A Uniaxial Damage Accumulation Law for Time-Varying Loading Including Creep-Fatigue Interaction

A differential form for damage accumulation is proposed wherein the rate of damage accumulation is equal to a product of two functions. The first depends only on the present damage and the second has the present value of the controlled quantity (stress or strain) and its time rate as its argument. Such an equation leads to the usual linear damage accumulation law and is incapable of modelling sequence effects. A power function form is proposed and integrated for a trapezoidal waveform with unequal tension and compression holdtimes. Using total strains only this equation can model the separate influences of frequency, holdtime, unequal effects of tension and compression holdtime, as well as mean stress (strain) effects. It includes Coffin’s frequency approach as a special case. The equation is shown to correlate low-cycle fatigue data with holdtime of type 304 stainless steel at 1200°F (649°C) and of cast IN 738 at 1600°F (871°C). It is then used to predict the lifetime for other tests for which no experiments have yet been reported.