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Fatigue crack growth rates have been measured in different biaxial stress fields for a variety of stress ranges at two temperatures. It is shown that a negative T-stress accelerates the crack propagation, the increase in growth rate being greater for high stresses. Different methods of determining plastic zone size are compared, and a crack growth correlation with crack-tip plasticity is proposed for remote loads not greater than the yield stress.

A high-strain, biaxial (tension/torsion) fatigue study has been conducted on Type 316 stainless steel at 550°C. The tests were under strain control, some with dwell periods up to 10 min at the positive strain peaks. The deformation and fracture behavior under these conditions revealed a strengthening effect due to the fast carbide precipitation which extended life in some cases. Also it is shown that the form of the multiaxial fatigue failure criterion, modes, and directions of cracking were functions of hold time. The multiaxial cyclic stress-strain curve is best described in terms of shear stress and strain components on the maximum shear plane for a given hold time.

A number of studies of the influence of multiaxial stresses and strains on low-cycle fatigue life have been published during the past 20 years. Development of correlations is reviewed and comparisons are made with high-cycle fatigue theories. It is recommended that the more advanced criteria should be incorporated in design procedures to replace the octahedral equivalent strain approach currently used.