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Stress Relaxation In FEMAP with NASTRAN

Stress Relaxation In FEMAP with NASTRAN

Stress Relaxation In FEMAP with NASTRAN

(OP)
Hello forum members,

I have been experimenting with creep analyses in FEMAP 10.3.1b with NASTRAN NX.
I have been using very simple models to gain insight into how the program works.
I create an analysis set in FEMAP and then use the "Preview Input" option to create two subcases
with different NLPARM statements, the first without creep, the second with CREEP time.

I have been using a model with a single ROD element using a material with CREEP prameters defined.
When I apply a constant force to this element, the results look reasonable: the first subcase gives
the non-creep response, and the second subcase gives larger and larger displacements when I increase the time value in the NLPARM statement.

I also try stress-relaxation cases where I impose a constant displacement on the ROD.
Here I get results I find puzzling. The first load case gives the expected ROD force for
a non-creep enforced displaceemnt, a tensile force for a stretching of the ROD element.
I would expect the second CREEP subcase to give a smaller tensile force as the stress relaxes in the element with the constant enforced displacement. Instead the ROD force changes sign and is given as a compressive force.

Has anyone done similar problems in FEMAP with NASTRAN NX and can furnish some observations about what is happening?

I started out wanting to study the behavior of bolted polymer flanges. I am trying to determine
how much of the initial bolt preload remains as the stress in the polymer flange relaxes over time.
I resorted to simple 1 or 2 ROD element models to gain insight into the CREEP routines.

Thank you

Jerzy

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