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