Corrosion fatigue crack growth of AISI 4340 steel

Corrosion fatigue crack growth (CFCG) experiments of AISI 4340 high strength steel were conducted using pre-cracked compact tension specimens in dry air, distilled water, and 3.5% NaCl aqueous solution at different R-ratios (where R is the ratio of the minimum load over the maximum load in a loading cycle) and different loading frequencies. The material displays typical Type B CFCG behavior: significant environmental effect when the maximum stress intensity factor is above KISCC (the threshold stress intensity for stress corrosion cracking) and minimal environmental effect when the applied load is below the threshold value or above a large stress intensity factor. A new engineering model was developed to consider the interaction and competition between fatigue and environmental contributions. The R-ratio effect was considered using the Walker’s equation. The baseline data required for the model is the data from stress corrosion in a given environment and the data from crack growth of the material in dry air or inert environment. The new model was found to correlate very well the CFCG experiments of AISI 4340 steel.