<VV> Re: VirtualVairs Digest, Vol 4, Issue 112

[RoboMan91324 at aol.com]

Roger,
 
Great post.  The "compression trick" is used in many  applications.  
Pre-stressed concrete is a good example but the one we are  most familiar with is 
tempered glass.  In the manufacturing process, both  sides of the near molton 
glass plate/pane is cooled relatively quickly.   This shrinks the outside glass 
while the inside is still hot.  This is  usually done with oil.  You may have 
noticed "LOF" printed on a lot of  automotive glass.  This stands for "liquid 
Oil Float."  As the inside  cools relatively slowly, it also shrinks.  Since the 
outside has already  hardened and can't really shrink, it compresses.  As you 
point out, hard  and brittle substances will tend to crack and then propagate 
that crack if in  tension.  The compressed outside glass is very tough.  You 
may have  noticed the difference between regular plate glass and tempered 
glass if it is  struck.  If you want to break tempered glass relatively easily, 
tap it on  the edge with a hammer.  The inside layer is in tension and the crack 
will  propagate.  Wear safety glasses.
 
Just ask Mr. Wizard.
 
Doc
~~~~~~~~~~~~
In a message dated 5/17/2005 4:15:58 PM Pacific Standard Time,  
virtualvairs-request at corvair.org writes:

Message:  3
Date: Tue, 17 May 2005 17:49:49 -0500
From: "Roger Gault"  <r.gault at sbcglobal.net>
Subject: Re: <VV> Crank  Nitriding/Engine case question Question
To: "Corvair List"  <virtualvairs at corvair.org>
Message-ID:  <00cd01c55b32$bb905d20$6400a8c0 at GaultComputer>
Content-Type:  text/plain;    charset="iso-8859-1"

Chris,
<snip>
As for the nitriding discussion you found, I'm not  impressed.

<snip>
Nitriding is done to put the surface in  compression.  In order for a fatigue 
crack to start, it must get the  surface into tension.  Bending forces on the 
crank can do that.  If  the surface is nitrided, there is a built-in 
compression which must be overcome  before the surface goes into tension. So, the 
nitriding reduces  cracking.

<big snip>