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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subjected to a secondary loading at a stress amplitude of 630 MPa. Fig. 5 provides a comprehensive comparison of the fatigue lifetime of the specimens in terms of respective number of cycles to failure N f with the corresponding material condition, and these data points arise from the work by Shrivastava et al (unpublished results) and were experimentally determined as part of the joint project.

Fig. 5: Comparison of the lifetimes of IC specimens and PL specimens with and without reconditioning [data from Shrivastava et al., (unpublished results)]

The IC specimens (green circles; IC) are characterised by an average lifetime (green line) of approximately 2.9 × 10 6 cycles. Considering specimens already subjected to primary loading without reconditioning (red circles; without RC), a significant reduction in lifetime to approximately 8.5 × 10 4 cycles could be determined. Reconditioning, on the other hand, led to an increase in lifetime to approximately 1.06 × 10 6 cycles. In comparison to the IC specimens, this results in a reduced lifetime of 63.95 %, whereas an increase in lifetime by a factor of 11.42 was observed in case of reconditioned specimens (blue circles; with RC) compared to specimens that were not reconditioned. The surface treatment of the PL specimens therefore appears to be suitable in order to noticeably improve the fatigue lifetime compared to non-reconditioned specimens. Despite this improvement, the lifetime of reconditioned specimens was still demonstrably less compared to that of the IC specimens. In previous research by Raghuraman et al. (2023), an increase in fatigue life of reconditioned specimens was observed, although this presumably depends both on the parameters selected for the primary loading (stress amplitude and number of cycles for the primary loading) and for the secondary loading (frequency, stress amplitude), which will be systematically investigated in the future research. The RP can be derived from the ratio of the lifetimes concerning different PL specimen conditions to the lifetime of reference IC specimens, as illustrated in the Equation (2). This results in RP values of 0.360 and 0.029 for specimens with RC and without RC, respectively. These RP values provide a measure of the latent fatigue lifetime that can be still elicited through re-use of a material already subjected to loading that would otherwise remain untapped. 4. Summary and conclusion The re-use of materials, in particular for the quenched and tempered SAE 4140 steel investigated within this research, can be both economically and ecologically attractive. The StressLife method was used to generate a trend S-N curve to determine a stress amplitude σ a, PL = 720 MPa for primary loading at a testing frequency of 5 Hz. Several constant amplitude tests were performed at this stress amplitude in order to quantify the induced damage. Besides the fatigue behaviour, which was recorded in-situ using thermography and resistometry, the damage state was assessed using a (2)