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Determining Material Strength
Name: Chad B.
Status: student
Age: 30s
Location: N/A
Country: N/A
Date: 10/31/2004
Question:
I read that "pound for pound" wood is stronger than steel
in http://www.newton.dep.anl.gov/askasci/gen99/gen99707.htm
My co-workers (one being a hobby-metallurgist) find this a fact in dispute
and are not able to wrap their minds around how this could be true.
What is used to determine the "Strength" of a material? How is it that
"pound for pound" wood is stronger than steel? Their primary examples
include why steel is used in skyscrapers and wood is not - the same with
Naval warships.
Replies:
You have identified a key issue in the question -- specifically, what do
you mean by the "strength" of a material. Tension is the force associated
with the stretching of a material. Compression is the force associated with
the compressing of a material. Shear is the force associated with the
sliding of one section of a material against another. Torsion is the force
associated with the twisting of a material. In each case the direction of
the applied force may produce movement of the material in directions other
than the direction of the applied force. So, even formulating the question,
"What is strength?" becomes mathematically intricate. Young's modulus is
one of the simplest measures of elastic strength for small forces with
deformations only in the direction. The following web site compares values
for various materials. It looks like steel wins over wood:
http://www.engineeringtoolbox.com/24_417.html
Vince Calder
"Strength" is most commonly taken to be the stress required to permanently
bend or break a material. (Stress is force divided by the area of the test
object. ) Units are pounds-per-square-inch (in English system) or Pascals
(in metric system).
Typically you push or pull along the length of a bar of material with a
testing machine. Rarely is the testing machine used to bend a bar sideways.
In metals, the "yield" strength is the stress required to make the metal
permanently bend a small amount. At lower stresses, the metal bends, but it
is elastic like a piece of rubber. But at the "yield" stress, there is
permanent deformation. The "tensile" strength is the stress required to
make the metal actually break in tension, if you keep pulling.
Ceramics, glass, and other brittle materials can have large compressive
strength, but often small tensile strength. That is, if you squeeze the
material is it strong and does not break, but if you pull on the material,
it breaks easier. This is because microscopic surface cracks open up when
the object is pulled, and the object cracks apart. In compression, the
crack stays closed. The strength of a metal is about equal for compression
or tension.
The "toughness" of a material is the amount of energy required to break it.
Metals are tough because the bend a lot before breaking. Ceramics might be
strong, but they are not "tough" because they don't bend much before
breaking.
Metals are just about equally strong no matter which way you bend or push on
them. As mentioned, ceramics are strong in compression but weak in tension.
Composites, like plywood, fiberglass, carbon fiber composites (used in race
cars), and other materials, can have strength that is very different
depending on which way you stress them.
I have never heard that wood is stronger than steel. Nor are spider webs
stronger than good steel. The same goes for Kevlar and other polymers.
Bob Erck
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