Variation of dislocation density during the fatigue process of 0.1% aluminum-killed carbon steel was explored by the aid of transmission electron microscopy technique. The fatigue tests were conducted under a constant axial stress amplitude with zero mean stress at a frequency of 30Hz.
The results showed that the dislocation density of about 108/cm2 under the annealed state increased with cycles and reached a value of 5×109/cm2 at ten percent of the total life. This trend of growth well corresponded to that of plastic strain amplitude. By the analyses of multiplication and annihilation processes of dislocations, the change of dislocation density ρ with the number of stress cycles n was expressed by
ρ=α/β1/1+ke-αn, k=α/βρ0-1,
where α and β are the constants, and ρ0 is an initial density before fatigue tests. It was demonstrated that this equation was applicable not only to the present low carbon steel, but also to pure iron, pure aluminum and prestrained S15C steel.
