The interface is modelled with cohesive surface, and I tried to plot a traction-separation curve of my cohesive model. And I am struggling to plot this curve with a certain surface node:

1. The strain and stress​​​​​​​ I chose is LE12 and S12, and they overlap each other and the stress curve is a mess as below. This can be improved when more nodes were chosen together.

2. Then I turned to plot a displacement-force curve, hoping that can curve work. I used RF2 and U2, just found that RF2 is 0.

I am stuck here and any suggestion? Thanks!