Which is the most unfavourable crack orientation?

The most unfavourable orientation of a straight crack of given length is investigated, assuming mode I growth. It is shown that this orientation is not always the one perpendicular to the largest principal stress. If the smallest in-plane principal stress is compressive and its magnitude more than about one third of the largest (tensile) in-plane stress, then some other orientations are more unfavourable. Crack growth then takes place after kinking.