Statistical characteristics of propagation of small fatigue cracks in smooth specimens of aluminium alloy 2024-T3

Abstract Smooth specimens of aluminium alloy 2024-T3 were fatigued under a nominally elastic condition. The irregular growth behaviour of many small fatigue cracks was discussed in relation to the material microstructure. The distribution of crack growth rate was fitted well by the three-parameter Weibull distribution. The changes in the Weibull three parameters, standard deviation and coefficient of variation with crack length were determined. The relation between crack growth rate and crack length was divided into three regions: A, B and C. In region C, where the crack growth rate was higher than 10 −8 m cycle −1 , the crack growth data followed the d a/ d N vs. ΔK relation for large crakcs. In region A, where the crack length was less than 0.1 mm, cracks grew along slip planes and the scatter of the crack growth rate was large. In region B, the crack growth rate was perpendicular to the stress axis; the crack growth rate was higher than that predicted from the d a/ d N vs. ΔK relation for large cracks. Comparison of the behaviour of main crack propagation with that of average crack growth rate showed that the life prediction based on the average growth behaviour of cracks gave a dangerous estimate.