function  color_section_stress (i_p)
% Copyright 2006, J.E. Akin.  All rights reserved.
%  ------------------------------------------------------
%  Plot failure criterion over cirular or rectangular
%  of a shaft (or space frame ?)
%  ------------------------------------------------------
if (nargin == 0 )
  i_p = 4
end % if

%    Set control data: 
  load section_pt_stresses.tmp ;  
  CIR  = section_pt_stresses (1, 1) ;
  ELEM = section_pt_stresses (2, 1);
  SECT = section_pt_stresses (3, 1);
  RAD  = section_pt_stresses (4, 1);
  H_Y  = section_pt_stresses (4, 2);
  H_Z  = section_pt_stresses (4, 3);
  N_C  = section_pt_stresses (4, 4);

  if ( CIR == 1.0 )
    H_Y = RAD ; H_Z = RAD ;
  end % if

%   Build Z-y pts
  z(1) =  0.  ; y(1) =  0.  ;
  z(2) =  0.  ; y(2) =  H_Y ;
  z(3) =  0.  ; y(3) = -H_Y ;
  z(4) =  H_Z ; y(4) =  0.0 ;
  z(5) = -H_Z ; y(5) =  0.0 ;
  z(6) = -H_Z ; y(6) =  H_Y ;
  z(7) =  H_Z ; y(7) =  H_Y ;
  z(8) =  H_Z ; y(8) = -H_Y ;
  z(9) = -H_Z ; y(9) = -H_Y ;

%   area connections
  loop_1 = [1, 4, 7, 2] ;
  loop_2 = [1, 2, 6, 5] ;
  loop_3 = [1, 5, 9, 3] ;
  loop_4 = [1, 3, 8, 4] ;

%   Extract failure criterion values
  v(1:N_C) = section_pt_stresses (5:(4+N_C), i_p) ;

  if ( CIR == 1.0 )   ;
   v (6) = 0.5*(v(2) + v(5))   ;
   y (6) = y(6) * 0.707        ;
   z (6) = z(6) * 0.707        ;
   v (7) = 0.5*(v(2) + v(4))   ;
   y (7) = y(7) * 0.707        ;
   z (7) = z(7) * 0.707        ;
   v (8) = 0.5*(v(3) + v(4))   ;
   y (8) = y(8) * 0.707        ;
   z (8) = z(8) * 0.707        ;
   v (9) = 0.5*(v(3) + v(5))   ;
   y (9) = y(9) * 0.707        ;
   z (9) = z(9) * 0.707        ;
  end % if

  ymax = max (y) ; zmax = max (z) ; vmax = max (v) ;
  ymin = min (y) ; zmin = min (z) ; vmin = max (v) ;

  axis ([zmin, zmax, ymin, ymax])
  axis ('equal')                    % true shape style
  % caxis
  % caxis ([vmin vmax])
  clf
  hold on
  fill (z(loop_1), y(loop_1), v(loop_1))
  fill (z(loop_2), y(loop_2), v(loop_2))
  fill (z(loop_3), y(loop_3), v(loop_3))
  fill (z(loop_4), y(loop_4), v(loop_4))
  grid                              % add grid dots
  xlabel (['Z-coordinate, Section ', int2str(SECT), ...
           ', Element ', int2str(ELEM), ' '])
  ylabel ('Y-coordinate')
  format short
  if ( i_p == 1 )
    title ('Shaft Cross-sectional Area Principle Stress P1')
  elseif ( i_p == 2 )
    title ('Shaft Cross-sectional Area Principle Stress P2')
  elseif ( i_p == 3 )
    title ('Shaft Cross-sectional Area Principle Shear Stress')
  elseif ( i_p == 4 )
    title ('Shaft Cross-sectional Area Von Mises Stress')
  else
    error('Argument 1-P1, 2-P2, 3-Max Shear, 4-Von Mises')
  end % if
  colorbar
 
%       -depsc -tiff            % for an eps version
  %b print -dpsc color_el_at_a_node
  hold off
%b fprintf ('Created file color_el_at_a_node.ps \n')
% end of color_section_stress