a bug of 'plot_nodal_displacement' when use BEAM188

[FemLearner]

Before submitting the issue

• I have visited the Troubleshooting section.
• I have searched among the existing issues.
• I am using a Python virtual environment.
• I have a fully updated virtual environment (i.e. pip install --upgrade --upgrade-strategy eager ansys-mapdl-core)

Description of the bug

When I use the element beam188, there is an error:
'ValueError: data length of (11) != required length (31)', what does the ‘required length (31)’ mean?There is only 11 nodes actually.

Steps To Reproduce

Steps:

• First I did...
• ...

And I ran the following code:

from ansys.mapdl.core import launch_mapdl
mapdl = launch_mapdl()

# Insert your code here

Which Operating System are you using?

None

Which Python version are you using?

None

PyMAPDL Report

Show the Report!

# PASTE HERE THE OUTPUT OF `python -c "from ansys.mapdl import core as pymapdl; print(pymapdl.Report())"` here

Installed packages

Show the installed packages!

# PASTE HERE THE OUTPUT OF `python -m pip freeze` here

Logger output file

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# PASTE HERE THE CONTENT OF THE LOGGER OUTPUT FILE.

[mikerife]

Hi @FemLearner can you please post the code for review. Thanks,
Mike

[FemLearner]

# start pymapdl
import sys

from ansys.mapdl.core import launch_mapdl
import os
# Start mapdl and clear it.
path = os.getcwd()
path1 = path+'\workspace'
ansyspath = "D:/setup_position_1/ansys2021r1\ANSYS Inc/v211/ansys/bin/winx64/ANSYS211.exe"
mapdl = launch_mapdl(override=True, run_location=path1, exec_file=ansyspath,additional_switches="-smp")
mapdl.clear()

# Enter verification example mode and the pre-processing routine.
mapdl.verify()
mapdl.prep7()

# Type of analysis: static.
mapdl.antype("STATIC")

# Element type: BEAM188.
mapdl.et(1, "BEAM188")

# Special Features are defined by keyoptions of beam element:

# KEYOPT(3)
# Shape functions along the length:
# Cubic
mapdl.keyopt(1, 3, 3)  # Cubic shape function

# KEYOPT(9)
# Output control for values extrapolated to the element
# and section nodes:
# Same as KEYOPT(9) = 1 plus stresses and strains at all section nodes
mapdl.keyopt(1, 9, 3, mute=True)

mapdl.mp("EX", 1, 30e6)
mapdl.mp("PRXY", 1, 0.3)
print(mapdl.mplist())

w_f = 1.048394965
w_w = 0.6856481
sec_num = 1
mapdl.sectype(sec_num, "BEAM", "I", "ISection")
mapdl.secdata(15, 15, 28 + (2 * w_f), w_f, w_f, w_w)

# Define nodes
for node_num in range(1, 6):
    mapdl.n(node_num, (node_num - 1) * 120, 0, 0)

# Define one node for the orientation of the beam cross-section.
orient_node = mapdl.n(6, 60, 1)

# Print the list of the created nodes.
print(mapdl.nlist())

for elem_num in range(1, 5):
    mapdl.e(elem_num, elem_num + 1, orient_node)

# Print the list of the created elements.
print(mapdl.elist())

# Display elements with their nodes numbers.
mapdl.eplot(show_node_numbering=True, line_width=5, cpos="xy", font_size=40)

# BC for the beams seats
mapdl.d(2, "UX", lab2="UY")
mapdl.d(4, "UY")

# BC for all nodes of the beam
mapdl.nsel("S", "LOC", "Y", 0)
mapdl.d("ALL", "UZ")
mapdl.d("ALL", "ROTX")
mapdl.d("ALL", "ROTY")
mapdl.nsel("ALL")

# Parametrization of the distributed load.
w = 10000 / 12

# Application of the surface load to the beam element.
mapdl.sfbeam(1, 1, "PRES", w)
mapdl.sfbeam(4, 1, "PRES", w)
mapdl.finish()

mapdl.run("/SOLU")
out = mapdl.solve()
mapdl.finish()
print(out)

mapdl.post1()
mapdl.set(1)
mapdl.post_processing.plot_nodal_displacement('NORM', line_width=10,render_lines_as_tubes=True,
                        smooth_shading=True)

mapdl.exit()
sys.exit(0)

This is an example on the website "https://examples.mapdl.docs.pyansys.com/verif-manual/vm-002-beam_stresses_and_deflections.html#sphx-glr-verif-manual-vm-002-beam-stresses-and-deflections-py"
I try to use the command plot_nodal_displacement to plot the displacement result, but I got an error like this:

CRITICAL -  -  logging - handle_exception - Uncaught exception
Traceback (most recent call last):
  File "E:\PROJECT\NPIC\new_demo\test.py", line 91, in <module>
    mapdl.post_processing.plot_nodal_displacement('NORM', line_width=10,render_lines_as_tubes=True,
  File "D:\code_language\python3.8.10\lib\site-packages\ansys\mapdl\core\post.py", line 882, in plot_nodal_displacement
    return self._plot_point_scalars(
  File "D:\code_language\python3.8.10\lib\site-packages\ansys\mapdl\core\post.py", line 539, in _plot_point_scalars
    surf = self._mapdl.mesh._surf
  File "D:\code_language\python3.8.10\lib\site-packages\ansys\mapdl\reader\mesh.py", line 125, in _surf
    self._surf_cache = self._grid.extract_surface()
  File "D:\code_language\python3.8.10\lib\site-packages\ansys\mapdl\core\mesh_grpc.py", line 477, in _grid
    self._grid_cache = self._parse_vtk(force_linear=True)
  File "D:\code_language\python3.8.10\lib\site-packages\ansys\mapdl\reader\mesh.py", line 236, in _parse_vtk
    grid.point_data["ansys_node_num"] = nnum
  File "D:\code_language\python3.8.10\lib\site-packages\pyvista\core\datasetattributes.py", line 223, in __setitem__
    self.set_array(value, name=key)
  File "D:\code_language\python3.8.10\lib\site-packages\pyvista\core\datasetattributes.py", line 618, in set_array
    vtk_arr = self._prepare_array(data, name, deep_copy)
  File "D:\code_language\python3.8.10\lib\site-packages\pyvista\core\datasetattributes.py", line 805, in _prepare_array
    raise ValueError(f'data length of ({data.shape[0]}) != required length ({array_len})')
ValueError: data length of (6) != required length (14)

But the command plot_nodal_displacement is useful when LINK180 is used instead of BEAM188, why it doesn't work with BEAM188?

[FemLearner]

The same problem is mentioned in the question:#751, but i can't find a way to solve the problem by reading:#751

[mikerife]

Hi @FemLearner & @germa89
It looks like the fix for this issue was reverted back due to some other stability issue that came up. And so it has not ultimately been addressed...? Pinging Germa89 for visibility.

FemLearner - the underlying issue is that the quadratic shape function for Beam188 adds a mid-side node (to each element) at the time of solution. The cubic shape function adds two nodes. The PyMAPDL mesh grid property is returning all of the nodes when a list is requested, and not skipping these solver added nodes (and it is also returning the orientation node). However only results at the nodes that define the element in the normal pre-processing sense are available.

To work around the issue for now use the linear shape function - possibly at the expense of needing to mesh slightly finer depending on the applied load. So here:

# Element type: BEAM188.
mapdl.et(1, "BEAM188")

# Special Features are defined by keyoptions of beam element:

# KEYOPT(3)
# Shape functions along the length:
# Cubic
mapdl.keyopt(1, 3, 3)  # Cubic shape function

You can use instead

mapdl.keyopt(1, 3, 0) # Linear shape function

Mike