On crack paths

Crack paths are discussed from several different points of view. The remarkable reproducibility of crack paths, especially the frequent occurrence of straight cracks, is considered. The general dominance of mode I growth at mixed mode loading (via kinking or tilting of the crack edge) is examined, and it is shown that mode I growth can be suppressed by superposition of a high pressure. Mode I growth is scarcely combineable with mode II or III, because of different micromechanisms, whereas a mixture of modes II and III growth appears to be possible. Directional stability of straight crack paths is also analysed with special reference to the importance of a meaningful definition of the concept itself. Finally crack branching is briefly discussed.