A STUDY OF THE GROWTH OF SMALL FATIGUE CRACKS IN A HIGH-STRENGTH STEEL USING A SURFACE-ACOUSTIC-WAVE TECHNIQUE

An ultrasonic surface acoustic wave technique for studying the growth behaviour of small fatigue cracks is described. The technique allows c rack depth and opening stress to be monitored continuously during the course of a fatigue test. Results are given for a 1740 MPa yield stren gth, silicon-modified, AISI 4340 steel tested under zero-to-tension cy clic loading. Good agreement is shown between acoustically determined crack depth and that measured by post-fracture optical microscopy. The monitoring of changing crack depth-to-surface length ratios during te sts is also demonstrated. Acoustically determined crack opening stress es were found to be about 10% higher than values determined by measure ments of crack tip opening displacements by scanning electron microsco py. Effects on crack growth of two different specimen surface preparat ions, electropolishing and diamond paste polishing, are also reported. Growth rates in electropolished specimens were as much as an order of magnitude higher than in diamond paste polished specimens which had a shallow but significant layer of compressive residual stress.