How do I determine a desired Charpy/Izod result?

I have a design that we are seeing fractures in. We are changing the material in the design for better toughness. I have done FEA on the design and know the stresses. I know the approximate Yield and Ultimate strength that I am specifying. I would also like to specify a Charpy value but i can't find any information to correlate Charpy values to design parameters. Any help would be appreciated.

I have found an old Army Engineer article correlating charpy to the k1c value but again i run into the k1c value is a result of a test given a specific crack size etc... but i don't have "cracks" in my design unless surface roughness can be used in place of crack length.