Influence of environment on fatigue crack growth in the threshold region

Abstract

Low fatigue crack growth rates (down to 4 × 10 −14 m/ cycle) were produced using a high frequency 20 kHz ultrasonic fatigue testing machine. The influence of non corrosive (silicone oil) and corrosive (3.5% sodium chloride solution) environments was compared. Down to crack propagation rates of some 10 −10 m/cycle which corresponds to a crack growth rate of one lattice space per cycle no difference of crack growth rates was found. However, below this rate there seems to exist for non corrosive environments a threshold cyclic stress intensity, below which crack growth becomes diminishingly small, whereas no threshold was found for the corrosive environment. In the first case crack propagation is controlled by plastic deformation processes, in the second case these processes are markedly restricted. For this region, a transition in fracture mode from ductile transcrystalline to intergranular cracking was found.