Unit measure system selection for FEM with focus on dynamics



Selection of a consistent unit measure system is fundamental to get correct results in FE analysis. Usually, users need to pay particular attention to this aspect because the solver will digest whatever model it is provided as input, without giving any warning as it will not be able to recognize unit measure discrepancies.

In dynamic analysis this is more evident because inertia of the system need to be taken into account and wrong definition of input can give results with high error margins (natural frequencies of a system not displayed in Hz and with bad offsets are a classic example of poor unit measure management).

The below table shows examples of consistent unit measure selection for different disciplines.


Table 1: unit system definition examples


To complicate even more the situation, it is habit in certain industry sectors, like aerospace, to make large usage of inconsistent imperial unit measure system. If you have to work with FE models prepared by US/UK based companies, you will probably stumble in the classic [inch], [lbf], [psi], [s] system that is an inconsistent one. If you are not used to it this will appear as an odd choice and indeed it is a bad habit. So, how can this be corrected from the solver side to give you proper results? You will notice also that these models usually have a parameter called WTMASS with a value of 0.00259. This parameter has been introduced to correct mass matrix to overcome this inconsistency in the unit measure selection, and this specific numerical value comes from 1g acceleration correction that is 386.1 in/sec2 à1/386.1 = 0.00259.


The table below shows wtmass definition for different unit measure systems.


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Table 2: Wtmass examples


At the moment Hyperworks interface does not support an automatic unit measure system selection, but a project to add this capability is under development. Other pre-processors of Altair suite like Simlab already support this selection.


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Figure 1: Simlab units selection


In conclusion when dealing with unit measure systems:

  1. Be sure to adopt a consistent unit system;
  2. If point 1 is not possible add to your model a correct wtmass parameter.