Cold Work vs. Shot-Peening 1

[Neubaten]

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.

 

[MikeHalloran]

I'd imagine a steel coupon.


Mike Halloran
Pembroke Pines, FL, USA

 

[Neubaten]

Come on, I do the imagining, you do the evaluation!!

 

[rb1957]

i wouldn't shot peen a hole ... doesn't mean you can't.

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.

 

[hydtools]

Aluminum does not cold work very well. There will not be significant effect from cold working.

Increase the hole size.

Ted

 

[Neubaten]

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.

 

[rb1957]

but it (shotpeening) wouldn't be my 1st thought to improve a hole's fatigue life. and these are mainly improvement processes, ie the design has matured so that you can't increase the thickness (pad-up or boss) or the hole diameter.

and cold working includes freeze fit bushes (usually much easier to accomplish, as opposed to split-sleeve mandrels).

 

[TVP]

Cold working the hole would be preferable to shot peening for at least two reasons:

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.

 

[hydtools]

If you create residual compressive stress on the surface of the hole, you create residual tensile stress in the material around the hole. I don't think you have gained significant increase in fatigue life. Depending on the loads the resulting hole surface may alternate from compression to zero stress, but the material around the hole will alternate from higher tension to less conpression.

Ted

 

[ornerynorsk]

Ballizing? Also, what if the hole in the aluminum were to have a static steel liner put in place.

 

[rb1957]

hydtools,

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).

 

[Neubaten]

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?

 

[swall]

I would think that this would be a good application for flap peening. You would only touch the I.D. surface of the hole, whereas with lance type peening you would have to apply a mask to keep shot from the other surfaces.You would also have to rotate the part to get uniform peening with a lance. Also, flapper peening is well suited to one-off or low volume jobs, which is what you seem to have.

 

[rb1957]

you can expand the hole (with a split sleeve mandrel).

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 ?

 

[hydtools]

Remove the stress risers, the hole edges. Round over, preferred, or chamfer the edges.

Ted

 

[rb1957]

kt of a chamfer'd hole isn't much less than the plain hole

 

[Neubaten]

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.

 

[MikeHalloran]

Many of the useful aluminum products are already strain- hardened or otherwise hardened. Some of them will tolerate further abuse, some won't.



Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

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?

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.

 

[tbuelna]

Neubaten,

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".

http://books.google.com/books?id=L2...3toMAP&sa=X&oi=book_result&ct=result&resnum=6

Regards,
Terry