PSI - Issue 75

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

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Table 3 presents the mean fatigue crack growth rate (da/dN) ± SD (calculated using Equation 1) in the Paris regime for each surface treatment and load ratio (R), derived from three replicate tests (n=3). Table 3: Values represent mean ± standard deviation (SD) of three replicates (n = 3). SD values quantify specimen-to-specimen variability in crack growth rates Treatment R = 0.1 R = 0.3 R = 0.5 (×10⁻⁶) Untreated 4.8 ×10⁻⁶ ± 0.3 6.24 ×10⁻⁶ ± 0.4 7.68 ×10⁻⁶ ± 0.5 CSP 2.7 ×10⁻⁶ ± 0.2 3.51 ×10⁻⁶ ± 0.3 4.32 ×10⁻⁶ ± 0.4 LSP 1.5 ×10⁻⁶ ± 0.1 1.95 ×10⁻⁶ ± 0.2 2.4 ×10⁻⁶ ± 0.3 The experimental results demonstrate a clear dependency of the fatigue crack growth rate on both surface treatment and load ratio. Across all R-ratios, LSP-treated specimens consistently exhibited the lowest crack growth rates, followed by shot peened specimens, while untreated specimens showed the highest crack propagation rates. For example, at R = 0.1, untreated samples exhibited a crack growth rate of approximately 1.2×10⁻⁶ mm/cycle at ΔK = 5 MPa√m, increasing to 8.3×10⁻⁵ mm/cycle at ΔK = 21 MPa√m. In contrast, LSP -treated specimens demonstrated significantly improved performance, with da/dN values ranging from 5.0×10⁻⁷ mm/cycle at ΔK = 5 MPa√m to only 2.2×10⁻⁵ mm/cycle at ΔK = 21 MPa√m. This represents a reduction of over 70% in crack growth rates compared to untreated samples, highlighting the effectiveness of LSP in enhancing fatigue resistance. As the load ratio (R) increased from 0.1 to 0.5, all specimens exhibited higher crack growth rates. This trend is expected due to the reduction in the crack closure effect at higher R values, leading to increase the effective stress intensity factor ΔK eff (ΔK eff = ΔK – ΔK R ). However, the relative benefits of surface treatments remained consistent. The data at R=0.5 reveals a critical finding; the compressive residual stress field from LSP remains highly effective under elevated mean stress, where the benefits of many surface treatments diminish. While the performance of LSP at low R-ratios is well-established, its efficacy at high R-ratios like R=0.5 is less documented. The observed retardation, despite the suppression of plasticity induced crack closure, indicates that the primary mechanism is the direct shielding of the crack tip by a stable, deep compressive stress field. This field introduces a significant negative K R , persistently lowering the local ΔK eff and K max . The superior stability of the LSP-induced layer against relaxation under high K min conditions is a key outcome of this study, highlighting LSP's potential for applications involving high mean stresses. To further confirm this finding and directly link the mechanical performance to microstructural changes, future work will involve microhardness profiling of the treated specimens to quantify the work-hardened layer's depth and stability. Furthermore, a detailed analysis of the fatigue fracture surfaces will be conducted to correlate the crack path and growth rates with the underlying compressive stress field and identify any microstructural arrest features. Furthermore, the influence of crack tunnelling, a potential concern given the specimen thickness relative to the depth of the compressive residual stress layers, will be the subject of a subsequent investigation. A detailed fractographic analysis will be conducted to quantify crack front curvature using standardised methods ASTM E399. This future work will precisely determine the impact of tunnelling on the stress intensity factor and further refine the understanding of fatigue crack growth in surface-treated components. 5.1. Role of Residual Stress (ΔK R ) on Crack Initiation In addition, the fatigue crack growth data were analysed using the Paris power law: da/dN = C (ΔK eff ) m (3) where: da/dN is the crack growth rate, C and m are material constants, and ΔK eff is the effective stress intensity factor range, given by: ΔK eff = ΔK – ΔK R (4) In this formulation, ΔK represents the applied stress intensity factor range, while ΔK R accounts for the reduction in crack tip driving force caused by compressive residual stresses. The presence of such stresses introduces a negative contribution to the local stress intensity factor, thereby delaying crack initiation and retarding early crack growth. This understanding is consistent with previous studies, which have shown that compressive residual stresses induced by surface treatments such as CSP and LSP significantly influence fatigue crack propagation by effectively reducing ΔK at the crack tip (Suresh, 1998; Schijve, 2009).