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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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