help with creep power law

Sauldavies,
For the simple, steady-state, model of secondary creep, the strain rate only depends on stress. This is usually expressed as a power law:
espsilon(dot)=A * (sigma_equiv)^n

To define such a power-law with say A=3.2E-11, n=3 I would
specify the following as part of the material definition:

*CREEP,LAW=STRAIN
** A n m
3.2E-11 , 3.0 , 0.0 ,

where units of stress would be MPa and time in hours. So here there is no explicit time dependency of strain rate. All you need for your material are A and n in approriate units. One textbook on creep comes to mind by Webster and Ainsworth: "High Temperature Component Life Assessment", Chapman & Hall, 1994.
I hope this helps.

hi mrgoldthorpe

where or how can I find the A and n for my alloy.

If you've had creep tests done at specific stresses and temperatures try fitting the measured data against the Norton law. For example plot strain versus time results for a given stress (or strain rate versus time). If you plot a trial Norton law the strain rate will be independent of time for a given stress. Your measured results won't be in agreement; but by adjusting A and n you can get a Norton law which is approximately correct. You could also try to get two or three diffrent Norton laws which bound the measured behaviour, then do sensitivity analyses in your FEA.

Or ask the alloy manufacture if they know of any data at the temperature(s) you are interested in,

Or try a Google search for creep data for the alloy,

Or try a metals handbook (ASM),

Or commission some tests for the temperature(s) you are interested in, using a typical reference stress, or several different stresses that are typical of the component(s) you are interested in, then do a best fit to the Norton law at each temperature as above.

MRG