OS11W0316 On the formation of striations during fatigue crack growth

Abstract

A study of the fatigue crack growth process in a low carbon steel has been carried out over a wide range of crack growth rates in air and vacuum at R=0.05 using the techniques of stereo-microscopy and scanning-transmission electron microscopy(STEM). Examination of the profiles of striations indicated that they are of the stair-case type, as originally proposed by Schivje. In this steel the striations were not resolvable below a growth rate of 10^<-4> mm/cycle. In the crack growth rate range from 10^<-4>mm/cycle down to 10^<-8>mm/cycle a dislocation cell structure developed whose dimension in the direction of fatigue crack growth was approximately 1.5 x 10^<-4> mm/cycle, independent of the ΔK level. The constancy of this dimension has been interpreted to indicate that the fatigue crack growth process at low crack growth rates is discontinuous rather than cycle-by-cycle in nature. However the evidence gathered in this investigation supports the view that the fatigue crack growth process occurs on a cycle-by-cycle basis even at low fatigue crack growth rates. As is known, striations are difficult to find on the fracture surfaces of specimens tested in vacuum. The use of the STEM technique provides evidence that a striation is created in vacuum on unloading, but that due to the greater extent of crack tip blunting in vacuum as compared to in air, in the next loading cycle the previously created striation is swept into the flanks of the blunted crack and largely eliminated. For comparison purposes an aluminum alloy, 2519-T87, was also tested.