Novel Test Methodology for Peak Strain-Controlled Notched Fatigue Test

It is well understood that the fatigue crack initiation life of a structural component having a notch/discontinuity is primarily governed by two parameters: (i) the (i) magnitude of the localised peak equivalent strain amplitude at the notch tip and (ii) the extent of the strain gradient at the notch at/ahead of the notch tip. However, there hardly exists any standard test procedure for conducting notch fatigue tests. The current test practices existing in literature tend to bring out the combined effect of peak equivalent strain and strain gradient on fatigue life instead of the individual effects. The present study aims to develop a new test methodology where the individual effects of (i) peak equivalent strain and (ii) strain gradient on fatigue crack initiation life have been brought out explicitly by conducting fatigue tests on tubes of C-Mn steel. Axial/torsion strain-controlled tests have been performed on a tube having a one-sided, through-thickness circular hole at the centre of the gauge region. Different hole diameters representing different strain gradient conditions have been investigated. The test methodology makes use of the pretest finite element analyses in such a way that it results in a common peak equivalent strain amplitude for different gradient conditions (different hole diameter) or different peak equivalent strain amplitudes for a common hole diameter. The remote strain/relative displacements outcome from the pretest finite element analyses has been used as a controlled parameter in the actual test to get the desired peak strain amplitude at the hole tip. The elastic-plastic pre-test FE analyses use three decomposed Chaboche material models and have been calibrated from the saturated hysteresis loops obtained from uniaxial low cycle fatigue tests on solid unnotched specimens.