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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diameter of 1.2 mm at a velocity of 40 m/s (motor speed: 1,435 RPM). These parameters were selected to ensure uniform surface coverage and adequate stress development while minimising excessive roughness. Post-peening, the specimens were examined by visual inspection using a digital microscope at 100× magnification to confirm surface integrity and absence of defects. Surface roughness was measured via contact profilometry, yielding an average Ra = 5.1 ± 0.3 μm (SD, n = 9 measurements, 3 per specimen). Compressive residual stresses were confirmed by X-ray diffraction (Section 3.4). 3.3. Laser shock peening (LSP) process The LSP treatment was applied to the notch region, front/back faces, and side surfaces of three CT specimens to ensure both fatigue crack initiation and propagation occurred within the peened area. A Q-switched Nd:YAG laser (1.064 μm wavelength) delivered 1 J pulses at 15 ns duration, producing a peak power density of 2.6 × 10⁹ W/cm². Laser spots (1.5 mm diameter) were arranged in a staggered grid pattern with 50% overlap for complete coverage, using a sacrificial ablative layer and confining water layer to optimise shock wave transmission and surface protection. Post-treatment characterisation included: (1) surface integrity verification (identical to CSP methods, 100× magnification), (2) roughness measurement via contact profilometry yielding Ra = 1.3 ± 0.4 μm (SD, n = 9 measurements, 3 per specimen) which is 46% lower than CSP’s 5.1 ± 0.3 μm, reducing notch root stress concentrations, and (3) residual stress validation by X-ray diffraction (Section 3.4). This configuration induced compressive stresses critical to crack initiation/propagation zones, enhancing fatigue performance. 3.4. Residual Stress Measurement The compressive residual stresses induced by CSP and LSP were quantified using X-ray diffraction (XRD), a non destructive technique renowned for its precision in residual stress analysis (Withers and Bhadeshia, 2001; Noyan and Cohen, 1987). XRD evaluates residual stresses by measuring alterations in the interplanar spacings of a material's crystal lattice, which change in response to internal stresses. By directing monochromatic X-rays onto the specimen, diffraction occurs when Bragg's Law ( nλ = 2d sinθ ) is satisfied, where n is the diffraction order, λ is the X-ray wavelength, d denotes the interplanar spacing, and θ represents the diffraction angle (Cullity and Stock, 2001). Variations in d spacings, indicative of elastic strains, lead to shifts in the diffraction angle θ . These angular deviations are recorded and analysed to calculate the corresponding residual stresses present within the material. The efficacy of XRD in residual stress measurement is well-documented (Fitzpatrick et al., 2005), offering reliable and objective data crucial for quality control assessments in aerospace, automotive, and structural applications. Fig 4 shows the average compressive residual stress profiles for CSP (red dashed curve) and LSP (light blue solid curve), measured along the centreline of the anticipated crack propagation path across three specimens (n = 3). The CSP curve exhibits a mean peak compressive stress of −149 ± 15 MPa (mean ± SD) at 0.32 mm depth but drops rapidly beyond 0.5 mm, reaching complete stress relaxation (0 ± 5 MPa, typical XRD uncertainty (Fitzpatrick et al., 2005) by ~0.56 mm. In contrast, the LSP curve achieves both a higher peak stress of −183 ± 12 MPa at 0.38 mm and sustained compressive stresses of −30 ± 1 2 MPa to depth of ~1.0 mm. The standard deviation (SD) values were calculated using Equation (1).

Fig. 4. Compressive residual stress measurements using XRD