INITIATION AND EARLY STAGES OF GROWTH OF CORROSION FATIGUE CRACKS IN A STRUCTURAL-STEEL

The initiation and growth of fatigue microcracks in a BS 4368-500 structural steel has been studied in laboratory air and in aerated 3.5% NaCl solution. Experiments were performed using constant load fully reversed bending at a frequency of 25 Hz. Particular emphasis has been placed on the role of inclusions in crack initiation and growth. The fatigue strength of the material reduced in the aqueous environment to 130 MPa compared with 290 MPa in air. In air, microcracks initiated at oxide inclusions by the debonding of the inclusion-matrix interface. In chloride solution, crack initiation occurred at corrosion pits associated with sulphide inclusions. The average crack growth rates in NaCl solution were higher than those in air, as determined by observation of the crack length at intervals throughout the tests. The crack path was controlled by the distribution of inclusions along the highly stressed portion of the specimen surface