Failure Analysis Screw Driver

i recently broke the tip off of my flat head crew driver trying to back out a rusty screw. i have to do a failure analysis of it but i have been having trouble finding the right relation ships between equations. i know that the metal is AISI M2 and

it also has a Rockwell hardness factor between 63 and 65

i can measure the area of the tip which happens to be .15 in (long ways) by ,02 in (width)

i want to calculate the torque that it should have failed at how much my had would of had to put on it to break it at the tip like it did as if the screw driver was brand new

since it is not new and there have been many cyclic loads the actual number will be lower but i am looking to just calculate what the load should have been.

thank you