# Understanding Creep and Creep Rate

jaredogden
So I'm looking through some material on creep for one of my courses. There is a graph of strain ε vs Time, t. Consisting of Primary creep, steady-state creep, and tertiary creep. I pretty much can follow that and understand why the graph looks the way it does.

However there is another graph under it that is ln(dεss/dt) vs ln(σ). I am trying to understand what the significance is of taking the natural log of stress and the steady state creep rate. What would a graph containing these things be telling us, and why the natural log?

Thanks for any help.

Staff Emeritus
In the elastic range, σ = Eε, where E is the elastic (Young's) modulus, i.e., it's linear as in 'linear elastic'. In most systems, the service domain is in the elastic range.

Secondary or steady-state creep involves inelastic or plastic deformation in which,

σ = Kεn, or ln σ = ln K + n ln ε.

and there is also cases where,

σ = K εn $\dot{\epsilon}^m$.

In the case of ln(dεss/dt) vs ln(σ), this implies a strain rate effect, i.e., strain hardening or strain rate (hardening) effect, e.g., σ = K $\dot{\epsilon}^m$.