So I'm dropping a 10kg mass down to the flange (very bottom) and I want to calculate the stress.

So I'm dropping a 10kg mass down to the flange (very bottom) and I want to calculate the stress. I know mgh, the total energy in the beginning and the total energy in the end is ((stress)^2/young's modulus)* volume. So I'm wondering do I take the volume of the entire system even the one on top, or only the volume below the mass? Can you also explain why. Thanks.