Grain Size Effect in the Low Cycle Fatigue of a Steel under Mean Strain

Low cycle fatigue tests and small crack propagation tests were conducted on two kinds of annealed low carbon steel having fine and coarse grained microstructures with a special interest in the influence of grain refinement and mean strain on the low cycle fatigue life and on the fracture mode transition. Results show that (i) a difference in the fatigue lives for the fine and coarse grained steels under mean strain e_m=0 is caused by a difference in a crack initiation life for both materials, (ii) a surface to internal fracture mode transition occurs easily with an increase in mean strain and grain size and, (iii) the fatigue life of the specimen failed in the surface fracture mode at a small plastic strain is not primarily controlled by the exhaustion of ductility. In such a regime, the characteristics of fatigue life under mean strain for both materials can be predicted by the calculation of a low cycle fatigue life on the basis of the law of small crack growth.