A unified equation for creep-fatigue

• Coffin–Manson equation Δεp=CoN-βoΔεp=CoN-βo is a special case of a unified creep-fatigue equation, Δεp=Cos(σ)c(T,f)N-βob(T,f)Δεp=Cos(σ)c(T,f)N-βob(T,f).
• The stress function s(σ) incorporates the stress-characteristic of creep.
• The creep functions c(T, f) and b(T, f) embody the time-temperature characteristic of creep.
• At the reference condition when creep is dormant, s(σ) = c(T, f) = b(T, f) = 1, the Coffin–Manson equation is recovered.

“Pure fatigue” is a special case of creep-fatigue; and the Coffin–Manson equation, Δεp=CoN-βoΔεp=CoN-βo, is a special case of the general creep-fatigue equation, which is proposed to take the form: Δεp=Cos(σ)c(T,f)N-βob(T,f)Δεp=Cos(σ)c(T,f)N-βob(T,f). The functions, s(σ), c(T, f) and b(T, f), embody the stress–time–temperature characteristic of creep. At the reference condition when creep is dormant, s(σ) = c(T, f) = b(T, f) = 1, the Coffin–Manson equation is recovered. At the extreme condition when c(T, f) = 0, creep-rupture occurs without fatigue. In between these two extreme conditions whence 0 ⩽ c(T, f) ⩽ 1, creep-fatigue prevails.