From: roo.caltech.edu!johnck (John C. Krowas)
Subject: Re: Drillium
Originator: johnck@roo
Organization: California Institute of Technology, Pasadena
Date: Thu, 13 Aug 1992 18:32:55 GMT

In article <1992Aug13.151408.24659@col.hp.com> arnie@col.hp.com  
(Arnie Berger) writes:
> I'd have to go back to my old metallurgy texts to verify this, but  
there 
> was a rule of thumb called "St. Venet's Principle", which dates  
back to
> a monk in the 15th or 16th century. Anyway, Fr. Venet noticed that  
when 
> a hole was drilled into something, the strength of the material was 
> affected in an area roughly equal to the diameter of the hole all  
around
> the hole. Thus, if you have a 1" diameter hole in a metal plate,  
you could
> say that there is a 3" diameter region with the hole as its center  
that is
> weakened by the presence on the hole itself. To put that in cycling  
terms,
> if you drill a hole in the seat post, the diameter of the hole  
should not
> be comparable to the dimensions of the seat post. In other words,  
drill
> small holes.
> 
>                                                            Arnie
> 
> 
> -- 
> Arnie Berger
> arnie@hplsdvt
> Telnet 590-5642
For a plate with a crack in it, the resultant stress is given by the  
applied stress multiplied by (constant)*(a/r)^0.5, where a is the  
crack length (here the diameter of the hole you've drilled) and r is  
the crack tip radius (here the radius of the hole you've drilled).  
The value of the constant varies with the theory, but sometimes it's  
taken as 2. So, by drilling a round hole in a plate, you may be  
increasing the stress in the plate by a factor of 3. A safety factor  
of two is common in engineering design, so decide for yourself if you  
want to risk it. 

John Krowas
 


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