support and creep of lead block

[hobbs101]

Hi

I'm designing a mechanism that has a lead block. Block is 230 x 100 x 100mm. It is made from lead with 4% antimony. Weighs about 30kg. The block is used to shield x-rays.

I need to support the block off one of it's 100 x 100 ends. The other end is unsupported. The block will be used in various orientations.

I'm concerned that over time the lead material may creep under it's own weight. I'm struggling to find any information about this material and it's creep characteristics.

Does anyone know of a source of information for this?

Hobbs101
Mechanical Design Engineer

 

[Tmoose]

http://www.eng-tips.com/viewthread.cfm?qid=180873

 

[btrueblood]

Googling "creep data for lead" gave:

http://www.sciencedirect.com/science/article/pii/0167577X89901894

http://vulcangms.com/2015/03/20/material-creep-mechanical-properties-lead-creep/

https://books.google.com/books?id=T...dEQfd#v=onepage&q=creep data for lead&f=false

http://www.dtic.mil/dtic/tr/fulltext/u2/005978.pdf

The third link had some data for lead alloys up to 1.2% antimony. You might contact the people at the 2nd link and see if they can suggest a source.

 

[btrueblood]

There are also (at least) a couple of patents that contain creep data for lead/antimony alloys.

http://www.google.com/patents/US3888703

http://www.google.com/patents/US2570501

 

[JStephen]

I have read that pure lead can recrystallize at room temperatures.
However, I have never seen any kind of lead that showed any kind of deflection under it's own weight in a case like yours, and can't imagine that it would be an issue. Perhaps you could detect some deflection with a good micrometer, but it's not going to just sag down like a beanbag, either. I've got some civil war bullets around here somewhere, and they sure don't show any droopiness after lo these many years. I've never heard of lead pipe or lead caulking having problems from this.

Similarly, I have read that glass isn't truly crystalline, and that old windows, like 200 years old, will actually be thicker at the bottom than at the top because of this. But it's not an effect that most of us need to worry about, either.

 

[tbuelna]

Based on the very modest compressive stress (4.2psi at 1g) the lead block base will experience, I can't imagine any appreciable amount of creep occurring, unless there are elevated temperatures. Even if the lead block experienced a few micro inches of cold flow over the course of several years, would it create a hazard or result in the lead block being unable to perform its shielding function? Probably not.

 

[hobbs101]

Thanks for all the contributions.

btrueblood, that 3rd link looks good. Haven't managed to find the full download.

The lead block is actually used to collimate the x-ray beam, so the edge of the block needs to remain in a stable position, hence my concern about creep. My FEA shows a stress of 0.24 MPa (35psi) - tbuelna, the block is fixed as a cantilever. In addition to this I'm looking at a tie rod style fixing to mount the lead block to it's frame. They will apply a compressive stress along the length of the cantilever, increasing the overall stress.

http://www.ila-lead.org/UserFiles/File/factbook/chapter2.pdf

states

However, studies have found that creep can occur in lead (of 99.99% purity) at stresses as low as 0.7 MPa (though by a very small amount, elongation of 0.06% after 500 days at 30°C; double the stress produced a 2% extension in the same period (Blasket and Boxall, 1990)). Extrapolation from other tests shows that, under normal conditions, no appreciable creep will be expected at stresses below 1.72 MPa (or if in compression, 2.75 MPa) for 99.9% pure lead (LDA technical notes 1992). Differences in results are assumed to be because of differences in temperature and other test conditions. Processing conditions and some alloying additions influence the creep behaviour of lead; small additions of copper (widely used for lead sheet) and also silver can improve creep resistance.

The 4x tie rods can be preloaded to a total of 12kN to give a stress of 2.75MPa. The addition of 4% Sb should improve this. (Note, the tie rods will have springs so that preload is not lost if compressive creep does occur).




Hobbs101
Mechanical Design Engineer

 

[3DDave]

Glass flow is a myth. Uniformity of thickness wasn't that good and only became good in mass manufacture in the relatively recent float-glass process.

http://www.cmog.org/article/does-glass-flow

 

[tbuelna]

hobbs101- Adding 4% Sb to 99.9% pure lead more than doubles its tensile strength.

 

[JStephen]

Thanks, 3DDave, that's why I prefaced that with "I have read"!

In regular house windows, if that occurred, I would think it would be fairly obvious, in that you'd get the same pattern of distortion in each pane, rather than random waviness. I've never seen windows like that, never saw a picture of any like that, though.