EFFECT OF LOAD RATIO ON THE DEFORMATION-BEHAVIOR OF FATIGUE-CRACK TIPS

To investigate the change in the manner of plastic deformation at a fa tigue crack tip under different loading conditions, the fresh surface area which is produced in one fatigue cycle at the crack tip was measu red by an electrochemical method under various load ratios. The fresh surface area produced in each fatigue cycle is in proportion to the cr ack propagation rate in the Paris regime. This indicates that the crac k tip geometry at K(max) maintains a similar profile through the Paris regime. Furthermore, the fresh surface area is independent of load ra tio at a given da/dN in the Paris regime. This suggests that the shape of the fatigue crack tip at K(max) should be independent of load rati o at a given da/dN, and that the shape at K(min) is also independent o f load ratio, whereas the size of the deformed region at the crack tip is in proportion to da/dN.