Delayed information due to residual stress 2

[rob768]

Is there anyone who can shed some light on the subject of residual stress and delayed plastic deformation under static condition? I am of the opinion that the distribution of any residual stress in a cast or welded construction is stable, and that plastic deformation can only occur when stress due to externally applied mechanical or thermal load, together with the internal stress, exceed the yield stress causing plastic deformation.
However, some claim that this can also occur in a short period after casting or welding. This is supported by their claim that large cast parts such as engine blocks are stored for a year before further machining, just to make sure that the internal stress level is levelled.

Thanks for any input, and in particular literature references are very welcome.

 

[metengr]

rob768;
Now would be a good time to introduce you to the search feature above, which is a powerful tool. Just typing in residual stress results in over 100 posts that you can review. You can refine the search based on key words. The topic of residual stress distribution, magnitude, measurement and reduction has been discussed in previous posts.

 

[rob768]

I actually did, and found lots of posts on residual stress and deformation due to it. However, none gave me the information i need, namely deformation due to external stress without any applied load or additional stress. Image an object lying in the workshop. Perfectly straight one day, and bent or warped the next day (or weeks later for that matter) without anyone touching it.

And of course I should have typed delayed DEFORMATION instead of delayed INFORMATION in the header.

 

[arunmrao]

"stress relaxation phenomenon" google search

 

[metengr]

Image an object lying in the workshop. Perfectly straight one day, and bent or warped the next day (or weeks later for that matter) without anyone touching it.

The above can be explained by self relieving or relaxation of residual stresses over time within the material and not external stresses.

 

[swall]

There are numerous anecdotes about castings cracking just sitting around on the shipping dock. Also, Don Wulpi has a good example in one of his books--a large steel beam split while laying on the shop floor after having been cut to length the previous day.

 

[rob768]

I have heard some of those anecdotes. I am looking for a scientific explanation. Also: everything i find with a google search on stress relief is related to either heat treatment or alternating load. Apart from anecdotes on deformation during idle periods, there is actually no explanation (or i am reading the wrogn articles, which of course is also possible).

 

[redpicker]

Talk to anyone who has straightened much pipe and they will tell you that they can straighten a piece of pipe to be "perfectly straight" (whatever that means) and it will "go crooked" just sitting on the rack in a few days or weeks (or even overnight).

I believe this is a combination of stress relaxation/redistribution and is a result of both plastic compressive and tesnile strains being introduced during the straightening. Becasue of the Bauschinger effect, the yeild stress in areas that have had a plastic tensile strain is different from those that have had a plastic compressive strain. The residual stress levels are at the yield stress in these areas, but since the yeild stress is not constant, the residual stress levels are not constant. Add to this some thermal expansion and contraction, they no longer balance and you get movement.

I would guess it would be difficult to find much in the literature on this since it is very difficult to quantify, which makes an anaylitical analysis tough. Since, with pipe anyway, you are looking at L/d ratios of aorund 100 or so, just quantifying straightness is hard to do.

rp

 

[rob768]

Guess i have to live with that than. thanks for the replies.

 

[FeX32]

Thank a look at this study.
Pretty interesting.


Fe

 

[FeX32]

NUTS!!
That should have read "Take a look at this study".


Fe

 

[rob768]

I've printed it out and will read it somewhere today. However, aluminium is sure to be different from wrought and welded steel. Thanks anyway.

 

[FeX32]

Yes Al. should behave different in some ways. Although the fundamentals should coincide in one way or another.
I may have something on steel. But, I doubt welded.



Fe