Mass Scaling

caeguy

Mass Scaling
  
Mass-scaling refers to a technique whereby nonphysical mass is added to a structure in order to achieve a larger explicit timestep.
  
Anytime you add nonphysical mass to increase the timestep in a dynamic analysis, you affect the results (think of F = ma). Sometimes the effect is insignificant and in those cases adding nonphysical mass is justifiable. Examples of such cases may include the addition of mass to just a few small elements in a noncritical area or quasi-static simulations where the velocity is low and the kinetic energy is very small relative to the peak internal energy. In the end, it's up to the judgement of the analyst to gage the affect of mass scaling. You may have to reduce or eliminate mass scaling in a second run to gage the sensitivity of the results to the amount of mass added.
  
One can employ mass scaling in a selective manner by artificially increasing material density of the parts you want to mass-scale. This manual form of mass scaling is done independently of the automatic mass scaling invoked with DT2MS in *control_timestep.
  
When DT2MS is input as a negative value, mass is added only to those elements whose timestep would otherwise be less than TSSFAC * |DT2MS|. By adding mass to these elements, their timestep becomes equal to TSSFAC * |DT2MS|. An infinite number of combinations of TSSF and DT2MS will give the same product, i.e., timestep but the added mass will be different for each of those combinations. The trend is that the smaller the TSSF, the greater the added mass. In return, stability may improve as TSSF is reduced (just as in non-mass-scaled solutions). If stability is a problem with the default TSSF of 0.9, try 0.8 or 0.7. If you reduce TSSF, you can increase |DT2MS| proportionally so that the product/timestep is unchanged.  
  
To determine where and when mass is automatically added, write GLSTAT and MATSUM files. These files will allow you to plot added mass vs. time for the complete model and for individual parts, respectively. To produce fringe plots of added mass in parts comprised of shell elements (DT2MS negative), set STSSZ=3 in *database_extent_binary. You can then fringe the added mass (per element) using LS-POST by choosing Fcomp > Misc > time step size. (Here, the label "time step size" is really the element added mass.)  
  
The difference between using a positive or negative number for DT2MS in *control_timestep is as follows:  
  
Negative: Mass is added to only those elements whose timestep would otherwise be less than TSSF*abs(DT2MS). When mass scaling is appropriate, I recommend this method.
  
Positive: Mass is added or taken away from elements so that the timestep of every element is the same. My opinion is there is no advantage to using this method over the negative DT2MS method and I find it harder to rationalize.  
  
You can use ENDMAS in *control_termination to stop the calculation after a certain amount of mass has been added (active for automatic mass scaling only).  
  
See also: Long Run Times , Quasistatic .
  
Quasistatic vs. Dynamic Relaxation
  
Dynamic relaxation is not intended for general quasi-static analysis. Dynamic relaxation is okay for applying preload when the preload produces only small elastic strains or for initializing a system to a prescribed geometry but it's not good for much else.
  
You can do a quasi-static analysis the old way by running a regular explicit simulation, invoking time- and/or mass-scaling as necessary to crank out the results in a reasonable timeframe, but this approach can be tricky. You have to keep an eye on the kinetic energy in the system as you want to minimize the inertial effects. Basically, the kinetic energy should remain small relative to the internal energy. (By time-scaling, I mean applying the load more quickly than in the quasi-static experiment in order to reduce the simulation time.) See the paragraph above, Mass Scaling , for more on mass scaling.
  
Or, you can try an implicit, static analysis using v. 960 of LS-DYNA. See the commands *control_implicit_... and Appendix M in the 950 User's Manual. There are examples of implicit analysis on our "user" ftp site in the ls-dyna/example directory.