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Coldness, Hardness and Breakage
Name: Deanna
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: April 2003
Question:
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?
Replies:
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
Deanna,
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.
Regards,
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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Update: June 2012
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