## 2 Materials and Experimental Procedures  
### Materials and Samples  
This research was conducted using the 7475 aluminum alloy with a T7351 heat treatment. These alloys are widely used in aeronautical applications where the combination of high strength, fracture toughness, good fatigue crack propagation and corrosion resistance are required. The chemical composition is shown in Table 1. The material bars from which the specimens were produced had following the dimensions in mm: 4000 \(\times\) 1000 \(\times\) 250. According to the material manufacturer, the ultimate tensile stress and yield stress are \(\sigma_{\text{UTS}}\) = 490 MPa and \(\sigma_{\text{YS}}\) = 414 MPa, respectively. To study the surface shot peening effect on crack propagation, two CT specimen batches were prepared: one with shot peening (SP) and another without shot peening but with the lateral surfaces mechanically polished (MP). MP specimens were polished to ensure a good visualization of crack propagation. Manual grinding was done with a LaboPol-5--Struvers A/S, DK-2750, machine passing progressively the grinding papers 240, 320, 600, 1000 and 2500. After specimen grinding, diamond paste of 3 \(\upmu\)m and 1 \(\upmu\)m were used to give the specimens a mirrored surface aspect. Shot peening was done at OGMA Industria Aeronautica de Portugal S.A. Company (Alverca do Ribatejo, Portugal) with a large experience in producing components and aeronautics repair. Both sides of the specimen were subjected to a manual shot peening process, using a SURFATC machine, asshown in Figure 1a, and an Almen strip type A, according to SAE J443 standard [26]. Coverage assessment was done by the surface visual inspection, using a 10\(\times\) magnifying lens. One hundred-percent coverage was achieved when this analysis showed a completely attained surface by particles. The beads used in current study were of the type S170 with 0.43 mm diameter and Almen type A with intensity 0.20 A (mm), according SAE AMS2430 standard [27] for aluminum alloys. Figure 1b shows one sample after the shot-peening process. The studies of fatigue crack propagation were performed using the standard Compact Tension (CT) specimen with the geometry shown in Figure 1c, according with ASTM E647 standard [28]. For each batch of specimens, two different thickness (B) were manufactured: 4 mm and 8 mm. The specimens were machined in the longitudinal transverse (LT) direction from laminated plates. The loading specimen's direction coincides with bars lamination direction (Figure 1b,c). For each test condition, three specimens were used. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Zn** & **Ti** & **Others** & **Al** \\ \hline
0.1 & 0.12 & 1.2–1.9 & 0.06 & 1.9–2.6 & 0.18–0.25 & 5.2–6.2 & 0.06 & 0.15 & Remaining \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the 7475-T7351 aluminum alloy (% Weight). The surface roughness was evaluated according to DIN EN ISO 4288 standard [29] using a Surftest SJ-500 Mitutoyo, surface roughness measuring system. The evaluated parameters for each superficial treatment were: roughness average R\({}_{\text{a}}\), root mean square (RMS) roughness R\({}_{\text{q}}\) and mean roughness depth R\({}_{\text{z}}\). Table 2 summarizes the roughness parameters showing an increasing of more than 300% in the three roughness parameters for the peened surfaces. To analyze the material microstructure, some samples of the specimens were selected to observe their cross section. The specimen's surface was gradually polished with several silicon carbide papers. The papers' granulometry ranged from high to low. Afterwards, 1 \(\upmu\)m diameter diamond particles were used until specimen's surface became mirror-like. Surfaces were then etched with Keller reagent (2.5% HNO\({}_{3}\), 1.5% HCl, 1% HF, and 95% H\({}_{2}\)O (volume) (Coventry, UK)) and taken micrographs using an optical microscope Leica DM 4000 M LED (Wetzlar, Alemanha).