
Critical bending moment  
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Posted by: IMoo ^{®} 06/09/2005, 09:26:31 Author Profile Mail author Edit 
Anyone any idea of the formula to calculate the critical bending moment of a hollow cylinder (eg. a plastic tube) or could point me to text book where I could find this info. Cheers. 
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Re: Critical bending moment  
Re: Critical bending moment  IMoo  Post Reply  Top of thread  Forum 
Posted by: zekeman ^{®} 06/10/2005, 09:46:21 Author Profile Mail author Edit 
You probably mean the yield stress, so you use the formula
s=Mc/I where I=moment of inertia, in^4 M= bending moment, lbin c= outer radius of cylinder s= yield stress in lb/in^2 For the cylinder, I=pi/64(Do^4Di^2) C=Do/2 If you find the yield stress s, then M=sI/c If this is a design problem you have to put in factor of safety, at least 3, so that your design stress is 1/3 of the yield stress or less and therefore the allowed M is 1/3 of the value calculated above. 
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Re: Critical bending moment  
Re: Re: Critical bending moment  zekeman  Post Reply  Top of thread  Forum 
Posted by: ex03134 ^{®} 11/26/2005, 09:12:18 Author Profile Mail author Edit 
hi,im doin something similar, basically im investigating a type of can crusher. i need to know how to calculate a force acting down on a can that is positioned on its side..it looks like a simply supported beam however because its cylindrical in shape i dont know the formula to calculate the force for this...i was wondering if u can help me..thank you 
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Re: Critical bending moment  
Re: Re: Critical bending moment  ex03134  Post Reply  Top of thread  Forum 
Posted by: ex03134 ^{®} 11/26/2005, 09:16:30 Author Profile Mail author Edit 
sorry again..there was a formula you gave to Imoo.Since i'm from england im unsure of what your symbols mean..
I=pi/64(Do^4Di^2) C=Do/2 what is Do, Di and C? 
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Re: Critical bending moment  
Re: Re: Critical bending moment  ex03134  Post Reply  Top of thread  Forum 
Posted by: devitg ^{®} 11/27/2005, 17:59:37 Author Profile Mail author Edit 
about formulae It shall be as follow S=M * C/I where
If you find the yield stress S then
Modified by devitg at Sun, Nov 27, 2005, 18:00:18 
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Re: Critical bending moment  
Re: Re: Critical bending moment  zekeman  Post Reply  Top of thread  Forum 
Posted by: IMoo ^{®} 06/15/2005, 12:44:36 Author Profile Mail author Edit 
Thanks for the reply zekeman. What I am doing is kink testing plastic tubing. The tubing is bent into a semicircle and placed between 2 parallel plates. The top plate moves downwards until the tube collapses /kinks. If I know the force when the tube collapses, the distance between the plates and the cross sectional area of the tube what would be the correct formula to calculate critical bending moment? 
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Re: Critical bending moment  
Re: Re: Critical bending moment  IMoo  Post Reply  Top of thread  Forum 
Posted by: devitg ^{®} 11/27/2005, 18:05:51 Author Profile Mail author Edit 
Your aplication is far from a bending, despite you BEND it .
It is near a BUCKLING , and maybe you are asking for the force to be excerted by the plates. I sugest to do a try and error procedure or fit the plates on a hydarulic press , read the pressure need to bend it , and get the force by the area of the press cylinder.

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