AbstractTorsional tests under different mean shear stresses were conducted at room temperature using a 1.99% NiCrMo steel. A technique was developed to control both the mean and the alternating shear stresses. It is shown that the mean shear stress plays a fundamental role in promoting stage I, mode II short crack growth. The effect of mean shear stress on the plastic deformation at stage II crack tips is discussed, which illustrates the mean shear effect on the transition between stage I and stage II crack growth. The polarity of mean shear stress is found to have no effect on the fatigue crack growth rate and the fatigue lifetime of an isotropic material. A model is proposed to account for the effect of mean shear stress on the fatigue lifetime under torsional loading.