Hello,

I am using traction-separation to model hydraulic fracturing. When I increase the material tensile strength by 3 MPa, I am expecting the breakdown pressure to increase 3 MPa as well, but it increases more (about 10 MPa). Also when I increase the confining stress by 5MPa, I am expecting the breakdown pressure to increase about 10 MPa, but again it increases more (about 24 MPa). I am wondering what may cause this? Any help is appreciated. 

Abaqus ​​​​​​​