Empirical corrosion fatigue life prediction models of a high strength steel

Crack initiation and growth behaviour in plain hour-glass shaped fatigue specimens of quenched and tempered silico-manganese spring steel (BS250 A53) having a mirror image was studied under fully reversed torsional loading conditions in both the laboratory air and the aggressive (0.6 M, aerated NaCl solution) environments. A surface plastic replication technique was used alongwith optical microscopy to monitor the early stages of environment-assisted fatigue. Non-metallic inclusions were observed to play a major role in crack initiation in both the environments. Debonding at matrix/inclusion interfaces and chemical pitting at inclusion sites were major processes in the early developmental stages of air and corrosion fatigue, respectively. A significant influence of microstructure, i.e. prior austenite grain boundaries, on defect development was also noted during air and corrosion fatigue cracking. Corrosion fatigue failure appears to be a multiple stage process namely; pit development, short crack growth, and long crack growth. Corrosion fatigue crack growth rates are predicted by employing models, which incorporate elastic plastic fracture mechanics parameters to characterise the influence of microstructure. Two empirical corrosion fatigue crack growth models, including a superposition model discussing the inert air and environmental terms involved in the corrosion fatigue process, are presented. A reasonable agreement was found between experimental and calculated lifetimes. (C) 2000 Elsevier Science Ltd. All rights reserved.