D3PLOT 22.1

WARNINGS On-Line Warnings About Beam-Plotting Pitfalls

WARNINGS On-line warnings about beam-plotting pitfalls

There are several different beam formulations in Ansys LS-DYNA, the principal ones being:
Hughes-Liu	Standard/arbitrary section, integrated at mid-span
Belytschko-Schwer	Standard sections, integrated at two ends
Truss	Axial only Belytschko-Schwer beam
Discrete	Zero-length, generalised spring-like behaviour
Cable	Tension only
Spotweld	A variation on discrete

Various data output options exist depending on material type, beam formulation, number of integration points and user-defined settings. Unfortunately the output files do not contain enough information to allow beam results to be diagnosed unambiguously, so WARNINGS provides on-line guidance. You should also read Beam element results which describes beam output.

Inconsistent sign conventions in Ansys LS-DYNA releases up to and including 970

Due to a bug in Ansys LS-DYNA versions up to and including LS970 exhibit the following inconsistent sign convention for beam output:

"Resultant" (typically Belytschko-Schwer) elements use one sign convention
"Integrated" (typically Hughes-Liu) elements use the opposite sign convention for 4 of the 6 output components.

The following table shows the results from releases 970 and earlier:

Component	Matching?
Fx	Same
Fy	Opposite
Fz	Opposite
Mxx	Opposite
Myy	Opposite
Mzz	Same

Which is right?

Sadly there is no "right" for beam output, as different users have different conventions. The confusion arises because of the different ways in which the beam types work: integrated beams have integration points at their centre, whereas resultant beams have (potential) hinges at their ends. The former reports force in the beam, and the latter reactions at the supports.

D3PLOT attempts to draw bending moment diagrams on the tensile side, but depending on which beam type you have used this may or may not be the case.

Sign conventions are consistent from Ansys LS-DYNA release 971 onwards

At some stage during the development of LS971 this problem was fixed, and results now use the "integrated" convention for all beam types.

Unfortunately D3PLOT cannot tell with certainty which Ansys LS-DYNA version was used to generate a set of results, so it cannot correct for this automatically. This has consequences for cut-section force extraction from beams, described in FORCES Computing Forces and Moments on the Cutting Plane

Update : From approximately 2009 onwards output from Ansys LS-DYNA 971 reports its version number correctly in output files, and D3PLOT is thus able to determine that the sign convention problem described above has been fixed.

Interpreting beam results requires knowledge about the original beam and material formulations that is not available in the database ( .ptf ) files. As a consequence there are some ambiguities in the interpretation of results, especially for "extra" data, that D3PLOT is unable to resolve for you.

It is your responsibility to determine what your results mean, and to interpret them correctly.

If you need advice or help please contact Oasys Ltd Support.

Previous | Next