S–N plots and related phenomena of ultrafine grained copper with different stages of microstructural evolution

• Fatigue life at stresses corresponding to lifetime from 2 × 104 to 2 × 107 cycles depended on ECAP-pass-number.
• The fatigue life decreased according to the sequence of pass-numbers 8-4-12.
• The difference in fatigue life occurred from the crack initiation life.
• The short crack initiation life in 12-passed sample resulted from rapid generation/growth of recrystallized grains.

To elucidate the effect of the number of passes in equal channel angular pressing on the fatigue strength of ultrafine grained copper, fatigue tests of cylindrical specimens were conducted and the formation behavior of surface damage during cyclic stressing was studied. With the exception of extremely high- and low-stress amplitudes, the fatigue life depended on the number of processing passes and decreased according to the following sequence of pass-numbers: 8-4-12. The difference in fatigue life resulted from the crack initiation life. The physical background of different crack initiation lives among samples is discussed from the viewpoint of slip band formation and the growth of dynamically recrystallized grains.