Estimation of the size of GBF area on fracture surface for high strength steels in very high cycle fatigue regime

The sizes of the ‘granular bright facet’ (GBF) areas on fracture surfaces of specimens subjected to very high cycle fatigue for two high strength spring steels were measured in a field emission scanning electron microscope (FESEM) and compared with the estimated values, and a good agreement was found between the two sets of data. A criterion was proposed to estimate the sizes of GBF areas. It was assumed that when the increment of crack length in one applied stress cycle (numerically equal to crack growth rate, d a/d N) is just equal to the plastic zone size in front of crack tip, r p, the GBF stops developing. It means that hydrogen accumulating in the plastic zone under applied stress influences mostly on the fatigue behaviors which results in the morphology of GBF area quite different from the conventional fatigue fracture surface. The estimated sizes of GBF areas were also in accordance with the sizes measured from fracture surfaces of other high strength steels provided that the hydrogen concentration in those steels was not much higher than that in present spring steels.