fatigue and re-heat treating
fatigue and re-heat treating
(OP)
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
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





RE: fatigue and re-heat treating
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.
RE: fatigue and re-heat treating
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...
RE: fatigue and re-heat treating
RE: fatigue and re-heat treating
Maui
www.EngineeringMetallurgy.com
RE: fatigue and re-heat treating
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
RE: fatigue and re-heat treating
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
RE: fatigue and re-heat treating
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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