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Understanding Creep and Creep Rate 
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#1
Oct2512, 09:04 PM

P: 79

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, steadystate 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(dε_{ss}/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. 


#2
Oct2512, 10:55 PM

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P: 21,887

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 steadystate creep involves inelastic or plastic deformation in which, σ = Kε^{n}, or ln σ = ln K + n ln ε. and there is also cases where, σ = K ε^{n} [itex]\dot{\epsilon}^m[/itex]. In the case of ln(dε_{ss}/dt) vs ln(σ), this implies a strain rate effect, i.e., strain hardening or strain rate (hardening) effect, e.g., σ = K [itex]\dot{\epsilon}^m[/itex]. 


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