[MBDyn-users] Some questions regarding structural internal forces.

Rudi Jaeger rudijaeger at yahoo.com
Fri Jan 16 13:54:50 CET 2009


Dear Dr. Masarati,

I am trying to use structural internal forces. My first problem is that the .frc-output-file
contains 12 colums though the input manual suggests that there are only 9.
Based on the input file listed below, colums 7-9 and 10-12 are the locations
where the forces are being applied. Why is the information in colums 10-12
listed this way? Shouldn't this information go into a separate line, starting
with the appropriate node number?
The forces (just on node1) - colums 4-6 - are listed in a local coordinate system.
Right?
As far as I can see, there is no way of updating the direction of the forces. So
constructions like:

    force: 
        f_rod01, 
        absolute internal,
        n_base01,
        model::xdistance(n_base01,n_body01),
        model::ydistance(n_base01,n_body01),
        model::zdistance(n_base01,n_body01),
        null,
        n_body01,
        null,
        const, 2.0;

will not lead to internal forces that will always act in the direction
of a straight line that connects both nodes.
Or do I understand something wrong?

Cheers,
Rudi




# Problem definition    *******************************************************************************

begin: data;
    problem: initial value;
    integrator: multistep;                    # integrator type
end: data;


# Numerical    Configuration **************************************************************************

begin: initial value;
    initial time: 0.;
    final time: 2.;
    time step: 1e-3;
    tolerance: 1.e-6;
    max iterations: 10;
end: initial value;


# Control Data *************************************************************************************

begin: control data;
    structural nodes: 2;
    rigid bodies: 2;
    joints: 1;
    forces: 1;
end: control data;

# Substitution Variables ***************************************************************************

set: integer r_body01 = 1;
set: integer r_body02 = 2;

set: integer n_body01 = 1;
set: integer n_body02 = 2;

set: integer b_body01 = 1;
set: integer b_body02 = 2;

set: integer j_body01_body02_rod = 1;

set: integer f_rod01 = 1;

set: integer cl_rod01 = 1;

set: real dx_body01 = -1.0;
set: real dy_body01 = 0.0;
set: real dz_body01 = 0.0;

set: real dx_body02 = 1.0;
set: real dy_body02 = 0.0;
set: real dz_body02 = 0.0;

set: real frc_rod01 = 2.0;

# Reference Frames *********************************************************************************
    
reference:
    r_body01,                                                                    # refsys label
    reference, global, dx_body01, dy_body01, dz_body01,            # location vector
    reference, global, eye,                                                    # orientation matrix
    reference, global, null,                                                # translational velocity vector
    reference, global, null;                                                # angular velocity vector
    
reference:
    r_body02,                                                                    # refsys label
    reference, global, dx_body02, dy_body02, dz_body02,            # location vector
    reference, global, eye,                                                    # orientation matrix
    reference, global, null,                                                # translational velocity vector
    reference, global, null;                                                # angular velocity vector

# Node Definitions *********************************************************************************

begin: nodes;
        
    structural: n_body01, dynamic,
        reference, r_body01, null,
        reference, r_body01, eye,
        reference, r_body01, null,
        reference, r_body01, null;
        
    structural: n_body02, dynamic,
        reference, r_body02, null,
        reference, r_body02, eye,
        reference, r_body02, null,
        reference, r_body02, null;

end: nodes;

# Constitutive Laws ********************************************************************************
                   
constitutive law:
    cl_rod01,
    1,
    linear elastic generic,
    1.0e+1;

# Element Definitions ******************************************************************************

begin: elements;
        
    body: 
        b_body01, 
        n_body01,
        1.e-1,
        null,
        diag, 1.e-6, 1.e-6, 1.e-6;
        
    body: 
        b_body02, 
        n_body02,
        1.e-1,
        null,
        diag, 1.e-6, 1.e-6, 1.e-6;
        
    joint:
        j_body01_body02_rod,
        rod,
        n_body01,
        n_body02,
        from nodes,                            # rest length
        reference, cl_rod01;
        
    force: 
        f_rod01, 
        absolute internal,
        n_body01,
        1.0, 0.0, 1.0,                        # direction
        null,
        n_body02,
        null,
        const, frc_rod01;
        
end: elements;




      

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