**1. Introduction**

Laser Shock Peening (LSP) is a technology that makes use of shock waves induced by a laser to improve the mechanical properties of a metallic component. Short laser pulses (1–50 ns) with a high-power intensity are shot at the surface of the component. The laser beam vaporizes a superficial layer of the treated material, with the local formation of high-pressure plasma, as noted by the authors of [1]. Fabbro et al. [2] studied the use of a transparent overlay as an effective method to confine the generated plasma. This results in the formation of intense shock waves, which induce high residual stresses in the surrounding material, as shown by Sano et al. [3].

Conventional laser peening usually employs an ablative layer (i.e., a coating applied on the surface of the material) to prevent damage to the metal surface. Another technique also exists, called Laser Peening without Coating (LPwC), in which no ablative layer is used and the treated specimen is immersed in water during exposure to laser pulses. Compared with conventional laser peening, LPwC does not require a specific surface preparation and can be performed by commercial Nd:YAG lasers, which makes it particularly appealing for a number of applications.

The effects of the LSP-induced residual stresses on the fatigue of aluminum alloys are well documented in the literature. A review by Montross et al. [4] reported an improved fatigue life of Al 2024 and Al 7075 specimens treated with laser peening, while the authors of [5] show an effective reduction in the fatigue crack growth rate in a laser-peened 6061-T6 aluminum alloy. Gao [6] noted that the superior performances of laser shock peening compared with mechanical shot peening are due to deeper compressive stresses and a better surface finish.

Similar improvements of the fatigue behavior have been reported for laser peening without coating. In 2006, Sano et al. [7] observed a substantial prolongation of the fatigue life of an LPwC-treated AC4CH Al alloy, even though an increase of the surface roughness was found. Similar results were obtained by the authors of [8] for AA7075-T73 open-hole specimens. A recent review [9] reports enhanced fatigue properties for a wide class of metals treated with LPwC, including 6061 and 6082 Al alloys.

Geometry is known to play a role in the e fficacy of LSP, as shown in the literature [10], where the effect of LSP on thin AA2024 panels typical of aeronautical applications were studied, and retarded crack propagation was observed. A work by Troiani et al. [11] highlights the potential drawbacks of the LSP of thin panels, depending on the selected peening path. Of particular interest is the influence of geometric discontinuities on the peening process. Yang [12] reported an improved fatigue life of Al 2024 specimens with pre-existing holes treated with LSP; on the other hand, Ivetic et al. [13] showed that the interaction between laser peening and an open hole in aluminum panels could potentially result in a decreased fatigue life. Dorman [14] addressed the presence of scribe defects and their e ffects on the fatigue life of LSP-treated 2024 aluminum alloys.

Although curved notches are often critical for the nucleation of fatigue cracks, only a few works have addressed the e ffects of laser shock peening on a curved surface. Notably, Peyre [15] studied the e ffects of laser shock peening in specimens with a curved notch. The authors reported extended fatigue lives of laser-peened specimens compared with mechanically shot-peened and untreated ones. They also measured high compressive stresses around the notch. Vasu and Grandhi [16] analyzed the residual stress field induced by laser peening on a curved surface by means of a finite element model. The compressive stress was found to increase in a concave geometry, as compared with a flat or convex surface. The increase was related to the radius of the curvature, in that the smaller the radius, the higher the compressive stress.

This work aims to study the e ffects of laser peening without coating applied at a circular notch, as shown in Figure 1. In particular, the e ffects of the process on the fatigue behavior are investigated by three-point bending specimens, while the residual stresses at the notch are computed by means of a finite element model.

**Figure 1.** Application of the Laser Peening without Coating (LPwC) process to a notched specimen.
