[Insight-users] ITKFEM element stiffness matrix in simple truss element

[Barbara]

Hello All,

I went  to Insight/Testing/Code/Numerics/FEM/itkFEMElementTest.cxx and 
use itkFEMElementTest to run a simplified version of the truss.fem input 
file.  I have included below and it is just one truss element using 
<Element2DC0LinearLineStress>. 

itkFEMElementTest prints out the stiffness matrix, but since it is only 
one element, the stiffness matrix = the element stiffness matrix.
Description: Error: A diagonal element is not positive

I believe this cases other larger FEM models to have nonsymmetric 
matrices. 
Please check it out.
Thanks Barbara.


% Simple truss
                                                                                                                                         

<Node>
        0       % Global object number
        2 0 0   % Node coordinates
<Node>
        1       % Global object number
        2 0 2   % Node coordinates
                                                                                                                                         

<END>  % End of nodes
                                                                                                                                         

                                                                                                                                         

<MaterialLinearElasticity>
        0                   % Global object number
        E  : 1910       % Young modulus
        A  : 1.0         % Beam crossection area
        I  : 1.0          % Moment of inertia
        nu : 0.3        % Poisson's ratio
        h :  1            % Plate thickness
        RhoC : 1       % Density times capacity
        END:            % End of material definition
                                                                                                                                         

<END>  % End of materials
                                                                                                                                         

                                                                                                                                         

<Element2DC0LinearLineStress>
        0       % Global object number
        0       % Node #1 ID
        1       % Node #2 ID
        0       % MaterialLinearElasticity ID
                                                                                                                                         

<END>  % End of elements
                                                                                                                                         

% We apply only one nodal force. This is not the in book.
<LoadNode>
        0       % Global object number
        0       % GN of element on which the load acts
        1       % Point number within the element
        2 0 -20 % Force vector (first number is the size of a vector)
                                                                                                                                         

% Essential boundary conditions are applyed, so that the system is
% fixed and we can solve for displacements. In book the BCs are applied
% in form of MFCs, and are different than these here.
<LoadBC>
        1       % Global object number
        0       % GN of element
        0       % DOF# in element
        1 0     % value of the fixed DOF
<LoadBC>
        2       % Global object number
        0       % GN of element
        1       % DOF# in element
        1 0     % value of the fixed DOF
                                                 
<END>  % End of loads