PSI - Issue 68

Erik Calvo-García et al. / Procedia Structural Integrity 68 (2025) 809–814 Erik Calvo-García et al. / Structural Integrity Procedia 00 (2025) 000–000

810

2

1. Introduction The 6000 series of aluminium alloys not only have a high strength-to-weight ratio, but they also present high corrosion resistance, weldability and formability. Therefore, these alloys are highly demanded in the automotive and aircraft sectors for the manufacturing of components such as steering knuckles, chassis or aircraft body sheets, as reported by Chang et al. (2018) or Polmear et al. (2017). It is a fundamental issue that these components present an acceptable fatigue behaviour, as they are subjected to cyclic loading throughout their service life. Severe plastic deformation (SPD) techniques like shot peening or laser shock peening have proven to be successful in improving the fatigue life of aluminium alloys, as investigated by Su et al. (2020) or Troiani and Zavatta (2019). Shot peening consists in projecting small particles at high velocity onto a component surface. The kinetic energy of these particles is transferred to the target surface and causes plastic deformation, thus inducing strain hardening and compressive residual stresses in the surface of the material, as reported in the work of Segurado et al. (2018). Compressive residual stresses are beneficial for fatigue behaviour, as they increase the resistance to crack initiation and slow down crack propagation. Moreover, Wang et al. (2023) indicated that the increase in surface hardness can hinder crack initiation due to a higher resistance to plastic deformation. Nevertheless, shot peening can also increase substantially the surface roughness of the treated material, which may have detrimental effects on its fatigue behaviour due to the appearance of stress concentrators, as reported by Ferreira et al. (2020). There are many variables involved in a shot peening process, such as air pressure, peening time, stand-off distance, angle of impingement or shot medium. Therefore, it is a challenging task to define the shot peening conditions that lead to an improvement of the fatigue strength of aluminium alloys. Amongst the different SPD processes, shot peening stands out for being simple, economical, adaptable to different shapes and not involving any chemical or thermal change in the material. As a result, many research works have dealt with the improvement of fatigue properties of aluminium alloys by means of shot peening, like those from Calvo García et al. (2024), Krishna et al. (2018) or Wu et al. (2019). These research works evaluated the fatigue life of aluminium alloys in the high-cycle fatigue domain, where most of the fatigue life can be spent in crack initiation, as pointed out by Liang et al. (2012). However, the works that investigate the effect of peening techniques in crack propagation are scarce in the literature. Ferreira et al. (2018) did not observe a significant effect of shot peening on the crack propagation rate of an aluminium alloy 7475. Bergant et al. (2016) applied the laser shock peening technique on a 6082 alloy, but it led to a reduction in fracture toughness and an increase in crack growth rates. Assuming that flaws can exist in any structure, analysing the service life of components in terms of the crack propagation stage is of great importance. This investigation deals with the effect of shot peening treatments on the crack propagation rates and fracture toughness values of an aluminium alloy 6060 T6. Three-point bending tests were carried out using SE(B) specimens according to the ASTM E1820 standard. These specimens were tested in three conditions: (i) with no surface treatment, (ii) shot peened with silica microspheres and (iii) shot peened with alumina particles. Crack lengths were measured with an extensometer applying the compliance method. Fracture toughness values were obtained by means of the resistance curve method. The differences observed between each surface treatment condition were discussed. 2. Materials and Methods The material used in this research is an aluminium alloy 6060 T6. The chemical composition of this alloy is given in Table 1. As for the mechanical properties, the yield strength was 240 MPa, the ultimate tensile strength was 290 MPa, and the ductility was 14%.

Table 1. Chemical composition of the 6060 alloy (% wt.). Si Mg Mn Fe Cu Zn

Other

Al

0.36

0.53

0.05

0.18

0.02

0.02

0.03

98.81

Shot peening treatments were performed with a compressed air gun at 8 bar, a stand-off distance of 100 mm and a peening time of 60 s. Two types of shots were used: silica microspheres and alumina particles, which are shown in