Multiaxial fatigue of titanium including step loading and load path alteration and sequence effects

Abstract Load sequence effects on low cycle multiaxial fatigue behavior of titanium are investigated in this study. Constant and variable amplitude axial, torsion, as well as in-phase and out-of-phase axial–torsion strain-controlled tests were conducted on tubular specimens made from pure titanium and titanium alloy BT9. Load non-proportionality had no effect on deformation behavior and cyclic stress–strain data were correlated by von Mises equivalent criterion. However, non-proportional loading resulted in significantly shorter lives and fatigue data could not be correlated by the von Mises criterion. The Fatemi–Socie (FS) critical plane parameter was found to correlate constant amplitude data of both titanium grades very well. von Mises criterion and the critical plane parameter in combination with the linear cumulative damage rule were also employed for fatigue life prediction of both materials under various block loadings with different combinations of axial, torsion, and 90° out-of-phase strain paths. Predicted low cycle fatigue lives based on linear damage rule and the FS critical plane approach were in good agreements with the experimental observations. Predicted failure planes by the critical plane parameter were also found to be consistent with experimentally observed failure planes for all the loading blocks used in the experimental study.