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Date: Wed, 28 Feb 2007 12:28:04 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: engine mount 4130 vs 304 SS
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Larry Mac Donald wrote:

> However,Stainless is a very very soft metal. So you know what will
> happen if you hook it up to something that vibrates. HTH.
>
>  

304 Stainless is very soft,  for some value of 'very'.   It's nominal 
yield strength is about the same as most mild steels, and that is still 
STEEL.   But that is in it's annealed state.  The importance here is 
that stainless work hardens much faster than other steels, and vibration 
== cold working.  The strength to weight ratio of 4130 is a wonderful 
thing, but its not the be all and end all of design considerations.  To 
truly engineer a 4130 fuselage from a light plane would require many 
tension members that would be so thinwalled or so small that they would 
be difficult to work with, or would be susceptible to impact damage.  I 
believe (because I don't have an actual design to do a comparison with) 
that a typical 4130 fuselage that someone would build in their garage, 
would only weigh a few pounds more if engineered from stainless.  
Engineered, as in all the members would have more than adequate 
strength.  Most of the few (ie, less than ten) pounds would be recovered 
from not having to paint.  That's a saving of money, time, weight and 
possibly health.  Even more work will be saved 20 years hence when the 
fabric needs replacing.

From: http://www.azom.com/details.asp?ArticleID=1181

An unusual feature of annealed austenitic stainless steels is that the 
yield strength is a very low proportion of the tensile strength, 
typically only 40-45%. The comparable figure for a mild steel is about 
65-70%. As indicated above a small amount of cold work greatly increases 
the yield (much more so than the tensile strength), so the yield also 
increases to a higher proportion of tensile. Only a few % of cold work 
will increase the yield by 200 or 300MPa, and in severely cold worked 
material like spring temper wire or strip, the yield is usually about 
80-95% of the tensile strength.

As engineering design calculations are frequently made on yield 
criterion the low yield strength of austenitic stainless steels may well 
mean that their design load cannot be higher than that of mild steel, 
despite the tensile strength being substantially higher. Design stresses 
for various grades and temperatures are given in Australian Standard 
AS1210 "Unfired Pressure Vessels".


-- 
         ,|"|"|,              Ernest Christley       |
----===<{{(oQo)}}>===----    Dyke Delta Builder      |
        o|  d  |o        http://ernest.isa-geek.org  |