making curvature

I made 1mm slits with 9mm gap and 19mm gap, and then I curved the cardboard. If the gap between slits is less than the cardboard thickness, the cardboard is curved with smooth curvature. For keeping strength of cardboard, the slits should be on the inside of the curve which you want to make. If you make the slits on the outside of the curve, it is easy to break.