Threshold stress for fatigue crack initiation from a small crack of 70/30 brass.

Abstract

The critical stress σwi for crack initiation from a small crack and the effective threshold stress intensity factor range ΔK<eff.th> of 70/30 brass were determined by the annealing method. The specimens containing small cracks were annealed in order to relieve residual stress. Subsequent fatigue testing was performed again at different stress levels. The length of the nonpropagating crack Δl from the initial crack was measured, and by extrapolation to Δl=0, the critical stress σwi for fatigue crack initiation from an ideal crack and ΔK<eff.th> were determined. The experimental results are compared with those of a medium carbon steel (0.46%C) which were obtained in the previous study. The values of ΔK<eff.th> for 70/30 brass are dependent on crack size in the range from 150μm to 2600μm, although those for 0.46%C steel were independent of crack size in the range from 100μm to 1100μm. The relationships between ΔK<eff.th> and the crack length l are expressed as follows: ΔK<eff.th> ∝ l1/3 for 70/30 brass, and ΔK<eff.th> = const. for 0.46%C steel. The difference in the dependence of ΔK<eff.th> on crack size in 70/30 brass and a medium carbon steel may be attributed to the coaxing effect at the crack tip in carbon steels caused by strain aging. The dependence of ΔK<eff.th> on crack size in 70/30 brass must be considered a crucial factor which complicatedly influences the variation or scatter of the cumulative fatigue damage D under variable amplitude loading.