Laser shock peening (LSP) fires a short, intense pulse through a transparent water layer at an opaque overlay on the metal surface. The overlay flash-vaporizes into a confined plasma; because the water tamps the expanding plasma instead of letting it vent freely, pressures reach several GPa instead of the sub-GPa achieved by open-air ablation.
Fabbro shock-pressure model (Fabbro et al., 1990):
P(GPa) = 0.01·√(α/(2α+3))·√Z·√I₀
α ≈ 0.2 (thermal-to-mechanical conversion, confined regime)
Z = 2 / (1/Z_water + 1/Z_target) [reduced acoustic impedance]
I₀ = laser intensity (GW/cm²)
Because Z_water is far smaller than any metal's impedance, Z — and so P — is set mostly by the water and the laser, only weakly by which metal is hit. Toggling confinement off removes the tamping layer and the achievable pressure drops sharply, the key reason LSP is always run underwater.
Plastic deformation (and the compressive residual stress that gives LSP its fatigue benefit) only occurs where the shock pressure exceeds the material's Hugoniot Elastic Limit (HEL) — its dynamic yield strength under shock loading. The deeper the pressure stays above HEL and the longer the pulse, the deeper the plastically-affected layer.
Plastic depth requires: P > HEL
Residual stress must self-equilibrate through the thickness:
∫ σ(z) dz ≈ 0 → a compressive skin is always balanced
by a smaller tensile region deeper in the part
- Intensity / duration sliders — raise P and widen the plastic layer.
- Material — changes HEL and yield strength, so the same P produces a different depth and peak stress.
- Confinement toggle — shows the water-tamping effect that makes LSP practical.
Real-world use: LSP is applied to turbine blades, landing-gear components and welded joints to drive deep compressive residual stress into the surface, closing off the tensile stress that would otherwise drive fatigue-crack growth — extending service life several-fold without changing part geometry (Peyre & Fabbro, 1996).