Nearly all component failures are due to fatigue or stress corrosion cracking (SCC) initiating at the surface. Fatigue failures generally initiate from local stress concentrations caused by surface damage, including FOD, corrosion pits, or fretting-induced shear cracks. SCC occurs when the exposed sensitive surface is in tension exceeding the SCC threshold level. The integrity of the surface is therefore highly dependent upon the surface and near surface stress state, the sum of residual plus applied stress.
Introducing a layer of residual compression designed to have sufficient magnitude and depth greater than the surface damage can prevent failure. The compressive layer prohibits both crack initiation at the surface and crack propagation through the depth of the layer. The integrity of the surface is restored by treating the root cause of failure – local tensile stresses resulting in crack initiation and propagation to failure.
