Abstract: It is proposed that mean stress effect should be divided into two aspects, i.e.static effect and dynamic effect for fatigue under complex loading. The static effect affects only ultimate fracture condition (the critical residual strength), while the dynamic effect affects damage evolution and residual strength degradation rate. So only the dynamic effect needs to be considered in cumulative fatigue damage calculation. Besides, the present paper proposes an amending formula for calculating "equivalent damage stress", with which the effect of mean stress on cumulative fatigue damage and strength degradation under complex loading can be amended.

Key words: fatigue damage, complex loading, mean stress effect, dynamic effect, static effect