Reaction force depending on meshing abaqus

Hello everybody,

I am trying to do a flexural test over Abaqus. I applied a displacement at the middle of my part and I would like to know when it will break. For that I wanted to plot the curve RF vs displacement. It works very well. I can see the max force. But the only problem I have is that this force depends on my meshing. I mean if I use a mesh 10*10mm I get 490N for the max force but with a mesh 5*5mm I get 250N. I checked before my VM stress merged(around 89 MPa with mesh 5*5 and 10*10). I really don't know where this variation comes from ! If somebody can help me. Thanks a lot.