yielding of SS 304 part 2

[tlee123]

For a test, I loaded some SS 304 parts and found that they didn't yield at 50 lbs and did yield at 75 lbs. Therefore, I only know the yield strength is between 50 and 75 lbs.

A customer has returned some parts (same design as above). The parts hadn't failed but they have plastically deformed. The parts were subjected to 20 Hz vibration where the max load should be less than 20 lbs.

My question is this: When parts see very high cycles at stresses below the yield strength, is there a mechanism that can (or does) cause eventual yielding of the material?

My thinking is that while the parts might fracture at less the yield strength, they shouldn't be significantly yielding.

Thanks for any help.

Tom

 

[metengr]

What was the original condition of the 304 SS parts - were they in an annealed condition or cold worked prior to testing?

 

[CoryPad]

How about fatigue? That is a mechanism that can explain your results.

Regards,

Cory

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[tlee123]

Metengr: OK I made a mistake. The material is 302 SS, not 304. The drawing calls for annealed material, but it is a small sheet metal piece that is bent up and has a generally spherical shape so there must be some cold working during forming.

CoyPad: I know that the fatigue can eventually cause the part to break but can it also cause the yielding that we're seeing? My limited understanding of fatigue failure is that there is little or no warning which I take to mean that there is little to no yielding prior to failure. Does this sound correct?

Thanks,

Tom

 

[metengr]

The reason for my asking about prior cold work is that you could be seeing something similar to a vibratory stress relief in service; this would affect the dimensions of the piece.

 

[swall]

I would take some of the customer returned parts and do a dye penetrant test (either visible dye or fluorescent dye) to check for presence of fatigue cracks. Then you can rule in or rule out fatigue.

 

[tlee123]

I sincerely appreciate all the replies, but it seems that I'm not getting my question across clearly enough.

Let me try to rephrase it.

If you take a steel part and subject it to cycling loading below the yield stress, can the part plastically deform (yield), other than completely fracturing? If instead it was a static load of the same magnitude, we know that there will be elastic deformation only.

I know the part can fail due to a stress that is below the yield strength due to fatigue.

Thanks again,

Tom

 

[CoryPad]

Yes, the part could deform due to stress cycling below the static yield point.

Another possible mechanism is distortion due to residual stress.

Regards,

Cory

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[EdStainless]

Remember that yield strength is not a physical reality, it is an engineering construct. For a highly ductile, low strength material like stainless. the actual pure elastic portion of the stress strain curve is extreamly small. There will be some permanent set at low loads, and vibration will make it more so. This is a huge issue in spring applications.

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[MintJulep]

What is the actual stress in the part in service? How is the part loaded? What is the temperature?

What is the nature of the deformation? How does it compare to the deformation experienced during overload?

What is the stress gradient? What are the residuals after forming?

Metengr has hinted that residual stresses being relieved may be the cause. This certainly seems worth further investigation.