3D dynamic simulation

Equation of state models (Mie-Gruneisen equation of state etc) have been widely used to study hydrodynamic response of material during explosive deformation, ballistic impacts for a long time. As far as I understand, a hydrodynamic response is required to be incorporated in a model if the pressure of impact leads to an increase volumetric strength. That is the stress wave speed in the material exceeds the speed of sound in the material (v = sqrt(elastic modulus/density)).  Ballistic impacts occur at strain rates close to 10^9 to 10^12/s.

Hello,

I have to export stiffness and mass matrices from Abaqus (dynamical simulation)

and I have been using the commands *Matrix Generate and *Matrix Output

I need to export the matrices for different  instants of time  (time intervals) and I have noticed that the entries

of the MASS matrices are not equal at different  time points.

Is that correct or should the MASS matrices be constant for a dynamical simulation?

many thanks