Energy Density of Porous Rocks
Eureka! I and my prof figured it out. We found something interesting - that the energy density actually depends on a number of internal variables, namely, the displacements of the outer and inner walls, and porosity (which we already knew). So, when we specified the outer displacement to always be a fraction of the radius of the outer sphere, that seemed to fix the problem.

Thanks to all who attempted to help me out! :)