6. Location of the fields#

In the rest of the document, the location of the fields will not be explicitly specified. Indeed, the location is given in the name of the field (and therefore of the option):

  • Field by element: x_ ELEM

  • Field with Gauss points by element: x_ ELGA

  • Field with nodes per element: x_ ELNO

  • Field with knots: x_ NOEU

The fields are, for the most part, calculated natively at Gauss points (x_ ELGA).

The fields at the nodes per element (x_ ELNO) are obtained by extrapolation from the field at the Gauss points (method detailed in [R3.06.03]).

The fields at the nodes (x_ NOEU) are obtained from the fields at the nodes by element by taking a simple arithmetic mean (not weighted by the size of the cells) of the values encountered on the elements in a given node.

Note 1

For the calculation of equivalents, the fields at the nodes per element (x_ ELNO) are not obtained by extrapolation from the field at Gauss points. Extrapolation is done on the stress or deformation field and then the equivalent field is calculated.

Note 2

Averages at field nodes calculated in local coordinate systems are only legal if the angles between these coordinates are small. If not, they don’t make sense.

Note 3

When the keyword GROUP_MA is entered to calculate an x_ NOEU option, the arithmetic mean is done on the selected cells. Thus, this result is different from that obtained by doing a global calculation and then restricted to the selected cells only.

Example: Consider a structure whose shear stress \({\sigma }_{\mathrm{xy}}\) is equal to:

_images/10000201000002230000016B7DC9A07AEB355496.png

In the overall calculation, \({\sigma }_{\mathrm{xy}}\) is zero out of \(\mathit{M1}\cap \mathit{M2}\) as the average of two opposite values. These values are far from zero, as shown by the calculation on \(\mathit{M1}\) alone. The values on the border of the requested domain should therefore be interpreted with care.

For structural elements that have sub-points (plates, shells, multi-fiber beams, pipes), the fields of type x_ ELGA and x_ ELNO are calculated on all sub-points. To obtain a field on a single sub-point (a layer and a level for example), you must do an extraction using the POST_CHAMP operator (options EXTR_COQUE, EXTR_PMF and EXTR_TUYAU). Moreover, this intermediate operation is essential to calculate a field of type x_ NOEU for these elements of structure: fields of type x_ NOEU in fact never have a subpoint.

Finally, energy calculation options never produce fields at a sub-point. In fact, for structural elements, the field is integrated into the thickness (integration made on the sub-points).