MadSci Network: Physics

Re: What would happen colliding stretched nanotube with meteorite?

Date: Fri Feb 27 15:38:29 2004
Posted By: Joseph Weeks, Engineer
Area of science: Physics
ID: 1073511979.Ph

Carbon nanotubes have a number of interesting properties.  At: you can find a good summary 
of nanotube properties.

Your question doubtless has to do with the fact that nanotubes appear to 
have very high tensile strength and high elastic modulus.  Their maximum 
tensile strength is thought to be about 30 GPa, about 10 times higher 
than any other material.  So, at first glance, it might make sense that 
since the tensile strenght is so high, it might be able to slice through 
any softer object, like a nickel/iron meteor.  The US Government, for 
example, is looking into using nanotubes for ballistic protection.  See: 

There are a range of complex issues involved in your question.  First, 
let's say that you have a nanotube wire (or any wire) stretched between 
two supports, and you drop a ball of clay on the wire.  As the clay hits 
the wire, the weight of the clay distributes itself along the wire.  So 
the first question is whether the wire is strong enough to support the 
weight of the clay?  I suspect that you can use regular physics to 
calculate the forces on the wire.  Nanotubes are very strong, but they 
are also very tiny, some only 1 nM in diameter.  So an individual 
nanotube can only support a very tiny force.  Obviously, if you have a 
big enough bundle of nanotubes, you can support the weight of the clay.

The next question has to do with toughness and impact speed.  Let's again 
say that you impact a nanotube wire with a ball of clay.  If the clay is 
traveling slowly, the force of the clay against the wire will be 
gradually and uniformly distributed across all of the nanotubes.  If the 
clay is moving pretty fast, the force of the clay will not be distributed 
uniformly, but will load the nanotubes in front part of the wire first, 
while the back of the wire is not providing any support.  Nanotubes in 
addition to having high strength are very, very stiff (high elastic 
modulus).  In other words, they don't stretch very much.  So, the 
nanotubes in the front of the wire are going to tend to break before the 
nanotubes in back have even started to be loaded.

Two other considerations before the nanotube wire is going to be able to 
cut through the meteor.  First, erosion of the wire by the meteor.  There 
doesn't seem to be a lot of information about the hardness of nanotubes; 
maybe they are close to the hardness of diamond.  Nevertheless, the 
meteor is going to be grinding pretty hard against the nanotube wire.  
Unless the nanotube wire is very hard, the meteor is going to grind away 
at the wire, weakening it as it attempts to pull through the meteor.  
Which brings me to the final consideration.  If the nanotube wire is 
going to cut through the meteor, it will generate a lot of heat due to 
friction.  The wire is going to get hot; then the nickel and iron will 
chemically react with the carbon.  We can look to diamond cutting tools 
to verify this phenomena.  If you have a diamond tipped sawblade cutting 
through a bunch of concrete, it can perform very well.  If, however, 
there is a significant amount of steel reinforcing bar in the concrete, 
the saw blade will quickly wear away, even though the steel is softer 
than the concrete.  That is why cubic boron nitride is used in place of 
diamond when cutting stee.  So, when cutting an iron-nickel meteor, the 
carbon nanotube will simply dissolve away.

Summarizing, because of potentially low impact resistance and chemical 
interaction, my money is on the meteor breaking through the nanotube 
wire.  Even so, it is an interesting thing to think about.

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