I misunderstood your question to regard the onset of plasticity which is the general interest in a material above the density ratio you identified. There is actually a change in Young's Modulus but the amount is hardly appreciable. Total strain energy data and lower q3 values are what I expect...
Why not just use the Gurson material model that is available within Abaqus? There should be a simple way to modify the pore size within this material model that can meet your goal directly?
Unless the code has changed dramatically, which is possible, the Eulerian strains in the UMAT routines do not update the Langrangian vectors in the time step forward. They do address the virtual work functions and the first pass is where you need to introduce any virtual work energy change. You...
