
                             SLFFEA 1.3 Non-Linear Truss README

                    Copyright (C) 1999 San Le

                    email:     slffea@yahoo.com
                    homepage:  http://www.geocities.com/Athens/2099/slffea.html

For this non-linear element, the results for the large deformation calculation are
almost the same as that of using the regular small deformation element.  

The possible reasons for this are the following:

1) Unlike the brick, the only deformation( and stress) is in the axial
   direction of the truss.  This means there are no rotation rate terms
   which are added to the total deformation as in the brick element.

2) Because of 1), the truss goes through a process similar to polar
   decomposition when its element matrices are assembled.  This also means
   that stress is calculated in the local axial coordinate system based
   on axial displacement.  So deformation is represented by a pure axial
   elongation followed by a rotation.

While the similarity between small and large deformation results seem to
make this element unnecessary, it is worthwhile to include this element
because of its time dependent formulation which may allow for plasticity.

To improve results further, stress updating has been replaced with calculation
of the stress using the actual change in length of the element:

   stress = Emode*(L/Lo - 1.0)

The above is used because it is the most correct calculation of the
stress.  For dynamic relaxation, using traditional stress updating gives
stresses which are very close to the ones calculated from the equation
above.  This is not the case for when the Conjugate Gradient method is
used, so I have decided to go with the equation above.

