An output beam, roughly 20 Joules at 20 nanoseconds (i.e. 1000,000,000 Watts peak) from a Nd:glass laser is projected onto a work piece creating a shock wave that induces deep plastic compression. The area to be peened can be covered with material to act as an ablative layer and simultaneously as a thermal insulating layer. In some applications peening is done directly onto the base metal generating a thin (few microns) recast which if needed may be polished off without significantly affecting the deep peened layer. Peening is robotically controlled with repeatable pattern placement accuracy in the range of 0.004 inches.
A thin stream of water is flowed over the surface to act as an inertial tamping layer. The laser light transparently passes through the water and the leading temporal edge of the laser pulse reacts with the metal surface or ablative layer rapidly ionizing and vaporizing surface material, forming and heating a plasma.
The pressure of the heated plasma builds to approximately 100kBar (1.5 million pounds per square inch) with the water serving to inertially confine the volume. This rapid pressure rise is highly controlled to be one to two times the dynamic yield stress of the metal, plastically straining the material as it penetrates. Eventually the water is accelerated off the surface but only after the shock wave has propagated into the metal.