Learning Forum

[sorena shahbazi]

Hi dears I am doing a Creep analysis and I feced a problem. My problem is that my time step for Creep is long and time increments are small and don't increase significantly. Creep Limit Ratio parameter was setted for 1 and I increase that to 10. Conditions become better but still is slow and I want to increase this parameter to high values for example 1e6 or else. I want to know how can I justify this parameter that it be reasonable? what is the best way to extract a value for that an how can I check that my value is okay or not? In many refferences I see that the it's formulation is related to Elastic Strain and Creep Strain but is there Elastic strain in Creep step? Isn't it the case that the elastic strain is related to previous step that in it the static analysis have been done?

[John Doyle]

Elastic strain is still in the model and can still develop or might remain constant, separate from creep, depending on how the loading changes. The Creep Limit Ratio is the ratio of the equivalent creep strain increment to the equivalent elastic strain.  It is intended as a quality control tool to limit how much path dependent creep strain develops within each time increment.  A solver bisection is triggered when this limit is exceeded.  The default value of 1.0 in Mechanical is completely arbitrary.  If the elastic strain value in your application is very high (relative to creep) it might make sense to reduce this limit to a small fractional number well below 1.0.  If the elastic strain is very small, it might make sense to increase this limit to well above 1.0.   Setting it to zero just turns off this trigger altogether, which might also make sense, if the solver bisections are just slowing the run down for no good benefit in capturing better path dependence in the creep calculation.   Refer also to CUTCONTROL command in the MAPDL Command Reference Manual.