*2.1. LP Process*

The basic LP process is illustrated in Figure 1. In typical applications, an ablative coating, such as aluminum tape or black paint, along with a transparent overlay, usually water, are applied to the surface of the target material prior to peening. When used together, the confined ablation increases the intensity of the plasma pressure, which results in a higher intensity shock wave in the target material. During the peening process, the component is exposed to a very high intensity (1–9 GW/cm2) laser pulse, typically from an Nd:YAG or Nd:glass laser system operating at a FWHM of about 6–30 ns per pulse with a beam dimension of less than 10 mm [6,28].

**Figure 1.** (**a**) A high-intensity, pulsed laser vaporizes an ablative layer on the surface of the component, producing a rapidly expanding plasma. (**b**) The plasma is constrained by a confining medium, which creates a high-amplitude pressure pulse and induces a shock wave in the component.

The resulting pressure pulse generally lasts about 2–3 times the duration of the laser pulse and has a peak magnitude of about 1–10 GPa [28]. The spatial distribution of the plasma pressure can either be uniform or variable, depending upon the laser system employed. In order to generate the near-surface plasticity required to induce a residual stress, the plasma pressure needs to exceed the HEL of the target material for a su fficient amount of time, with an optimal value typically about 2–2.5 times the HEL [29].
