PSI - Issue 71

Shreebanta Kumar Jena et al. / Procedia Structural Integrity 71 (2025) 34–41

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circumferentially grooved shaft of SUS304 steel material under uniaxial and multiaxial remote loading conditions. Gladskyi and Fatemi [2013] studied fatigue behaviour of low carbon steel notched tube specimens with a single sided through thickness hole at the center of the gauge region while subjected to axial and torsion loading conditions. It is observed that despite of the fact stress concentration factor is higher in case of torsion loading condition, notch effect is more pronounced in case of axial loading condition. Gates and Fatemi (2014) studied the effect of notches and multiaxial loading on fatigue crack initiation life by performing load-controlled tests on thin tube specimen 2024-T3 aluminum specimens with a single sided through thickness hole at the gauge region while subjected to pure axial, pure torsion, in-phase axial-torsion and out of phase axial torsion loading conditions. They have brought that under same remote loading conditions, fatigue life for torsional and in phase loading condition is higher than that of axial and out of phase loading conditions. Gates and Fatemi (2016) studied the effect strain gradient and multiaxial loading on fatigue crack initiation life of notched specimen by conducting load-controlled tests on three different type of notched specimens (tube specimen with a trough thickness hole at gauge region, stepped shaft and circumferentially grooved shaft) of 2024 T3 aluminum alloy and AISI 1141 steel under completely reversible loading conditions. Wei Su and Hongmei Zhu (2022) have studied the effect of strain gradient and multiaxial loading on fatigue life by conducting stress-controlled tests on tube specimens of different with through thickness single sided hole at the gauge region of 2198-T8 Al-Li alloy material. In order to study the notch effect three different sizes of hole diameter has been considered. However, all these tests have been performed under same remote loading conditions though different gradient conditions exist owing to different notch dimension. This in turn results different peak equivalent strain amplitude and strain gradient at the notch tip for different notch dimension due to different strain amplification factor. Therefore, these existing test methodologies fail to explicitly bring out the individual effect of strain gradient and peak equivalent strain amplitude on fatigue crack initiation life. However, it has been demonstrated by Jena et al (2023, 2024) that there can be cases where structural components subjected to a given peak equivalent strain at the notch tip/discontinuity (variable remote loading condition) but results in different fatigue life due existence of different strain gradient ahead of the notch tip. Further, this study also brought out the individual effect of peak equivalent strain and stain gradient on fatigue life. This is achieved by conducting tests on notched tube specimen under remote pure axial and pure torsion loading condition. This study also highlights the importance of pretest FE analyses before conducting actual test. In view of this, present study focuses on proposing a new test methodology for notched tube specimen to bring out the individual effect of strain gradient and peak strain on fatigue life under different remote axial/torsion loading conditions. 2. Selected steel tensile and fatigue properties This section gives a brief idea about the selected test material grade, its chemical constituents and test specimen details. Additionally, test setup, calibration of the Chaboche model parameter and its suitability on unnotched tube specimen has also been discussed here. 2.1. Material selection & design of test specimen SA333 Gr. 6 low C-Mn steel, the primary piping material of Indian Pressurised Heavy Water Reactor (IPHWR) has been chosen as material for fabrication of test specimens. It spreads from the reactor core to the steam generator for transferring heat in the primary loop. The piping material has 610 mm outer diameter and thickness of 39 mm. It has been used for machining test specimens in such a way that the axial direction of specimens is aligned with pipe axial direction. Standard solid tensile and low cycle fatigue specimens have been designed in accordance with the ASTM standard E08 (2024) and E606 (2021) . Further, unnotched tube specimens and tube specimens with a single sided through thickness hole in the gauge region has been fabricated in accordance with ASTM standard E2207 (2015).The dimensional details of solid tensile and Low Cycle Fatigue (LCF) specimens (Unnotched solid, tube & notched tube) has been presented in Figure 1. During fabrication of the LCF specimen, extra care has been given to maintain a mirror-polished surface finish in the gauge as well as the transition region of the test specimen.