The effects of low frequency load cycles on crack initiation in low alloy steels

The stress corrosion behaviour of low alloy steels was investigated with low frequency cyclic loading in 2 M (NH₄)₂CO₃ at 70°C. The tests were carried out using triangular and sawtooth load cycles with a constant critical strain-rate prevailing within periods of increasing stress irrespective of frequency. Since frequency changes with stress amplitude when using equal ?-values, the effects of frequency were also determined. The experiments were concentrated on crack initiation and initial crack propagation. The results show that the threshold stress which causes stress corrosion cracking (SCC) in ammonium carbonate solution is considerably reduced for all steels tested during low-frequency load cycles. In contrast to tests with constant load, propagating stress corrosion cracks can be initiated below the lower yield strength. The effects of stress/strain amplitude and of the magnitude of deformation on crack propagation were determined.