PSI - Issue 71

Shreebanta Kumar Jena et al. / Procedia Structural Integrity 71 (2025) 103–110

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(a) (b) Fig. 3 Strain path for (a) remote in-phase axial torsion (b) remote out-of-phase axial torsion loading condition

3.2. Localized strain measurements The localized strain field at the notch tip has been captured using DIC technique. DIC is a non contact type optical surface strain measuring technique which works on the principles of continuum mechanics. The measurement of the complete strain field information at and ahead of the hole tip is performed by comparing the images recorded in undeformed configuration with their corresponding set of images in deformed configuration. Before using this technique for measurement of surface strains ahead of holes, it has been extensively validated for a wide variety of known/ applied strain fields on un-notched tube samples Jena et al. (2023). The in-house DIC set up is capable of measuring strains of 0.1% (or higher) with reasonable confidence level. The same set up has been used for capturing the fatigue crack initiation event corresponding to surface crack size of 1mm. However, this system can measure surface crack of nearly 0.2mm. A prior indication of sub-mechanical crack (~0.2 mm) is reflected in terms of amplified strains from its saturated value. A typical localized strain contours measured using DIC shows the crack initiation site for remotely proportional axial torsion condition (refer Fig. 4). In the present study, the test was continued till crack size becomes 1mm. 4. Results and Discussions The effect of remote proportional and non-proportional applied strains has been brought out on localized strains ahead

of the hole. Further, the influence of these remote loading conditions on fatigue life have been discussed for tube with hole and without hole geometries. Additionally, the effect of hole diameter has been studied on fatigue life for a given set of remote axial and torsional strain amplitudes under in-phase conditions. 4.1. Localized measured strains under remote proportional/ non-proportional conditions Before understanding the combined effect of axial and torsion loading condition on notched tube specimens, first the results of plate with a central hole have been revisited here. It is well understood and reported that, a simplistic infinite plate geometry when subjected to remote pure axial traction (as shown in Fig. 5), results in maximum (and equivalent) stress/ strain peaking at = 0°/180° , Sadd (2014). However, this peak stress/ strain location changes to =45° / 135°, if remotely subjected to pure shear state as reported by Sadd (2014) and Jena et al. (2022, 2025).

Fig. 4. Typical DIC strain contours indicating crack initiation location and size under remote proportional axial-torsion conditions