PSI - Issue 68

S.R. Raghuraman et al. / Procedia Structural Integrity 68 (2025) 769–775 S.R. Raghuraman et al. / Structural Integrity Procedia 00 (2025) 000–000

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which indicates a softening of the material as a result of the primary loading. Additionally, the increase in the change in temperature (Fig. 3 (b)) evidently confirms the assumption regarding the cyclic softening behaviour. Microstructural changes can also be demonstrated by an increase in the MBN amplitude of the PL condition compared to the IC. These findings are consistent with research results proposed by Sagar et al. (2005) for unalloyed and low-alloyed carbon steels in different material conditions.

Fig. 3: (a) MBN butterfly curves for the IC (black) and the PL (red) conditions; (b) comparison of the material responses for the IC and the PL conditions

The specimen surfaces were assessed in terms of IC and PL conditions using SEM imaging, as depicted in Fig. 4. The electropolished surface of the specimens in IC with a surface roughness of R z = 0.05 µm is represented in Fig. 4 (a). Areas including intrusions and extrusions resulting from cyclic loading, as well as a microcrack, are illustrated in Fig. 4 (b) and Fig. 4 (c).

Fig. 4: SEM-images of electropolished specimens of SAE 4140 steel (a) in IC; (b) in PL condition at σ a, PL = 720 MPa; (c) a microcrack due to primary loading

The PL specimens were reconditioned by mechanical polishing followed by electropolishing. After the primary loading of the specimens, the RP was estimated by a secondary loading to the up to the transition to the VHCF regime at a test frequency of 1 kHz. For this purpose, IC specimens and PL specimens with and without reconditioning were