Characteristics of the process zone at sharp notch roots

The classical Williams solution for the state of stress at the tip of a semi-infinite notch is re-visited and the two-term singular solution re-written in a form making the mode mixity and load magnitude explicit. This is used to show that, for a 270° solid angle, the majority of notch problems exhibit a process zone which is entirely or substantially mode I in character, which in turn means that the notch strength may practically be governed by a single elastic parameter. A method for finding the practical limit on the load and stress intensity ratio where this holds is described.