D3PLOT 22.1

Coordinate System and Centroid of Cut Forces

Coordinate system and centroid of cut forces

The coordinate system and centroid depend upon the plane's space system:

In DEFORMED space the following is used, all axes being in the plane local axis system .

Fx is in plane X force (effectively shear force)

Mxx is moment about plane local XX axis

The plane centroid is at the plane's origin, and its local X, Y and Z axes are as defined by the user. These axes may be visualised by turning plane display on. Unless "cut follows nodes" is turned on the centroid and axes remain fixed as the model deforms.

In particular note that moments are calculated about plane local axes acting through the plane origin.

Fy in is plane Y force (also shear force ) Myy is moment about local YY
Fz is Z force normal to the plane. Mzz is moment about local ZZ

In BASIC space the following are used, all axes being in the global model system .

Fx is force in the global X axis

Mxx is moment about the global XX axis

The plane centroid at any given state is the average coordinate of the cut elements, this means that it moves as the model deforms.

In particular note that moments are calculated about plane global axes acting through n the current plane centroid as calculated from the average of all cut elements.

Fy is force in the global Y axis Myy is moment about the global YY axis
Fz is force in the global Z axis Mzz is moment about the global ZZ

Changing the coordinate system in which results are reported

Deformed space cut forces and moments can also be rotated to the global coordinate system for reporting puposes using the "System" popup menu.

This affects all reporting of forces and moments, both in the Cut sections panel and in Write and XY_Plot

Note that this is simply a geometric transformation of the coordinate system in which results are expressed, rotating them between global and plane local systems.

Although forces in the global system are reported as [Fx, Fy, Fz] they still represent a normal force and two shear forces in the original system of the plane. Forces on a plane are not the same as forces at a point in space!

In order to obtain "direct" (not shear) forces through the structure in all three global axes it will be necessary to create three cutting planes aligned with each of the global axes, and to collect results from each in turn.

Compatibility with *DATABASE_CROSS_SECTION output from Ansys LS-DYNA

Ansys LS-DYNA uses the lagrangian approach for cross-sections, and computes their forces using the equivalent of the BASIC method above. Results from Ansys LS-DYNA should match those from D3PLOT closely when BASIC space is used..

From D3PLOT 10.0 onwards, D3PLOT is capable of displaying any *DATABASE_CROSS_SECTION definitions in the input deck, and also extracting the forces reported by Ansys LS-DYNA in these. For this to work all of the following must be true:

  • You must be running D3PLOT 10.0 or later
  • It must have read a ZTF file generated by PRIMER 10.0 or higher (in order to determine the geometry)
  • It must have read the "binout" file generated by Ansys LS-DYNA (in order to extract the cross section forces)
  • Cross-section output *DATABASE_SECFORC must have been turned on, and binary output (to the binout file) turned on.

D3PLOT Cut sections, and Ansys LS-DYNA Cross sections are separate and different within D3PLOT:

  • D3PLOT Cut sections, as described in this manual section,

    • Are user-defined and can be modified dynamically during post-processing.
    • Cut dynamically through the model using graphics calculations to display the cut structure.
    • Calculate forces and moments from a limited subset of elements using the forces, moments and stresses reported in those elements.
    • Force and Moment calculation can be in local or global systems, and the user can control dynamically (by blanking) the elements in which it takes place.
    • Only a single cut section can be active at a time, although any number may be stored for later retrieval.
  • Ansys LS-DYNA Cross sections, as defined under *DATABASE_CROSS_SECTION in the Ansys LS-DYNA user manual:

    • Are defined in the original keyword input deck.
    • Have their forces and moments calculated by Ansys LS-DYNA during the analysis, and reported to ASCII secforc and/or binout files
    • The section geometry and elements which are cut are stipulated in the input deck, and cannot be changed during post-processing.
    • The force system of the results is always global, and the cutting space lagrangian (basic). Neither can be changed during post-processing.

    • Can have their geometry imported and displayed in D3PLOT via a ZTF file
    • Can have their results, which are always in the global system, extracted from a binout file and displayed in D3PLOT.

    • Any number of cross sections may be defined, and all can be displayed in D3PLOT if read as described above.

It is possible to use the geometry of an Ansys LS-DYNA Cross section definition to define a D3PLOT Cut section, which will overlay the two definitions. Selecting Basic space for display and the reporting of forces and moments should give very similar results. The results will probably not be identical for one or more of the following reasons:

  • D3PLOT extracts results from the PTF files, and the time of a given state may not match exactly the nearest time written at "time history frequency" in the binout file. If forces are varying rapidly this can give rise to significant differences.

  • D3PLOT can only work with the limited subset of element data available in the PTF file. (No bending data in thick shells, no spring output, etc)

  • D3PLOT cut sections calculate forces from all unblanked elements that are intersected, whereas Ansys LS-DYNA cross sections can control the elements considered both by defining a subset of parts and by limiting the extent of the section in its local XY plane. Careful blanking could be used to reproduce the same result, but it can be difficult to get exactly right.

Consider the following example of a cantilever cut along its length:


In this example a cantilever made of solids is loaded downwards at its free end.

There is a cutting plane defined in DEFORMED space roughly half way along its length, with the positive side (+ve Z axis) being the free end.


The force acting on the cut plane from its +ve side acts downwards.


In the plane local coordinate system this is in the negative Y sense.


The moment acting on the plane is in the positive sense about the local XX axis.


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