Growth of fatigue cracks in steels

Abstract

An extensive study of fatigue-crack growth in BS15 mild steel has been made, in laboratory air at 21°C and at two cycling frequencies, namely 0.25 and 35 Hz. A wide range of cyclic growth rates, starting from the region associated with fatigue thresholds (10⁻⁷ mm/cycle) up to the region of rapid fracture, was investigated using contoured DCB specimens which provided constant stress-intensity factors over the crack lengths of ∼150 mm. It was found that for BS15 a relationship between cyclic growth rate da/dN and the tensile loading levels of the form da/dN = aϕ^α was applicable. a and α are constants and ϕ a parameter dependent upon the loading conditions and material properties. The proposed equation is shown to be dependent upon the range of crack-opening displacement, the plastic zone size, and the J-contour integral. The relation has also been successfully applied in analysing published crack growth rate data obtained from four different steels as well as some Al alloys. Threshold values of stress intensity, varying with its mean, have been established, but they do not correlate with crack closure. Instead an explanation based upon the inclusion of the threshold term in the above equation is proposed.