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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4. The fatigue crack growth testing To comprehensively evaluate the fatigue crack growth behaviour of 2024-T4 aluminium alloy, constant amplitude fatigue tests were performed on CT specimens per ASTM E647. Fatigue crack growth was evaluated for three surface conditions (untreated- baseline, CSP, LSP) across three stress ratios (R = 0.1, 0.3, 0.5). Three specimens were tested per combination of surface treatment and R-ratio, totalling 27 tests (3 treatments × 3 R-ratios × 3 replicates). This design captures Paris-regime behaviour while assessing interactions between residual stresses (from treatments) and mean stress effects from R-ratios. All tests were conducted using a closed-loop ±25kN servo-hydraulic machine equipped with high-precision electronic control for load applications. Cyclic loading was applied at a frequency of 10 Hz under load-controlled conditions, with load amplitude regulated to within ±1% of the nominal values, ensuring consistency and repeatability throughout the test series. All tests were conducted in laboratory air at room temperature. Crack length was monitored using synchronised compliance method (COD) clip gauge mounted at the crack mouth. The COD provided real-time stiffness-based crack length calculations. This setup enabled accurate, continuous, and non-intrusive measurement of crack extension throughout the test duration. High-resolution cameras were positioned on both sides of the specimen to monitor crack growth and confirm compliance with ASTM E647 straightness requirements (<10° deviation from centreline). This setup ensured valid da/dN measurements by detecting asymmetric propagation or branching in real time. Fig 5 shows the crack growth test rig setup used in this study (Safran Electrical & Power, Internal Report, 2024).

Fig 5: crack growth test rig set-up (Safran, internal report, 2024)

Crack growth was analysed based on linear elastic fracture mechanics (LEFM) principles. The crack growth rate, da/dN (mm/cycle), was evaluated as a function of the applied stress intensity factor range, defined as: ΔK = K max − K min (2) where K max and K min denote the maximum and minimum stress intensity factors during each load cycle, respectively. The applied ΔK ranged from 5 to 21 MPa√m, encompassing the Paris regime for fatigue crack growth in this material system. The inclusion of shot peening and laser shock peening treatments aimed to investigate the effect of compressive residual stress fields on fatigue crack propagation. 5. Results and discussion Based on the experimental measurements, the average crack growth rates (da/dN) for each surface treatment condition and load ratio are plotted in Fig 6 (a – c) versus the applied stress intensity factor range (ΔK) .

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Fig. 6. Crack Growth Rate vs. Stress Intensity Factor Range for Different Surface Treatments, (a) R = 0.1, (b) R = 0.3 and (c) R = 0.5