Barlow's law applications

[laithrahim]

i am new to barlow's law,
we are trying to determine the min wall thickness of HDPE bottle utilizing Barlow's law we would like to hear your comments on such approach is it valid to use such equation to our purpose also we would like to know what does design stress mean in barlow's law and how can we calculate it or obtain it

thanks a lot

 

[robyengIT]

may be it can help

 

[laithrahim]

actually this document that highlighted barlows law for me
but do you have any info concerning the design stress
thanks a lot

 

[robyengIT]

design stress considerations at

http://www.sinvacmining.co.za

 

[LittleInch]

The document from robyeng is very useful and should give you sufficient pointers.

Your key consideration is how long do you want the item to last / be under pressure or stress of any sort and what temperature are you going to operate it at.

Then use the graphs and data from the stress graphs listed in the document.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[laithrahim]

thanks a lot littleInch do you think Barlow's law is applicaple to determine the thickness of bottles which is intended to carry tablets or capsules

 

[btrueblood]

Not sure anybody knows how to apply "Barlow's law" to the design of a water bottle...googling got me:

http://en.wikipedia.org/wiki/Barlow's_law

the first sentence of that link is "Barlow's law was an incorrect physical law proposed by Peter Barlow in 1825 to describe the ability of wires to conduct electricity."

Can you elaborate for us on what "Barlow's law" means to you?

 

[laithrahim]

well barlow has many laws one of them is the one you talked about and that is wrong
the one that am asking for is in the following link

http://en.wikipedia.org/wiki/Barlow's_formula

this formula tries to relate the internal pressure with the wall thickness of pipes

 

[LittleInch]

Barlows law as normally understood, at least in terms of pipe, is to resist internal (or external) pressure over a reasonably long distance before end caps (probably at least 5D). I don't think filling bottles with tablets is what was envisaged. Sounds like some empirical testing needed to me. It might get you to an initial start point, but axial load, external load, and torsional load (screwing a cap on and off) will have much greater impact on thickness than internal pressure IMO.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rb1957]

yeah ... never knew hoop stress = pr/t was a "law".

filling bottles with tablets ... how does that produce an internal pressure ?

or are you tlaking about the bottle moulding process ?

Quando Omni Flunkus Moritati

 

[dvd]

It seems like buckling would predominate - pill bottles have much thicker walls than soda bottles.

 

[LittleInch]

Sorry, my point should have read internal pressure only. Tubes are much less strong in external buckling or collapse than they are in resisting internal pressure. If you've every had a "value" bottle of pressurised drink which is relatively strong until you release the pressure and try and hold it to pour some of it out, then this is what we seem to be talking about, i.e. it buckles, collapses and can break easily just because someone only looked at holding the contents in whilst it was pressurised and standing upright.

I suspect the approach here would result in the same thing, a bottle which would hold the tablets etc ok, but then buckle and collapse as soon as you tried to hold it and remove the child proof resistant cap on the top of it....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[GregLocock]

"is it valid to use such equation to our purpose also we would like to know what does design stress mean in barlow's law and how can we calculate it or obtain it "

a) No

b) From a material data sheet from the supplier.

As alluded to above, you probably have stiffness issues more than strength ones. Mr Barlow's Law (thanks for that, interesting that it was derived so long ago) only talks about stresses as a result of uniform internal or external pressure.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[laithrahim]

the problem that we are facing is that after we ship our bottles to our sister company in NJ USA the some of the bottles are found to be paneled upon proper investigation it was found that this case is happeing due to pressure differance between orgin and distination
the orgin location is 1000m over sea level while NJ is at sea level so there is pressure deffirance betweent he two locations
we are trying to find a preventive action to prevent this phonomenon from occuring again in other products and thats what led us to barlow's law to find the optimal thickness that can prevent the bottles from paneling

 

[GregLocock]

OK, but that isn't Barlow's Law, or hoop stress as we always called it.

What you are describing sounds like local skin buckling of a thin walled pressure vessel, the great news is that it is well understood, the bad news is that it is a bit more complex than Pd/2t.

http://www.eng-tips.com/viewthread.cfm?qid=91966

The net result is the same, reduce the pressure or increase the wall thickness.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rb1957]

ok, the containers are collapsing because of the air pressure difference between 3,000' and SL, something less than 2psi.

Odd for this to be the critical design load ... i'd've thought the the crusshing load from someone opening the container would be more. the problem if that i think FE will not predict buckling of the container under external pressure.

I think you may need to do some tests, with a vaccuum machine, develop a 2psi vaccuum inside the container.


Quando Omni Flunkus Moritati

 

[btrueblood]

Corrective action for an external pressure collapse in shipping...hmm, let's see...how about not capping the container for shipment?

 

[btrueblood]

Buckling collapse can be studied with FEA, but not the standard package, you need to spend a few more dollars on the fancier solver software to be able to handle eigenvalue problems. Even then, the actual collapse pressure is often a fraction of the predicted value, due to material imperfections, varying wall thickness, un-anticipated non-uniformity of loading, etc.

 

[laithrahim]

for not caping the bottles this is not possible as it contains pharmaceuticl drugs and this will effect the quality of the product
we have tryed to simulate the sea conditions by placing the bottles in a pressurized vessel paneling has occured hoewever when the size of the bottle was reduced no paneling was observed (from 625 to 500 cc) we are trying to find a preventive action to stop this from happening to other bottles.

 

[rb1957]

wasn't sure if the eigenvalue solution would work for pressure ... guess it justs scales the load applied. if so, reasonably basic problem and model.

if you get good results for the smaller bottle, then you know an acceptable stress (pR/t).

so the 625cc bottles should have a wall thickness 1.25* the 500cc bottles, yes?

haven't we made pill containers for a long time ? don't we know from experience what work and what doesn't ?

Quando Omni Flunkus Moritati