Stress concentration factors are nothing more than educated guesses. Therefore, make your own number and design by it. If the plate immediately cracked after loading, but not right after welding (which probably actually isn't the case, as you probably have several crack after welding that you can't see with the naked eye), then divide the stress the part should hold in an ideal situation by the stress induced and there's your concentration factor, or at least a starting point. If you were able to watch the piece during a slow load, and repeat this several times, you will have a much more accurate number, as this is where the "ideal" stress concentration factor numbers you are finding in books comes from.

If the load was dynamic, you have some investigating to do, and you will have to look at how many cycles the part went through, what amplitude it was, etc. Basic answer is, there is no definite answer. You have to find it, not in a book, but in a real world test trial.

We have found that a lot of the time, those ideal magic numbers don't exactly apply to some stuff, so be careful and play it safe, especially if this plate's failure could cause injury. Good luck and hope that helps!

gls