PSI - Issue 75

Hayder Y Ahmad et al. / Procedia Structural Integrity 75 (2025) 245–253 Hayder Y Ahmad et al. / Structural Integrity Procedia (2025)

252

8

The extent of this retardation is treatment-dependent: • Untreated: ΔK R ≈ 0 → no retardation.

• CSP: ΔK R ≈ 0.5 − 2 MPa√m • LSP: ΔK R ≈ 2 − 4 MPa√m

These findings are attributed to the delayed crack initiation combined with a significant reduction in the fatigue crack growth rate, which agrees with the effect of LSP (Yang et al., 2001; Sticchi et al. 2015). The present findings in fig 6 (a-c), which demonstrate reduced da/dN rates in both CSP and LSP treated specimens across all R-ratios, are in strong agreement with these earlier investigations. Moreover, correcting the applied ΔK to an effective ΔK eff was essential to characterize fatigue behaviour more accurately in the presence of residual stress fields. As supported by Withers and Bhadeshia (2001), this adjustment provides a more physically meaningful representation of the actual crack tip conditions, particularly in materials subjected to surface enhancement techniques. Therefore, the observed rightward shift in the crack growth curves and the corresponding reduction in crack growth rates are well explained by the incorporation of ΔK R in the Paris Law analysis. 5.2. Influence of R Ratio on ΔK eff and Paris Behaviour The load ratio (R) exerts a substantial influence on both crack closure behaviour and the effectiveness of residual compressive stresses in retarding fatigue crack growth. This relationship is well-aligned with previous studies (Elber, 1971; Suresh, 1998; Schijve, 2009), which demonstrated that variations in R affect the extent of crack opening and consequently the effective stress intensity factor range (ΔK eff ). • At low R-ratios (R = 0.1), crack closure is highly pronounced, allowing residual compressive stresses to exert a dominant retarding effect. This results in a greater reduction of ΔK eff , especially in treated specimens. As a result, although the Paris law exponent m remains relatively unchanged, the constant C is significantly reduced due to delayed crack growth, this trend also reported by Schijve (2009) and Withers and Bhadeshia (2001). • At intermediate R-ratios (R = 0.3), the extent of crack closure decreases, and residual stresses at the crack tip begin to relax due to the elevated minimum load. Under these conditions, CSP continues to provide moderate retardation, while LSP remains highly effective due to its ability to generate deeper, more stable compressive stress fields (Karash, 2022). • At higher R- ratios (R = 0.5), crack closure effects are largely absent, and the influence of residual stress on ΔK eff diminishes. The higher K min reduces the shielding effect provided by compressive residual stresses. Consequently, ΔK eff approaches the applied ΔK, particularly in untreated and CSP -treated specimens. However, LSP still demonstrates measurable crack growth retardation under these conditions, confirming its superior stability and effectiveness even under elevated mean stress levels. These findings reinforce the conclusion that the influence of residual stresses on fatigue behaviour is highly dependent on the applied load ratio and the depth and magnitude of the induced compressive stress field. 6. Conclusion Incorporating residual stress effects into the Paris Law framework provides a more accurate understanding of fatigue crack growth behaviour in the 2024-T4 aluminium alloy. Surface treatments such as CSP and LSP enhance fatigue resistance by introducing compressive residual stresses that reduce the effective stress intensity factor range (ΔK eff ), thereby slowing crack propagation. Among these treatments, LSP proves superior due to its ability to generate deeper and more stable compressive stress fields. This leads to increased fatigue thresholds (ΔK th ) and significantly lower crack growth rates across the tested range of R-ratios (0.1, 0.3 and 0.5). As the R-ratio increases, it indicates higher mean stresses and the crack tends to remain open for a greater portion of the loading cycle. This reduces the beneficial effects of crack closure and diminishes the influence of surface treatments. However, the deeper compressive layer from LSP remains effective even at higher R-ratios, whereas the shallower stresses induced by CSP are more susceptible to relaxation. Overall, these findings highlight the critical interplay between mean stress, residual stress, and crack growth behaviour, reinforcing the value of LSP as a more robust fatigue enhancement technique for high-performance applications.