The study is devoted to the prediction of crack path direction in polymodal fatigue. First experimental and theoretical results published in the literature are reviewed. It is shown that proportional and nonproportional loading conditions should be distinguished. Then, the experimental results obtained in cyclic Mode III and biaxial nonproportional Modes I + II loading are presented. These tests were carried out on two alloy steels and an aluminum alloy. The biaxial tests were performed on cruciform type specimens.

In Mode III loading, it is shown that, at low applied torque, the crack surface adopts a “factory-roof” appearance with facets characteristic of local Mode I extension. It is suggested that the flat surfaces corresponding to apparent macroscopic Mode III extension observed in this study as in other ones might be an artefact. In Modes I + II loading, it is shown that the two investigated materials exhibit very different crack bifurcation angles. The results are discussed in terms of possible extended forms of the maximum tangential stress criterion, Δk_Imax*, K_Imax* and (da/dN)_max. It is shown that, as a general rule, the latter criterion gives results in better agreement with the experiments. The difficulties associated with crack closure effect are also shortly discussed.