyclic yield. Crack tip opening hysteresis loops in nickel do not show closure due to cyclic hardening. Local yield behaviour at the crack tip was observed only after reducing the influence of the cyclic pre-history by annealing.