How can I cut a triangular section at an angle (<90 degrees) to the face of a part?

[Julian2w2w]

I want to cut a triangular section thru a part at an angle to the face (not perpendicular)
Can you tell how can I do that please?
Here is the line through which I wannna make the cut:

Here is my trial:
1)I drew an axis at the start of the cut, on the face of the part.
2)I make a plane along the cut
3)I make a plane perpendicular to the previous plane thru the axis.
4)I drew the section on that plane with a through-all depth. It did not work

 

[Julian2w2w]

I was able to do the cut through the part. But there was few things that don't match what I wanted.
I wanted the triangular section to cut through the part. However, cut through the entire part. Look hoe the part is cut from down to up. How can I do it properly? My section cut implies only a cut through, not a cut through the hole part, that divides the part into 2.

 

[Julian2w2w]

In fact the part is a shell element, could it be the reason why?

 

[FEA way]

Try offsetting the sketch plane a bit (in the direction opposite to the cutting direction).