Laser shock peening is a novel technique for introducing beneficial residual stresses into components
The controlled introduction of compressive residual stresses into the surface layers of components can effectively delay the onset of crack initiation and slow down crack growth rates. Laser shock peening is one such technique of introducing these residual stresses.
It has been well established that most fatigue cracks initiate from stress concentration sites found on the surfaces of components, subject to cyclic fatigue loading. The introduction of compressive residual stresses into the surface layers of components, through various means including shot peening and laser shock peening, can result in a resistance to both crack initiation and propagation thus leading to an increase in the fatigue life of the components.
These locally induced residual stresses counteract applied local stresses, thereby delaying crack initiation and also slowing down the overall fatigue crack growth rate. This typically results in an overall increase in the fatigue life of the treated component.
With the ever increasing demand for lower operational costs, higher safety measures and better performance, significant pressure has been placed on manufacturing systems and component surface processing technologies to produce components which are near defect free and require as few processing steps as possible before completion.
Compressive residual stresses can be introduced into the surfaces of engineered components through various means, including shot peening and laser shock peening. Both treatment processes introduce these residual stresses by plastically deforming the surface layers of the treated component. In the case of shot peening the surface is bombarded with small spherical (shot) particles while in the case of laser shock peening the deformation is induced by shock waves. Laser shock peening utilises high speed, high powered lasers to focus short duration coherent energy pulses onto the surface of the component to be peened, creating a shockwave between a thin film of water, which acts as an 'inertial tamp', and the surface of the component. This shockwave propagates into the surface of the component, deforming the surface material plastically to depths of up to 1 mm and in so doing inducing compressive residual stresses in this surface material.
The laser shock peening process can be adjusted and controlled in real time through computer controlled robotic systems, whereby the energy per pulse can be measured and recorded for each location on the component being peened. Regions inaccessible to shot peening, such as small fillets and notches, can also be treated effectively through laser shock peening, so long as there is a line of sight between the laser and desired peening location. Laser shock peening has a minimal effect on the surface roughness of the peened component, with hardly any thermal or mechanical (surface roughness) changes occurring at the surface as a result of the treatment process.
Published in Technical Tips by Origen Engineering Solutions on 1 August 2016