Is there a Time Dependent Strain for material under constant stress at ambient temperature

[johnchrc]

Is there a time dependent strain condition when a material is under stress (alloy steel 4140 Q&T) that could cause permanent strain increase over time at ambient temperature?

I have a collet that is collapsed into a sleeve ID under very high bending stress for the material. After 1 year the collets were pulled and the OD did not return to their pre-stress diameters. They had experienced permanent strain as much as 7%. The OD of flared Collet Fingers were 4.813 and collapsed into a 4.335 ID (see pic). The OD after removing from Sleeve ranged from 4.463 to 4.803 over 11 parts. This was discovered after disassembling tools that had been in inventory for over 1 year.

I looked at Creep but that only occurs with temperature. Is there a time dependent deformation under stress that could cause these parts to undergo permanent strain? Fatigue should not be an issue as the parts are on the shelf.

Any help would be appreciated. Thanks.

- CJ

 

[btrueblood]

Presumably the same tool, if disassembled immediately (minutes instead of months/year), does not show the same permanent deformation?

But the answer to your question is yes.

http://imechanica.org/files/An expe...laxation of AISI type 304 stainless steel.pdf

http://www.sciencedirect.com/science/article/pii/S0261306900000741

 

[slashragnarok]

But I've read that weightlifting barbells (usually made from 4140 HT, 4340 etc.) return to original straightness even if it has been stored loaded for months.

 

[metengr]

As mentioned by btrueblood are you sure that during initial installation secondary stresses did not exceed the YS of the material resulting in local permanent deformation? This seems to be a more reasonable cause and effect you observed. For 4140 material, the amount of creep strain would be too low to measure at ambient temperature.

 

[EdStainless]

And remember that you don't have to exceed the 0.2% OS Yield stress in order to get a set. The stress strain curve will start deviating from a straight line at lower strains than that.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[johnchrc]

Correct Blueblood,

If part is pulled immediately there may be some hysteresis from elastic yielding but this is the kind od reduction I'm seeing after 1 year+

4.802/4/882 As machined
4.429 to 4.502 after pulling from 4.335 ID for ~1 year

LPI shows cracks is QPQ surface that is 5-8 mil thick. It does not extend into base material. Could it relieve the OD ~7%

Thanks for your input.

Bye the way, I have never seen a barbell return to shape once overloaded. However, I don't know if it was done over time or if a bada** just loaded too much weight on it.

- CJ

 

[johnchrc]

Another pic to help convey situation. I just learned I could only place one attachment at a time. This just shows collet collapsed in Sleeve.


- CJ

 

[CoryPad]

Your problem description sounds similar to stress relaxation of springs. There is a lot of information available on this subject.

Your image after magnetic particle inspection is worrisome. How deep are those cracks? You may have plastic deformation of the material in addition to cracking, which could explain the shape change.

 

[johnchrc]

Yes,

I do have a small amount of plastic deformation. My calcs show the beam outer fiber is overstressed. We have been running these for at least two years but until we disassembled parts that were sitting in inventory outside to clean up we never discovered the problem. I can see if I can reduce the stress by modifying part but I'm limited without redesigning entire tool. I can't increase the finger length. I can reduce the deflection and increase the number of fingers. I might be able to change the moment of inertia of finger. If I did that at base of finger I'm not sure that changes the stress through the rest of the beam. I think it would because the load would be reduced to deflect finger and thus bending moment.

The cracks shown have conflicting information. I see the diffusion depth of QPQ per drawing is 5-8 mils thick but report shows .4 mils. Crack does not extend into tempered martensite base material. Could this be why the OD has relieved by over .336" on diameter?

The spring analogy makes sense. I suppose I need to keep the part out of the plastic region and this problem should disappear.

- CJ

 

[johnchrc]

metenger,

What would create a secondary stress during installation and storage? No disrespect I just don't follow. Part is collapsed and installed inside Sleeve where it remains until it is pressured to 900 psi and shifts open at which point it is no longer under stress or used again unless the ID seat is milled up. No if the ID takes a set, when the collet moves down and expands there could be problems milling and/or pulling milling tool back through tool without catching on undersized Collet.

- CJ

 

[metengr]

The secondary stress would be from a biaxial stress state during installation, and not during storage. This would be local yielding on the front end and not during storage.