confusing problem

[Chen1]

Hello.
I used Abaqus explicit dynamic to simulate the uniaxial compression test.
First, I used a displacement control to compress a deformable rock sample using two rigid platens, where one of them is fixed and the other can move.

The movable platen is forced to move 0.02 in. i found the reaction force on the rigid platen is 14000 psi.

http://www.eng-tips.com/threadminder.cfm?pid=799

Second i used a load control to compress the rock sample, i used a pressure to squeeze the rock sample. i divide (14000 psi/ area of the rigid platens)

When i compare the external work, internal work and strain energy in the two cases (load and displacement control) i found a difference !!!! Why?

I was wondering why there is difference between the two cases (displacment and load control), i got the reaction force from the displacement control and use it in the load control, so why there is a difference between them.

I keep everything the same in the two cases except the mode of loading.

can anyone help me why there is a difference between the load and the displacment control.