Tom, your guess is partially correct. When thinking about stress concentration, it can be useful to imagine a liquid flowing through the part rapidly. If the smooth flow of the fluid is interrupted by abrupt changes, then this will predict where stress could concentrate.
So if you imagine your drillium hole scenarios, the two less abrupt changes of the 45 degree chamfer corners will interrupt the flow less than one 90 degree corner. Taken further, a smooth radius will smooth the flow still further and therefore reduce the possible stress concentration even more than a chamfer. But this radius is more difficult to form.... Typically, a larger radius = lower stress concentration and lower resultant stress.
I hope this helps!
Cheers, Fraser Cunningham. Cincinnati, Ohio.
4. Effects of countersinking drillium holes on
structural strength
(Tom
Sanders)