fatigue and re-heat treating 1

[Manifolddesigner]

General question about fatigue.
I'm an ME and I do not remember seeing in any text an answer to the following:

If I have a part that's cyclically loaded and after say ~50% est fatigue life, it's taken out of service, x-ray'd and checked for cracks. Then annealed and re-heat treated (and machined)...does this start the fatigue clock over?
Interested in aluminum and steel.

Jason Minahan
jminahandesigns@gmail.com

 

[metengr]

For exposure to high cycle fatigue crack initiation one can undo accumulated local slip strain by reheat treatment.

The problem with this approach lies in the ability to detect small cracks that have initiated from high cycle fatigue damage.

High cycle fatigue crack initiation ocurrs from accumulated slip strain that on a microscopic scale generates small cracks. This is why ~80% of high cycle fatigue life is spent forming cracks. This is where your approach can become very risky because if small cracks are generated and missed by NDT, you can possibly progress to low cycle fatigue crack propagation and have failure.

 

[Manifolddesigner]

To recap:
If I have a valuable part out of the assembly already, it may help significantly?
(Re-)Polishing high stress areas may help significantly as well (grinding off micro cracks)?
Think Connecting rod on engine.
What about cryo treatment instead of (or on top of)?
Shot peening instead of (or on top of) re-heat treating?

Hmm...

 

[metengr]

Surface treatment methods that induce residual compressive stresses on a component that has been exposed to high cycle fatigue can certainly prevent further damage provided past damage has been evaluated.

 

[Maui]

Unless surface cracks that have been initiated through fatigue are removed by grinding, polishing, etc., the remaining fatigue life of the part is already essentially determined. Keep in mind that the surface finish and surface condition of the part can be important considerations in preventing crack initiation in the first place. As metengr mentioned above, the generation of residual compressive stresses on the part surface through shot peening can be beneficial in prolonging fatigue life. Fine surface finishes obtained by polishing or lapping to remove grinding marks will smooth out asperities that can act as stress risers. This approach can also serve to prolong fatigue life.

Maui

http://www.EngineeringMetallurgy.com

 

[redpicker]

X-ray (Radiographic Testing, or RT) may not be the best test method to detect fatigue cracks. I would guess there are very few, if any, cases where it would be a preferred method. Mag Particle, Penetrant, or Eddy Current (or other electro-magnetic methods) would be better methods to use.

As others have stated, while the theory is sound, the practice is rarely successful.

Grinding existing cracks (micro or not) is not recommended since the fatigue damage exists well past where the cracks have developed.

Polishing and shot peening are successful methods of mitigating fatigue, but it is best performed on new parts; the cost:benefit ratio decreases on used parts.

Cyro treatment? I won't touch that one. Let's just say that I wouldn't rely on it.

The best solution is to design the part so the stress levels stay below the fatigue limit and, if that is not possible, provide for frequent inspection and replacement when damage is detected. Your thoughts on the matter are not new, but I have never seen these methods have a commercially successful applicaiton.

rp

 

[Metalhead97]

Manifolddesigner,

FPI inspect for cracks to determine if your material has any fatigue damage. This type of inspection requires a trained eye and some knowledge of how the component is loaded. As metengr said your biggest problem is the slip strain that produces intrusions/extrusions on the surface. These surface defects can initiate fatigue cracks if more severe defects are not present. Refinishing the surface will remove these defects, however I wouldn't be surprised if these defects show up again around the same spots because localized slip is really a question of intragranular microstructural differences. Surface treatments and other hardening treatments will certainly help.

MH

 

[EdStainless]

On top of these valid comments remember that Fe has a fatigue limit, Al does not.
You are dealing with two very different cases with these different materials.
I could see trying this in a Fe or Ni based alloy.
If it was a steel then perhaps inspect, re-temper, re-polish would get what you needed if the temper was at high enough temp.
While fully re-heat treating the part would be more effective you have issues with HT induced surface damage and having enough stock to do an adequate grind and polish.

I have seen this done with a PH stainless. The shaft was take out of service and re-aged, then polished. But it was done at 25% of the predicted fatigue life.

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