PSI - Issue 71

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

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(a) (d) Figure 8: FE tube model with a central hole (a) remote pure axial loading, (b) remote pure torsion loading (c) a schematic of the tube gauge section indication and angles, (d) typical mesh convergence study depicting the comparison of the principal and equivalent stress components with fine and coarse mesh along = 0° . 3.3. Notched fatigue test After successful completion of pretest FE analyses, the remote axial and shear strain fields corresponding to these peak equivalent strain amplitudes have been extracted from pretest FE analyses and are being used as a controlling parameter for the extensometer in the actual test. Remote pure axial and remote pure torsion strain-controlled tests have been carried out to investigate the individual effect of strain gradient and peak equivalent strain on fatigue crack initiation life. 3.3.1. Effect of strain gradient To bring out individual effects of strain gradient on fatigue crack initiation life, remote pure axial and remote pure torsion tests have been conducted on different notched specimens while subjected to a single peak strain amplitude condition. Typical comparisons of test fatigue lives for different gradient conditions (un-notched tube & tube with hole diameters of 3 mm & 8 mm) for given sets of peak equivalent strain amplitudes of 0.33% are compared in Figures 9 (a) & (b) for remote pure axial & pure torsion loading conditions, respectively. Figure 9 (a) & (b) clearly show that for a given peak equivalent strain amplitude, fatigue crack initiation life for unnotched tube specimens is lesser when compared with notched tube specimens. This is because, in the case of unnotched tube specimens, all the material volume present at any cross-section in the gauge region is experiencing uniform equivalent strain amplitude due to the zero strain gradient condition. However, for notched tube specimens due to the existence of a varying strain gradient condition at the notch tip, only the notch tip location is subjected to peak equivalent strain amplitude conditions, while the remaining material volume is experiencing a lower value of strain amplitude. Due to this reason, larger material volumes are experiencing fatigue damage in the case of unnotched tube specimens when compared with notched tube specimens. This in turn results in lower test fatigue life in unnotched tube specimens when compared with notched tube specimens. (b) (c)

(a) (b) Figure 9: Comparison of test fatigue lives for different strain gradients (or hole diameters) subjected to given sets of peak equivalent strain amplitudes (a) Remote pure axial loading (b) Remote pure torsion loading Further, it is also observed from Figures 9 (a) & (b) that among notched specimens, specimens having an 8 mm hole diameter (shallow strain gradient condition) has less fatigue life when compared with 3 mm hole diameters (steeper