The Metallurgical Effects
What role does the laser play in the process?
The rapid rise of pressure generated at the surface of the workpiece by the high energy laser pulse creates a shockwave in the part. The pressure pulse typically has a length of 2 - 2.5 times that of the laser pulse. This pressure pulse creates and sustains a shockwave.
What happens after the shockwave is generated in the work piece?
The shockwave consists of a strong primary front of compressive stress moving through the work piece. This primary wave front is trailed by weaker tensile and compressive waves that are important, but not necessarily contributory to this discussion. When the magnitude of the primary shockwave front is above the dynamic yield strength of the material, termed the HEL (Hugoniot Elastic Limit), it plastically deforms the microstructure and creates added dislocations in the material. For more information on what dislocations are in a material, take a look at Wikipedia.
As the shockwave travels into the target, some of the energy of the wave is absorbed during the plastic deformation and the creation of these dislocations. With diminishing energy, the peak stress of the shockwave is reduced (attenuates). Plastic deformation is induced in the material by the shockwave until the wave magnitude has attenuated below the HEL. This results in plastic deformation, or cold work, over a gradient of depth in the material with the peak amount occurring at, or near, the surface. The plastic deformation and increase in dislocations generate the compressive residual stresses and increase the strength of the material. When the shockwave magnitude drops below the HEL, the wave functions as an elastic wave in the material and does not input any further plastic deformation into the material.
How does plastic deformation generate residual stress?
The shockwave stresses permanently stretch the internal structure of the work piece. Surrounding material in the work piece responds elastically to the processed material and places it in a compressive residual stress state. To maintain equilibrium, the elastic compressive response is also balanced by a tensile response outside of the processed area. The residual stress magnitudes and depths in the work piece are a result of the gradient of plastic deformation as opposed to a very dense concentration of cold work. The dependency of the residual stresses on the gradient of cold work allows laser peening to reach compressive residual stress depths that are unrivaled.
Why are residual stresses beneficial?
Residual stresses are taken into account just as applied stresses are in determining damage tolerance. The total stress that is observed by a component is the sum of the applied stress and the residual stress. As a generalization, components fail when they are being pulled apart by tensile forces. When you apply compressive residual stresses to a component, the tensile stress that must be achieved to reach the failure point from degradation effects such as foreign object damage, high cycle fatigue, or stress corrosion cracking must overcome both the inherent material strength properties as well as the compressive residual stress.