[MBDyn-users] Constitutive law for a chain

Rudi Jaeger rudijaeger at yahoo.com
Wed May 7 17:03:42 CEST 2008

Marco Morandini <morandini at aero.polimi.it> wrote:
> I am trying to model something like a rope or chain using the rod-element.
> It is necessary to use the prestrain property to change the length of
> the rope arbitrarily. Force should only be produced if the distance between
> the two nodes connected by the rod exceeds the momentary length
> of the rod.
> Up till now I could not figure out a way to do that.
> Do you have some suggestions how to proceed?
> Cheers,
> Rudi

I'm not sure that it will work, but
I'd try to us a "Double linear elastic"
(or a "Scalar function elastic"
constituive law) arranged in such a way that
the force is == 0 for deformations <=0.

I tried the Scalar function approach with something like:

set: integer cl_rod = 1;

set: real high_force= 1.0e+16;
set: real cut_force= 0.0;
set: real eps= 1.0e-1;

set: real k_plus=  1.0e+5;
set: real k_minus= 1.0e-1;

scalar function: 
   "k_step_fun", multilinear,
    -high_force, k_minus,
    -cut_force-eps, k_minus,
    cut_force+eps, k_plus,
    high_force, k_plus;
constitutive law:                                    # constitutive law
     cl_rod,                                        # label
     1,                                            # dimension
     prestrain,                                    # prestrain
     single,                                        # tpl driver
     scalar function elastic isotropic,                # specific const. law

The constitutive law features a large change in the elastic constant,
but I am not sure if it actually does the right thing.
Anyway, I could not get it to work with various forms of the prestrain definition.


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