Effect of Superimposed Static Compressive Stress on the Multiaxial Torsional Fatigue Behaviour and Crystallographic Deformation Response of P558 Austenitic Stainless Steel

Components loaded in cyclic torsion are often also subjected to a static compressive stress, but whether this stress extends or shortens the fatigue life is not resolved. Double-cone specimens of the nickel-free, nitrogen-alloyed austenitic stainless steel P558 were tested under angle-controlled cyclic torsion at $R=0$ (rotation ranges of 5–20°, and 22.5° in one test) with a superimposed static compressive stress of 0, 250 or 350 MPa, and were characterised by SEM, electron backscatter diffraction (EBSD), Taylor factor and hysteresis-energy analysis. Compression more than doubled the fatigue life at 5° but reduced it by up to 64% at larger rotation ranges; at 20°/350 MPa the test was stopped by axial ratcheting after 244 cycles. A repeat test and two tests at $R=-1$ reproduced the lives within a factor of 1.25. The grain orientation spread (GOS) of the pure-torsion specimens remained at the level of the untested material, although the specimens at 10° and 15° were cycled plastically, and compression did not change it at 5°. At 10–22.5°, compression raised GOS to 2.4–4.4°, four to seven times the pure-torsion value, in line with the accumulated axial ratcheting strain (7–18%) rather than with the cyclic shear amplitude or the equivalent stress. The change in life followed the maximum principal stress: at 5°, compression lowered it by up to 47%, whereas at larger rotation ranges the compression-induced increase in the stabilised shear stress raised it by up to 79%. Whether superimposed compression is beneficial or detrimental therefore depends on the rotation range.