Vol. 11
March Issue
Year 2010

MFN Trainer Column

in Vol. 11 - March Issue - Year 2010
Compressive Residual Stresses To Laser Shock Peen or Not?

David Lahrman

Laser shock peening is a relatively new process for introducing compressive residual stresses into the surface of a component compared to the traditional shot peening process.  Both processes are used to reduce the propensity for surface initiated fatigue and increase fatigue resistance. The question most often asked is can laser shock peening be used as a replacement to shot peening? The answer depends upon the component and the requirement for the component. In general, the laser shock peening process cannot be used as a replacement to shot peening and is used in conjunction with shot peening to augment those areas where shot peening is not providing sufficient increase in fatigue resistance. 

Laser shock peening can generate compressive stresses that are 10 times deeper than those produced by shot peening which provides significant advantage in surface initiated fatigue. If laser shock peening is used as a complete replacement to shot peening, in most cases it will cause unacceptable distortion in the component. This difference in residual stress depth becomes critical in thin components like compressor airfoils where the compressive stress can be through the thickness of the airfoil. In airfoil components, the application of process is confined to the critical locations to provide the extra strength that is needed to prevent fatigue cracks from propagating from such locations as the leading edge where foreign object damage often occurs. Laser shock peening can and is tailored to minimize distortion in these thin components. In thicker components such as pilger dies that are used to manufacture seamless tubing, the dies have sufficient mass to support deep compressive stresses and the process could be applied to the entire surface of the die without causing any distortion. However, processing the entire area of the die does not provide any significant advantage. The critical location on the dies is in the groove section of the pilger die where the surface stresses are high due to the pilgering process. As a result, the laser shock peening is applied only to the critical location where the cracks initiate in the surface of the groove on the die. 

The best approach to employ laser shock peening on a component is to examine the component as a whole and determine if there are critical areas where shot peening may not be providing sufficient advantage in fatigue resistance. For existing component designs that have been in service for some time, this may require reviewing the history of the component to determine where the most frequent failures have occurred. In newly design components, this may require critically evaluating the analytical stress models prepared by the design engineer to determine if laser shock peening may be needed to increase fatigue life. In either case, it is a matter of determining where and how best to apply the process to provide the desired benefit.

The application of laser shock peening should be in locations where high cyclic stresses are generated during the use of the component such as in filet radii or blend areas. Once the critical area has been identified, the next step is to determine how much area needs to be processed and what processing conditions may prove beneficial. While the application of laser shock peening can be a balancing act between increasing fatigue resistance and minimizing distortion because of nature of some component, there is a growing base of experience that provides guidance for future parts. Laser shock peeing is a mature manufacturing process that can be applied to most metallic components and should be given serious consideration.

For questions contact: david@mfn.li

Author: David Lahrman
Trainer Column
by David Lahrman, 
Official MFN Trainer
More Information at www.mfn.li/trainers