EXAMPLE 201
Isotropic plane stress analysis.
Two unknown displacents per node.
Three active strain and stress components, S_xx, S_yy, S_xy
Four post processed stress components, above plus the
Von Mises' failure criterion

DATA_SET 01 
 title "2D STRESS PATCH TEST, T6 ESSENTIAL BC" ! keywords
 nodes       9 ! Number of nodes in the mesh
 elems       2 ! Number of elements in the system
 el_nodes    6 ! Maximum number of nodes per element
 b_rows      3 ! Number of rows in the B (operator) matrix
 shape       2 ! Element shape, 1=line, 2=tri, 3=quad, 4=hex
 el_real     3 ! Number of real properties per element
 post_1        ! Require post-processing
 example    201 ! Source library example number
 data_set    01 ! Data set for example (this file)
 exact_case  0  ! Exact analytic solution
Note: Patch test yields constant gradient and strains
   3--6---9    Mesh to left. Exact solution u = 1 + 3x - 4y
   :(2) / :                  du/dx = 3,    du/dy = -4
   2  5   8          Exact solution v = 1 + 3x - 4y
   : / (1):           dv/dx = 3,    dv/dy = -4
   1/--4--7  Thus answer at node 5 is u = v = -1
Strains: 3, -4, -1. Stresses: 3, -4, -0.5, for E=1, nu=0
Stresses: 2.13333, -3.46667, -0.4, for E=1, nu=0.25

DATA_SET 02 
"2D STRESS PATCH TEST, Q8, ESSENTIAL BC"
  5--8-13-16-21 (4,4)    Mesh shown to left. 
  :     :     :     Exact solution u = 1 + 3x - 4y
  4 (2) 12 (4)20    Exact solution v = 1 + 3x - 4y
  :     :     :     du/dx = 3,    du/dy = -4
  3--7-11-15-19     dv/dx = 3,    dv/dy = -4
  :     :     :     for E=1, nu=0:
  2 (1)10 (3) 18    e_xx = 3, e_yy= -4, e_xy = -1      
  :     :     :     s_xx = 3, s_yy= -4, s_xy = -1/2
  1--6--9-14--17 ->X

DATA_SET 03
"2D STRESS PATCH TEST, T3, ESSENTIAL BC"
 2\----/5    Mesh to left. Exact solution u = 1 + 3x - 4y
 : \ 3/ :                du/dx = 3,    du/dy = -4
 :1 3  4:          Exact solution v = 1 + 3x - 4y
 : / 2\ :           dv/dx = 3,    dv/dy = -4
 1/----\4  Thus answer at node 5 is u = v = -1

DATA_SET 04
2D STRESS PATCH TEST, Q9, ESSENTIAL BC METHOD
5--8-13-16-21 (4,4)    Mesh shown to left. 
:     :     :          Exact solution u = 1 + 3x - 4y
4 24  12 25 20         Exact solution v = 1 + 3x - 4y
:     :     :          du/dx = 3,    du/dy = -4
3--7-11-15-19          dv/dx = 3,    dv/dy = -4
:     :     :          s_ij = e_ij for E=1, nu=0
2 22 10 23  18         e_xx = 3, e_yy= -4, e_xy = -1      
:     :     :        
1--6--9-14--17 ->X

DATA_SET 05
STRESS PATCH TEST, T6, NODE 9 missing, reaction loads
   3--6--10   Mesh to left. Exact solution u = 1 + 3x - 4y
   :(2) / :                  du/dx = 3,    du/dy = -4
   2  5   8          Exact solution v = 1 + 3x - 4y
   : / (1):           dv/dx = 3,    dv/dy = -4
   1/--4--7  Thus answer at node 5 is u = v = -1
gauss of 3 dies in error estimator (rank defficient LSq?)
too few elements for a good patch?
Self equilibrating loads are actually reactions from
DATA_SET 01 with node 10 replacing old node 9.

DATA_SET 6
2D STRESS PATCH TEST, T10, ESSENTIAL BC METHOD
5  8 13 16 21 (4,4)    Mesh shown to left.
                       Exact solution u = 1 + 3x - 4y
4-24--12-25  20        Exact solution v = 1 + 3x - 4y
: (2)  / :             du/dx = 3,    du/dy = -4
3  7 11 15   19        dv/dx = 3,    dv/dy = -4
:   /    :             s_ij = e_ij for E=1, nu=0
2 22 10 23   18        e_xx = 3, e_yy= -4, e_xy = -1
: / (1)  :             UN-USED NODES: 5, 8, 13, 16-21
1--6--9-14   17 ->X

DATA_SET 7
Two element plane stress example
         2------4 ---> P = 1e4 N
  Fixed  : \ (2):   width = height = 2 m, thickness = 5e-3 m
  edge   :  \   :   E = 15e9 N/m^2, nu = 0.25
         : (1)\ :
         1------3

DATA_SET 8 
Circular hole in an infinite plane
exact_case  23