not smooth F-D curve_Simulation of indentation with elastic-plastic model

Dear all,

I have a problem now:

I simulated indentation on a elastic-plastic model with a rigid
indenter, the elastic ideal plastic with von mises stress model in

ABAQUS is used. A contact pair is applied to calculate the contact between the indenter and the sample.

The problem is that the obtained the force-displacement data in the loading part, which contains the plastic deformation
is not smooth, but in the unloading part is smooth. But if a model
without plastic is used, the force-displacement data is smooth.

I have tried to use a finer mesh and finer calculation increament step, but both method don't work.

If anyone can give me some suggestion or tips how to solve this problem?

Thank you all