RESULTS CALCULATE PERDIFF [Tolerance]

        

This command applies to element nodal, gaussian or invariant result data sets and allows the user to calculate the percentage difference between the results obtained by nodal averaging and the selected element results. The calculation is carried out on all components of the selected attribute.

Tolerance = This is an optional limiting percentage value used in the percentage difference criterion. If it is given, its value should be between 0 and 100, inclusive. If not entered, its default value of 5 is then used.

The percentage difference criterion, with the default tolerance, can be written as follows:

• if m $\gt$ 5% * R_max , then $P = \frac{max(\vert R_{i}-m\vert)}{m}$
• if m $\leq$ 5% * R_max , then $P = \frac{max(\vert R_{i}-m\vert)}{R_{max}}$

where

• m is the average value calculated at the node ($m = \frac{\sum_{i=1}^{n}R_i}{n}$)
• (R_i , i=1,n) are the 'n' element results attached to the node and
• R_max is the maximum result value in the element result data set.

The resulting data is stored as a new nodal result data set. A new attribute name is made for this new data set by attaching the letter P in front of the current attribute name, unless it already exists for nodal results. The user then has the choice of accepting it or typing a new attribute name.

When the PERDIFF command is applied to gaussian or invariant results, a results extrapolation is implicitly performed for the calculation of the average values (see RESULTS CALCULATE EXTRAPOL). No nodal averaging is performed across material boundaries and the resulting nodal data set is therefore material dependent. Only material discontinuities are considered in the present calculation, other discontinuities, such as difference in shell thicknesses or non co-planar shells, are not taken into account.

The restrictions on nodal averaging apply here, see notes below.

Notes:

1.

Loadcases
The percentage difference is calculated for the selected results for all of the selected loadcases.

2.

Coordinate systems
Percentage difference cannot be performed on results defined in more than one coordinate system (this is based on the value of the FEMVIEW system indicator ISYSTM). An error message is issued if several coordinate systems are encountered for the current data set.

3.

Record variants
The present command is only applicable if all results are the same variant record. If the command is applied to results that use more than one variant record, an error message is issued.

4.

Multi-surface results in multi-material models
For models with several materials and several surfaces, there might be a variation in the number of materials from one surface to the other at any one node. In this case a warning message is issued and the percentage difference is calculated only for the materials that are common to all surfaces.

5.

Gaussian results
As part of the averaging process an extrapolation is made and the extrapolated results will be valid only if the gauss point coordinates are given in their undeformed state. FEMVIEW assumes that this is the case and the user should take care to ensure that the command is only used in this situation.

6.

Use of PERDIFF with assemblies
It is not currently possible to calculate PERDIFF at the assembly level.