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=1 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.