Home Forums General Discussion Ask Dr. Peener relaxation of peen with cyclic stress?

Regarding titanium helicopter tail rotor parts that are peened: Is there reason to suspect that with exposure to cyclic stress the peen is somehow becoming ineffective? The reason I ask is Sikorsky has a requirement (albeit a very old requirement) to re-peen every 2000 flight hours. These are the load-bearing parts of the tail rotor head (hub, blade grips, etc), mostly exposed to varying magnitudes of tensile and bending stress. I could understand if there was exposure to high temperatures, but in this case there is not. Is there any guidance on the subject?

If the applied strain exceeds the yield strength then the part will be instantly stress-relieved! This can occur during cyclic stress if the level becomes excessive e.g. contact with a large bird.

There is no exceedance of the yeild strength whatsoever. We're talking normal loads here.

How can you be sure that the yield strength is not exceeded? The applied stress has to be added to the residual stress to get the actual stress being applied. High strain stress relief of peened parts usually occurs during the compressive part of the applied strain cycle. That is when the two compressive stresses are being added together in the peened surface layer.

With the extensive analysis and flight testing that has been done on these parts, including strain gauged flight testing, the concern over whether or not the parts might be yielding is not a big one, although I don't totally discount your point. However, the question I'm asking is if a part is under non-yielding cyclic loads (including the residual stress), is there a possibility of the peen layer relaxing over time? Does anyone know of any other manufacturers that requires regular re-peen of parts?

I can tell you that the almen test strips do relax overtime.