**1. Introduction**

The first discoveries leading to the development of modern-day laser shock peening (LSP) started in the 1960s with the spread of pulsed laser technology. The study of laser interaction with different materials led to pressure measurements on a surface ablated by a pulsed laser [1]. A major breakthrough occurred in 1970 when Anderholm discovered that the pressure delivered through a laser shock could be greatly improved by confining the plasma produced by placing a quartz overlay, transparent to the laser beam, on the target [2]. At the beginning of the 1970s, studies on the effect of LSP applied on metallic targets began at the Battelle institute in Columbus, Ohio and demonstrated an improvement of mechanical properties in the treated zone [3].

From there, LSP started becoming a competitor of the conventional shot peening for the improvement of the fatigue performance of treated components, and laser peening is now one of the most effective surface pre-stressing treatments used to enhance the fatigue strength of metallic structures. The laser-shock-peening-induced compressive residual stresses offer better fatigue life

properties [4] by retarding the crack growth and inhibiting the crack initiation [5–7]. In addition, LSP can significantly enhance the resistance of the treated components to stress corrosion [8].

Compared to the conventional shot peening, the affected depth is much larger for laser shock peening—up to ≈1.5 mm compared to ≈300 μm for shot peening [9] of Al alloy materials. In addition, the LSP-induced work hardening is generally limited (about +10% to +30%) compared to conventional shot peening [10]. This can be explained by the fact that the loading duration is very short, which consequently does not allow the activation of all the sliding systems of the material and thus generates fewer cross dislocations. Only cyclic hardening materials such as 304L and 316L have their hardness and their level of residual stresses increase with impact repetition. Nevertheless, it is not possible to draw conclusions regarding the amplitude of the induced compressive residual stresses, given that contradictions exist in the literature. Therefore, conventional shot peening may be unfavorable over a material's lifetime due to controlled deformation loading (high cycle thermal loading), reducing the beneficial effects of compressive residual stresses. For laser shock, such a risk can be avoided because of the low work hardening.

Today, LSP at an industrial scale is mainly applied in aeronautics for the treatment of sensitive areas on certain parts to increase their lifespan. Water is the usual confining material because it is cheap, transparent to the laser, and ensures contact with surfaces. Other areas are developing quickly toward industrial applications such as the treatment of parts produced by additive manufacturing. LSP treatment of these types of materials allows more shaping and forming possibilities as well as shape correction treatment due to the highly controlled nature of the process compared to conventional shot peening [10,11]. Coupled with the deeper levels of residual stress produced, it has shown to be highly cost effective despite its higher operating cost compared to conventional shot peening.

One of the obstacles to extending LSP's applications is the impossibility of using water in a reactive atmosphere or near electronic devices. A solution to this issue should be a solid material, as demonstrated by the pioneering work on laser shock [2]. However, the use of rigid glasses for the treatment of pieces presenting complex geometries such as the ones encountered in the aeronautics industry is impossible. In contrast, a soft polymer confinement, with its adaptability, shaping possibilities, and wide range of formulations and properties, is an ideal candidate for this type of need. Laser shock peening with polymer confinement has been studied only by Hong et al. [12]. Authors evaluated only the influence of the confining medium used on mechanical impedance, without carrying out a complete investigation of the performances exhibited by these materials.

This paper presents a study concerning the use of polymers for laser shock applications. It focuses on the capability of materials to be transparent to a laser beam at high intensity and to generate a high-pressure plasma when used for the laser shock peening process in a confined regime and without a thermal coating. Plasma pressure is the driving force of the process for evaluating the range of materials which can be processed and the boundary limit for simulation. In the case of confinement in specific conditions, only experimental approaches can determine this key parameter of LSP. We present optical transmission and the characterization of plasma pressures produced by the laser in a confined regime with a choice of polymer confinements (i.e., acrylate and polydimethylsiloxane). Experiments were performed from rear free-surface velocity measurements using the velocity interferometer system for any reflector (VISAR) on pure aluminum foils coupled with numerical simulations. Results are compared to the pressure produced with water confinement interaction. The first part of this paper presents the experimental setup and methodology, while the second part is dedicated to results and discussion.

### **2. Materials and Methods**

Figure 1 presents the experimental setup and methods used to determine plasma pressure from the velocity profile measured by VISAR.

The velocity profile was reproduced using a finite element (FE) model in which the plasma pressure was adjusted as an input condition. A direct correlation between rear free-surface velocity and the maximum pressure of the plasma was also extracted to provide fast signal analysis.

**Figure 1.** Setup used in the Hephaïstos platform to realize the pressure measurements. VISAR: velocity interferometer system for any reflector.
