Cold Work vs. Shot-Peening
Cold Work vs. Shot-Peening
(OP)
Imagine an aluminium flat coupon (2" thick) having a central hole under traction/compression alternate tension.
What process would you apply for increasing fatigue life:
Cold working the internal diameter by interference or shot-peening the hole?
Supose both process are optimized to their maximum potential.





RE: Cold Work vs. Shot-Peening
Mike Halloran
Pembroke Pines, FL, USA
RE: Cold Work vs. Shot-Peening
Come on, I do the imagining, you do the evaluation!!
;)
RE: Cold Work vs. Shot-Peening
shot peening is usually a surface preparation.
cold working is usually expanding the hole diameter.
both processes put the surface layer into compression which delays the onset of fatigue.
RE: Cold Work vs. Shot-Peening
Increase the hole size.
Ted
RE: Cold Work vs. Shot-Peening
Hmmm... I disagree with you, Hydtools. Experimentally, cold working does increase fatigue life for aluminum.
What I want to know is if you consider the effect of cold working better than the effect of shot peening.
I also disagree with rb1957, shot peening is a surface preparation AND a cold working process.
RE: Cold Work vs. Shot-Peening
and cold working includes freeze fit bushes (usually much easier to accomplish, as opposed to split-sleeve mandrels).
RE: Cold Work vs. Shot-Peening
1) the depth of residual compression would be deeper than for shot peening
2) The surface roughness would be much lower for cold working/hole expansion than for shot peening.
Shot peening of aluminum is probably most commonly done with glass beads instead of steel shot, which means that the depth of residual stress is usually quite limited. This is the reason that the aerospace industry, courtesy of Lambda Technologies, has developed techniques like Low Plasticity Burnishing and Laser Shock Peening. I agree with Hydtools that the response of Al to cold working is lower than that of steel, but it certainly can be used to improve fatigue life of Al parts with holes.
RE: Cold Work vs. Shot-Peening
Ted
RE: Cold Work vs. Shot-Peening
RE: Cold Work vs. Shot-Peening
cold working is a stadnard industry practice for improving the fatigue life of a details, google ForceMate, Fatigue Technologies Inc (not that i'm selling their products). you're right that the surface compression also creats sub-surface tensions but fatigue failures generally start at the surface and the surface stresses induced are much higher than the tension stresses ('cause they act on a smaller area). that being said, you need to be aware of the tension stresses, to avoid stress corrosion cracking (typically what happens if the tensions stresses "leak" out to a surface.
and generally cyclic compresion stresses don't cause fatigue, though they can cause a relaxation of the compressive stress field (hence loosing the effect you wanted in the 1st place).
RE: Cold Work vs. Shot-Peening
Well, maybe the depth of the compressed layer is greater, but does it garantee that the compressive stress on the surface (just where I need it for fatigue improvement) is going to be grater?
I mean, to my understanding what determines the growth of a crack (and thus, fatigue life) is the tension at the surface. I don't care how deep is the compressive layer as it gives to me the necessary compression I need at the inmediate surface layer to oppose traction stresses. Don't you think?
The surface roughness is a very good point.
The mean for obtaining the cold work by expanding the hole could very well be freeze fit, for example.
And what about flap peening?
RE: Cold Work vs. Shot-Peening
RE: Cold Work vs. Shot-Peening
you can get the same effect with a freeze fit bush, if you can afford a larger hole in your part.
is this reworking an existing piece/design, or designing from scratch ?
RE: Cold Work vs. Shot-Peening
Ted
RE: Cold Work vs. Shot-Peening
RE: Cold Work vs. Shot-Peening
Well the question is, given those conditions, choose between one or another.
Sure there are other alternative solutions, but I'm more interested in, given that initial setup, knowing which one would work better in fatigue and why.
RE: Cold Work vs. Shot-Peening
Mike Halloran
Pembroke Pines, FL, USA
RE: Cold Work vs. Shot-Peening
No, I don't think that. You want to suppress fatigue crack initiation and propagation, and a deeper level of compressive stress is better. You can review scientific literature on this subject to convince yourself.
RE: Cold Work vs. Shot-Peening
Holes in aluminum structures can have their fatigue properties improved through cold working. This is done quite commonly on shear fastener (ie. rivets, hi-locks, etc.) holes in aircraft structures by a process called "coining".
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Regards,
Terry
RE: Cold Work vs. Shot-Peening
However, data (proprietary) which I have seen indicates that a cold-worked hole is superior to shot-peened holes. This is most likely due to the fact that cold-working is a uniform process than with shot-peening.
jetmaker