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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widely used material in these sectors, is valued for its excellent strength-to-weight ratio and good fatigue resistance (Grover, 1966; Rambabu et al. 2017). However, its susceptibility to fatigue crack propagation under cyclic stresses necessitates the adoption of advanced surface treatment techniques to enhance its mechanical performance and durability (Clauer, 1996; Shukla at al. 2014). In the fatigue life of mechanical components, it is well established that the total lifespan under cyclic loading is typically divided into two main phases, frequently termed crack initiation and crack propagation. The crack initiation phase involves the nucleation and growth of microscopic cracks, while the crack propagation phase entails the growth of the dominant crack until it reaches a critical size, leading to failure (Schijve, 2009). To enhance the fatigue lifetime of mechanical components, the introduction of compressive residual stresses at the surface is highly recommended, as it can significantly influence fatigue behaviour by delaying crack initiation. (Clauer, 1996; ; Ivetica at al., 2011; Becker, 2017) indicate that surface treatments such as mechanical surface treatment or case hardening induce compressive residual stresses, which counteract the tensile stresses responsible for crack formation. Consequently, the crack initiation phase is prolonged, increasing the proportion of the total fatigue life spent in this stage. Studies have shown that, in certain cases, crack initiation can account for up to 88% of the total fatigue life, thereby enhancing the overall durability of the component (Bathias and Pineau, 2010). This study aims to investigate and compare the effects of Laser Shock Peening (LSP) and Conventional Shot Peening (CSP) on the fatigue crack growth behaviour of Aluminium 2024-T4 alloy, with particular emphasis on the influence of compressive residual stress intensity in delaying crack initiation and retarding crack propagation. To evaluate the impact of mean stress, fatigue tests are conducted at three load ratios (R = 0.1, 0.3, and 0.5), allowing for a detailed assessment of how different levels of tensile mean stress affect crack growth performance following surface treatment. By comparing the relative effectiveness of LSP and CSP, the study aims to generate insights into the optimisation of surface treatment strategies for fatigue-critical applications. The findings are expected to support the development of more durable and reliable structural components, particularly in safety-critical industries such as aerospace, automotive, and marine engineering. Nomenclature CSP Conventional Shot Peening COD Crack Opening Displacement CT Compact Tension LSP Laser Shock Peening ΔK Applied Stress Intensity Factor ΔK eff Effective Stress Intensity Factor ΔK R Residual Stress Intensity Factor ΔK th Threshold Stress Intensity Factor K max Maximum Stress Intensity Factor K min Minimum Stress Intensity Factor R Load Ratio da/dN Crack Growth Rate SD Standard Deviation 2. Surface treatment Surface treatments are extensively utilised to improve fatigue resistance by altering surface properties, primarily through the induction of residual stresses, surface hardening, and microstructural modifications. These treatments can be broadly classified into mechanical, chemical and thermal processes, each offering distinct mechanisms for enhancing fatigue life. This study will focus on mechanical surface treatment, a widely utilised technique for enhancing the performance and longevity of engineering components by modifying their surface properties. These treatments employ controlled physical mechanisms to alter surface conditions, leading to improved mechanical characteristics. In ductile metals, compressive residual stresses are primarily introduced through localised plastic deformation within the near-surface region, significantly enhancing fatigue resistance and structural integrity.