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Name: Deanna 
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: April 2003

I was reading one of the answered questions in your archives, and it said that the colder something is, the more likely it is to break. It was an experiment in which a hot dog was dipped into liquid nitrogen. Is it the same to say that the harder something is, the easier it breaks?

The more rigid something is, yes, the more likely it is to break instead of bend. But it is over-simplifying things to just say that harder things break more easily. For instance, while it is true that hard cast iron breaks more easily than softer steel, the steel will not break as easily as much softer wood. The wood might bend more before it breaks, but it takes more energy to break the steel.

It turns out that learning how materials break is an active area of current research, with obviously important and useful applications

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois


I do not believe that this is the case for all materials. I have seen the same demonstrations. When a material is lowered into the liquid nitrogen it gets cooled to a temperature that is below its particular T(db). T(db) is the ductile to brittle transition temperature. This magical little number will tell you the temperature at which a material will become brittle (if being cooled from the Tdb) or become ductile (if being warmed from the Tdb).

You may not make the generalization that all hard things are easier to break. Take for example carbon fiber. I believe it is carbon fiber woven together and bound with some sort of epoxy. This material once heated / cured is just ridiculously hard. And I am fairly certain that this material is very very hard to break as well.

Darin Wagner

The strength of a material and its hardness are not the same thing. Some materials -- like the hot dog or a flower -- cooled to the boiling point of liquid nitrogen 77 kelvins or about -196 C. are hard but brittle -- that is they do not have much strength. On the other hand, a material like leather is quite soft, but it is very strong and tough. Materials that cannot "absorb" some mechanical stress may be quite hard but break very easily -- glass for example. This is quite an oversimplification, because the strength also depends upon impurities, and dislocations in the material too.

Vince Calder

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