PSI - Issue 71

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

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strain gradient condition) for a given peak equivalent strain amplitude condition. The reason can be attributed to the fact that for the steeper strain gradient (3 mm hole case), lesser material volume is subjected to higher strain/stress amplitudes, resulting in lesser fatigue damage and exhibiting higher fatigue test life. However, in the case of a shallow strain gradient condition (8 mm hole case) higher material volume experiences larger strains/stresses (or 8 mm hole diameter/ un-notched tube), resulting in higher damage and shorter fatigue life. 3.3.2. Effect of peak equivalent strain Similarly, for a given strain gradient condition, the effect of peak equivalent strain on fatigue crack initiation life has also been investigated. It was observed that fatigue life decreases with an increase in peak equivalent strain amplitude. Figure 10 (a) & (b) present a typical comparison of test fatigue life data for different peak equivalent strain amplitudes for given sets of strain gradients for remote pure axial and remote pure torsion loading conditions. For a given strain gradient, test fatigue life reduces with an increase in peak equivalent strain amplitude. A higher peak equivalent strain amplitude at the notch tip expands the size of the cyclic plastic zone in front of it.

(a) (b) Figure 10: Comparison of test fatigue lives for peak equivalent strain amplitudes subjected to given sets of strain gradient condition (a) Remote pure axial loading (b) Remote pure torsion loading This expansion causes increased fatigue damage, ultimately reducing the material's fatigue life. This observation is consistent with fatigue life results observed under uniaxial strain cycling for unnotched standard low cycle fatigue specimens. 4. Conclusions The individual effect of strain gradient & peak equivalent strain on fatigue life has been brought out here. 1. Effect of Strain Gradient on Test Fatigue Life: Increases with the decrease in hole size. 2. Effect of peak equivalent strain test fatigue life: Decreases with an increase in peak equivalent strain. References Arora P et al J, Proposing an improved cyclic plasticity material model for assessment of multiaxial response of low C-Mn steel, Int. J. Fatigue, 2021,142, 105888. Arora P. A modified cyclic plasticity model for C-Mn steel and a new critical plane model to predict crack initiation under multiaxial cyclic loading. Ph.D. dissertation, Dept. of Eng. Science, Homi Bhabha National Institute, India, 2021. ASTM E8/E8M-22, Standard Test Methods for Tension Testing of Metallic Materials, ASTM E08, ASTM International, West Conshohocken, PA, 2024, 03.01. ASTM E606/E606M-21. Standard Practice for Strain-Controlled Fatigue Testing. (Designation: E 606 – 04), American Society of Testing, ASTM International, West Conshohocken, PA, 2021, 03.01. ASTM E2207-15(2021), Standard practice for strain-controlled axial torsional fatigue testing with thin-walled tubular specimens. 2015, 03.01 Gao Z et al. An investigation of fatigue of a notched member. Int. J. Fatigue 2010; 32: 1960–1969. Gates N, Fatemi A. Notched fatigue behavior and stress analysis under multiaxial states of stress. Int. J. Fatigue 2014; 67: 2–14. Gates N, Fatemi A. Notch deformation and stress gradient effects in multiaxial fatigue. Theory. Appl. Fract. Mech. 2016; 84: 3–25. Gladskyi M, Fatemi A. Notched fatigue behavior including load sequence effects under axial and torsional loadings. Int. J. Fatigue 2013; 55: 43– 53. Jena SK et al J. Fatigue experiments and life predictions of notched C -Mn steel tube. Int. J. Fatigue 2023; 169:1-15. Jena SK et al, Axial/torsional fatigue tests on notched tubular specimens of carbon steel and life estimation using theory of critical distance method, 2024, 60, 115-122, Procedia Structural Integrity Jena SK et al. Validation of notch stress estimation schemes for low C–Mn steel. Fatigue Fract. Eng. Mater. Struct. 2022; 1-20. Jena SK et al. Review of various hypotheses used to correct notch elastic stress/strains for local plasticity, Fatigue, Durability, and Fracture Mechanics. Lecture Notes in Mechanical Engineering, 2020, 121-134. Sakane M et al. Notch effect on multiaxial low cycle fatigue. Int. J. Fatigue 2011; 33: 959–968. Su W and Zhu H, Multiaxial fatigue behavior and life prediction of 2198-T8 Al-Li alloy notched tubular specimen based on DIC technique, Int. Jour. Dam. Mech., 31,957-974, 2022.