Re: New Member & Test (did this work??)

["Marsee Skidmore" <heyred@email.msn.com>]

A quick and flawed primer on Mechanics of Materials:

Almost all solid materials act like a spring. Pull on a chunk, and it
stretches. Let go, and it returns to its original length. If the material
happens to be something like steel, it takes a lot of force to stretch it,
and it doesn't stretch much. Just how much it stretches also depends on how
long the chunk is. To level the playing field, "strain" is defined as
stretch divided by overall length, and is measured in something like inches
per inch.

Just how much force it takes to produce a given strain depends on how thick
the chunk is. To level the playing field, "stress," is defined as force
divided by cross sectional area, and is measured in something like pounds
per square inch.

The ratio of stress to strain is a material property representing the
springiness or elasticity of a material. Just to confuse people, this
property is known interchangably as Tensile Modulus, Modulus of Elasticity,
or Young's Modulus. As you say, the value for steel is 3E7. Acrylic is
somewhere around 4.5E5, and is therefore much less "stiff" than steel.

It gets more complicated. If you stretch your chunk of something until it
doesn't snap back, you've exceeded the "Yield Strength" of the material. If
you continue to stretch it until it breaks, you've exceeded the "Ultimate
Strength" or "Tensile Strength" of the material. The amount of strain at
ultimate strength is the "Elongation at Break," which is a measurement of
the brittleness of the material.

As you pull on your chunk of material and stretch it in one direction, it's
going to shrink in the other direction. The ratio of strains from the
stretch direction to the shrink direction is "Poisson's Ratio." For most
solid materials, Poisson's Ratio falls in the range of .25 to .35. I'll be
damned if I can remember why it keeps showing up in formulae, but it does.
But if you do the calculation with .25, and then repeat it with .35, you'll
probably find that it doesn't make a hell of a lot of difference.

The new kid is learning this stuff even as we speak, so I'm depending on him
to keep me honest. :-o Joe
-----Original Message-----
From: Sean Walinga <swalinga@telusplanet.net>
To: personal_submersibles@psubs.org <personal_submersibles@psubs.org>
Date: Saturday, May 09, 1998 9:36 PM
Subject: Re: New Member & Test (did this work??)


Hi Joe,
I'll admit that I am no engineer. I do what I can with what I have. I do not
know the difference between these 4 things. In my calculations I have used
the math formula from others. Would you explain the difference of these 4
for me?

The elasticity works out to be 3E7 for steel. I am not sure of the
elasticity of acrylic. I am also unsure which elasticity this is...

ttfn
Sean

>I'll let the new kid search for numbers, but when you ask for elasticity,
do
>you mean Elongation at Break or Tensile Modulus? Also, density and
Poisson's
>ratio are two different things. :-o Joe