Featured Application: 7475-T7351 aluminum alloy are widely used for structural components in aerospace applications. Abstract: Shot peening is an attractive technique for fatigue enhanced performance of metallic components, because it increases fatigue crack initiation life prevention and retards early crack growth. Engineering design based on fatigue crack propagation predictions applying the principles of fracture mechanics is commonly used in aluminum structures for aerospace engineering.

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... main purpose of present work was to analyze the effect of shot peening on the fatigue crack propagation of the 7475 aluminum alloy, under both constant amplitude loading and periodical overload blocks. The tests were performed on 4 and 8 mm thickness specimens with stress ratios of 0.05 and 0.4. The analysis of the shot-peened surface showed a small increase of the micro-hardness values due to the plastic deformations imposed by shot peening. The surface peening beneficial effect on fatigue crack growth is very limited; its main effect is more noticeable near the threshold. The specimen's thickness only has marginal influence on the crack propagation, in opposite to the stress ratio. Periodic overload blocks of 300 cycles promotes a reduction of the fatigue crack growth rate for both intervals of 7500 and 15,000 cycles. rates based on the application of fracture mechanics theory is an important aspect of a structural damage tolerant assessment. Many metal components, such as turbines blades, used in aerospace and power industries are subjected to dynamic mechanical loading, leading to the initiation of fatigue cracks. One way to reduce the risk of fatigue crack initiation is to introduce compressive stresses in the region of higher stresses concentration, for example by shot peening. At the industrial level, this is a well-established surface treatment technology, despite generating a meaningful rougher surface and therefore surface defects [1] . Considering that most fatigue cracks initiate at the surface, the conditioning of the surface to resist crack initiation and earlier crack growth is a convenient method to enhance fatigue performance. The indentation of each impact, in shot peening process, produces local plastic deformation given rise to a field of surface compressive stresses. Studies by many researchers have shown a positive shot peening effect [2] [3] [4] , resulting from the introduction of residual compressive stresses in the subsurface layers of material. Depending on the peened material, there is an Almen intensity for which the optimum fatigue strength is achieved, corresponding to a certain balance between residual compressive stress field and surface roughness damage. Paris's law, which relates fatigue crack growth rate (da/dN) and stress intensity range (∆K) is the prime approach adopted for characterizing fatigue crack propagation in engineering structures. Fatigue crack growth of the aluminum alloys in the Paris's law regime is affected by microstructure [5-7] and by the crack closure induced by plasticity, oxidation and surface roughness, especially near threshold regime. Paris law characterizes the rate of crack advance per cycle: da/dN = C∆K m . The rate of crack advance per cycle is related to the stress intensity factor range ∆K. C and m are constants that depend on the material, environment and stress ratio. Crack closure is considered a very good approach to explain the influence of mean stress on the fatigue crack growth rate [6, 8] . Bergner and Zouhar [6] showed that crack growth rates of various aluminum alloys varied by a factor of about 20 for some ∆K values, suggesting that the main factor to explain that discrepancies was the crack closure effect and the environment. Fatigue cracks tends to grow into a material region which has experienced large plastic strains due to its location in the crack tip plastic zone. Typically, this material is deformed beyond its elastic domain in the direction normal to the crack flanks. The trace of the plastic deformation produced is left in the crack's path. It acts in the same way to an additional wedge stick between flanks, thus pre-straining them and partly protecting the crack tip from the action of posterior loads. This phenomenon is called plasticity induced crack closure and tends to decrease the effective stress intensity range thereby resulting in slower crack propagation rates [8] . In compact tension (CT) specimens, the crack progresses more rapidly in center than at the surface conducting to a crack tunneling effect, due to the prevailing tri-axial state of stress at the center, promoting plain strain in contrast with plain stress at surface. Striation spacing between beach marks on fatigue crack surfaces is also affected by shot and laser peening effect. Zhou et al. [9] have observed a decrease in striation spacing with increase in the number of laser peening impacts for Ti6Al4V specimens'. For the same alloy, Pant et al. [10] studied the effect of shot peening and laser peening on the fatigue crack propagation and compared with the untreated one with respect to the striation spacing; this was done using R = 0.1 and R = 0.7. Both peening surfaces presented a reduction on the striation spacing when compared to the untreated specimens. Overloads can lead to significant interaction effects on crack propagation, as has been reported in many studies [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] . Crack growth retardation can be explained by many mechanisms, including models based on crack closure, residual stresses, crack tip blunting, strain hardening, reversed yielding and crack branching. The residual plastic deformation effect leads to compressive stresses in the wake of the crack and raises the crack opening load on subsequent crack growth (crack closure), becoming the most important phenomena for what concerns the explanation for the variation of characteristic features of post-overload transients [19] [20] [21] [22] [23] . Donald and Paris [23] observed for 6061-T6 and 2024-T3 aluminum alloys that closure measurements produced good data correlation between distinct stress ratio crack growths obtained in tests with increasingly K conditions. However, in the near-threshold