Internal Energy

Internal energy is computed in LS-DYNA based on the six components of stress and strain (tensorial values).
The calculation is done incrementally for each element as follows:

(IE)new  =  (IE)old +  sum over all six directions of (stress * incremental strain * volume)

The internal energies of all the elements is summed to give the total internal energy.

In addition, energy dissipated due to stiffness damping (*DAMPING_PART_STIFFNESS) is added
to the internal energy if RYLEN=2 in *CONTROL_ENERGY.

The calculation of energies is done for user-defined materials outside the material subroutine.

Negative internal energy in shells:

To combat this spurious effect,

• turn off shell thinning (ISTUPD)
• invoke bulk viscosity for shells (set TYPE =-2 in *CONTROL_BULK_VISCOSITY )
• use *DAMPING_PART_STIFFNESS for parts exhibiting neg. IE in matsum. Try a small value first, e. g., 0.01. If RYLEN=2 in *CONROL_ENERGY, then the energy due to stiffness damping is calculated and included in internal energy.