PSI - Issue 71

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ScienceDirect

Procedia Structural Integrity 71 (2025) 34–41

Abstract 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. Shreebanta Kumar Jena a,b* , Punit Arora a,b , Suneel K. Gupta a,b , J.Chattopadhyay a,b a Reactor Safety Division, Bhabha Atomic research Centre (BARC), Mumbai, India b Department of Engineering Sciences, Homi Bhabha National Institute (HBNI), Mumbai, India 5 th International Structural Integrity Conference & Exhibition (SICE 2024) Novel Test Methodology for Peak Strain-Controlled Notched Fatigue Test 1. Introduction Fatigue is the most common mode of failure phenomenon, predominantly observed in the structural industry. Fatigue itself is a complex phenomenon, and its complexity is enhanced in the presence of notches/discontinuity. During operation, these notches/discontinuity locations experience load excursions due to amplification of stress/strain, ultimately becoming the site for fatigue crack initiation. Further, the local state of stress at the notch/discontinuity region becomes elastic-plastic, as the von Mises equivalent stress exceeds the yield strength of the material at these localised regions, though the elastic state of stress prevails at the remote locations. These differences in stress/strain amplitude values at localised (at notch tip) and remote locations set up a stress/strain gradient. This gradient effect is due to introduction of notch and discontinuity was studied by Jena et al (2020, 2022). It is well understood that the state of stress/strain as well as the extent of strain gradient at the notch tip tend to influence the fatigue life. Therefore, accurate modelling of these geometric discontinuities and their effects on fatigue life should be duely accounted at the design stage. Gao et al. (2010) studied the influence of notch size variation on fatigue life by carrying out four different constant amplitude load-controlled tests in ambient air environment for 16MnR steel. It has been observed that under same remote loading condition specimens having sharp notch radius results in less fatigue life compared to that of blunt notch radius. Sakane et al. (2011) investigated the effect of notches by conducting stress-controlled tests on Keywords: low cycle fatigue; strain gradient; Finite element analyses; Chaboche material model © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of SICE 2024 organizers

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of SICE 2024 organizers 10.1016/j.prostr.2025.08.006