"perhaps one of the engineers on the board can elaborate."
You are correct in your remembering.
One can visualize stress as flowing lines through a material, any abrupt transition causes these stress line to concentrate and increases the stress. If that increased stress is above the Ultimate Yield Stress than failure occurs. Most of these types of classes have a simple demonstration of how this occurs with the use of a lexan machined model and polarized glasses. Wearing the glasses one can see the 'stress' lines in the model and as this stress is increased and/or varied, the stress lines change and/or concentrate. Of course today with computer simulations, these are probably not used anymore.
Any abrupt change in the lattice structure of a metal has to viewed with caution and single use of the material may not provide a rupture but may rupture on multiple uses and there is simply no way to know once the material homogeneity has been compromised.-Dick
