Discrete beams, like discrete springs, require valid mass at the nodes to compute the timestep. Unlike continuum based elements, the length of the discrete beams, are not used in the timestep calculations. Hence realistic density in the discrete beam material model must be used. There are two methods this can be achived.
Density (RHO) in the material card is not equal 1.0
When the density in the material is anything but unity, it is required to set appropriate value for the parameter “VOL” in the SECTION_BEAM keyword. When VOL is non-zero and RHO in the material card is not unity, LS-DYNA computes the mass by simply multiplying these two parameters and lumps it equally to the two nodes that make up the discrete beam.
Density (RHO) in the material is equal to 1.0
Whne the density in the material card is equal to unity, LS-DYNA uses the VOL parameter in the SECTION_BEAM keyword as the mass value and lumps them to the beam-nodes.
For non-zero and zero-length discrete beams, both methods are valid but methods must yield realistic mass to provide a reasonable timestep.