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Stress Concentration

Failures, whether by fatigue or stress corrosion cracking, will always occur first at the locations of highest stress, which may be highly localized. The highest stressed points are usually stress concentrations in fillets, boltholes, key ways and similar structural features where the stress is inherently higher than in the uniform section of the component by as much as factor of two or more. A zone of residual compression can often be designed to cancel the effect of the physical stress concentration. The depth and magnitude of the compressive zone can be designed using the fatigue design analysis method developed by Lambda Research to calculate the mean stress distribution required at each point throughout a finite element model of the high stress region to overcome the reduced strength caused by the stress concentration.

Low Plasticity Burnishing (LPB®) has been effectively applied to eliminate stress concentrations caused by surface damage including FOD and corrosion pitting. This same technology can be extended to mechanical features of the component. Knowing the mean and alternating stress distributions in service, appropriate CNC processing code and pressure files can be created to introduce just the amount of residual compression necessary to mitigate the stress concentration. The precise CNC reproduceability of LPB® allows the designed compressive field to be introduced reliably and repeatably during manufacture or during repair and overhaul. It should be noted that only localized stress concentrations can be addressed in this manner. A layer of deep surface compression added to the surface of a uniformly loaded section will simply increase the interior tension, moving failure subsurface.

Introduction of designed compression by LPB® to overcome component weaknesses discovered in service is a common application of the new technology. Thousands of aircraft engine compressor vanes treated with LPB® are now flying with the stress concentration at the airfoil-platform fillet radii effectively eliminated. A fatigue life limiting feature discovered in service can be eliminated without changing either the material or component design.

For more information on how LPB® can help increase the life of your parts please call 1-800-883-0851 or email us.
 

Stress Concetration
Close-up of crack locations on F-16 wing cut out

FEA of F-16 wing Cut Out
FEA of F-16 wing cut out

Fatigue Life Increased after LPB Treatment
Fatigue life increase of F-16 wing cut out after LPB treatment

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