The new cutting plane filter method in FEMFAT max

Complex analyses in FEMFAT max can require great computational power. There are a number of options in FEMFAT for reducing this computational effort: 

  • Limiting the calculation to a corresponding groupDefinition of a node analysis filter
  • Cutting plane filtering using advanced Cutting Plane Parameters

Using cutting plane/node filters, the analyst can specify a limitation to 2D or 3D stresses on surface nodes or can analyse all nodes with 3D stresses. In addition, it is possible to define the angle between the cutting planes or to use certain filter methods. This allows to perform the analysis only for those cutting planes for which it is expected that they will have a significant influence on the result (cp. Figure 1).

 

Figure 1: Selection of the node and cutting plane filters in FEMFAT

Until now, the following options were possible with respect to the filter methods: 

  • Completely omit the filter (“None”), 
  • Select method 1 in which cutting planes are selected based on the maximum equivalent stresses of all channels, or
  • Select method 2 which utilizes the degree of multiaxiality.

What sometimes happened with method 1 was that equivalent stress histories with high mean stresses and low amplitudes compared with equivalent stress histories with low mean stress and high amplitude were assessed as being non-critical, although in connection with the Haigh diagram, they should be assessed as being critical.

A method 3 has now been introduced in order to increase the reliability of the filter methods. The procedure here is similar to method 1, except that instead of the upper stress, an inverse safety factor based on the biggest stress amplitude which occurs is used. As can be seen in Figure 2, the biggest inverse safety factor is calculated for all cutting planes. Those cutting planes are selected for the calculation whose inverse safety factor lies within the filter limit. The cutting planes to be calculated are derived from the filter limit in conjunction with the maximum inverse safety factor.

Figure 2: Inverse safety factor as a function of the cutting plane angle and filter limit

The use of filter 3 is not recommended for the special case in which an identity matrix is specified for the load-time histories in ChannelMAX (i.e. switching channels “on and off”). Instead, it is recommended to switch to filter 1 or to run the analysis without any filters at all.

Due to the generally increased accuracy as compared to method 1 (while retaining the same computational time savings), filter method 3 is the new default setting in FEMFAT 5.3.