Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Fuel flow to shuttle main engines...
Date: Fri, 12 Mar 1999 03:40:54 GMT

In article <36E863A3.2CF080E3@home.com>, Bruce P. Dunn <bpdunn@home.com> wrote:
>...Some metals,
>such as most conventional steels, become extremely brittle at cryogenic
>temperatures, and can't be used as they have little or no resistance to
>the propagation of cracks. Note that this is not the same thing as
>"strength"; a material can be strong (resist separating when pulled),
>but relatively brittle.

Indeed, it turns out that things like carbon steel become brittle because
they get *stronger* at cryogenic temperatures.

There are two different kinds of "strength".  "Ultimate strength" is how
much of a load the stuff will take before it breaks.  "Yield strength" is
how much it will take before it starts to yield, i.e. deform irreversibly
(the deformation won't go away if the load is removed).

Metals generally have ultimate strengths much higher than yield strengths.
That's good, because it means that in response to concentrated load, the
most highly-loaded portion will yield a bit, which often spreads the load
out more and eliminates the problem.  This is very important, because real
structures always contain small flaws which concentrate loads.  (Indeed,
for practical structural materials, flaw tolerance turns out to be far
more important than strength.)

As temperature drops, both kinds of strength *increase*.  However, they
typically don't increase at the same rate.  If yield strength grows more
quickly than ultimate strength, then eventually the two curves cross over,
and below the crossover temperature, the metal fails before it yields,
i.e. it has become brittle.

(This doesn't necessarily always happen at very low temperatures, either.
The steel originally used in the mass-produced WW2 "Liberty ships" could
become brittle at ordinary North Atlantic winter temperatures.)

This sort of brittle transition is actually not very common; relatively
few metals experience it.  The reason it's such a prominent issue in
cryogenics is simply that we are so fond of carbon steel as an engineering
material, and carbon steel happens to be one of those few.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)