Influence of grain size and yield strength on threshold fatigue behaviour of low-alloy steel

Abstract

Fatigue crack growth rates in air in the threshold region have been measured in an Fe-Ti-C steel heat treated to produce a range of ferrite grain sizes and yield strengths. At a constant ferrite grain size, the threshold value at R = 0 (∆K₀) was decreased by increasing the yield strength by precipitation hardening. Reducing the ferrite grain size decreased ∆K₀. In each microstructure the threshold value was reduced by increasing the load ratio, the load ratio sensitivity being greatest for low yield strengths. The results are consistent with a model, proposed by Ritchie, based on environmental interactions at the crack tip. The grain size effect on the threshold cannot be fully explained.