Influence of load and deformation-controlled multiaxial tests on fatigue life to crack initiation

Generally, areas of components from ductile steels with notches or geometrical transitions are critical because of the resulting stress/strain concentrations. In these areas due to the stress-gradients and constraint local deformations are displacement controlled even if the materialʹs yield stress is exceeded, as long as the deformations are below the structural yield point. Therefore, load controlled tests in the elasto—plastic region with unnotched specimens from ductile materials under combined axial loading and torsion are not suitable for the interpretation of a componentʹs behaviour because of uncontrolled local deformations. Thus, the influence of multiaxial stress/strain states on the fatigue behaviour of a compoment under elasto—plastic deformations can be determined reliably with unnotched specimens only by deformation controlled tests, if plastic racheting is not expected in critical areas. The modified effective equivalent strain hypothesis (MEES) for ductile materials renders a satisfactory evaluation of the in- and out-of-phase test results.