20 metres beneath the mortal world lies the Paris Catacombs.
Here, in the Crypt of the Passion this column of bones helps to stop the roof from caving in.
With regular building materials becoming harder to source and more expensive, could bone be
the next big thing for the construction industry?
In the late 1700s Paris’ graveyards were overflowing, often into neighbouring basements.
For two years Parisian authorities moved skeletons of the dead into old abandoned quarries, so
today these the remains of over 6 million people line these tunnels.
There are two forces to consider when selecting an engineering material.
Tension and compression.
Lets leave the dead to rest and head back above ground to see how these work.
This is the Eiffel Tower and a key example of our first force; tension.
If you don’t fancy climbing the 674 steps to the top of the tower, then you’re going
to have to take the elevator.
The passenger cabin is placed on top of a platform which is attached to a series of cables.
Tension in these cables pulls elevator up through the legs of the tower.
Tension force stretches the cables, making them longer.
By measuring how much stretching occurs, we can get a good gauge on whether the cable
is about to break or not.
Welcome back to the Leeds laboratory.
This Tinus Olsen machine is what we use to measure material stretch; more formally known
as strain.
Strain is calculated by dividing the change in length over the original length.
Lets try it out using this steel rod.
We begin by placing it sample between these two grips.
The machine is now going to pull these grips apart; applying tension to the steel in the middle.
As we add more and more of this tension, the steel begins to stretch.
Originally 64 millimetres long, the sample has now stretched to a length of 66 millimetres.
This extra 2 millimetres over the original 64 millimetres in length gives us a strain
value of just over 3 percent.
At the moment our rod has only undergone elastic deformation.
This means that, when we remove it from the machine, it reverts back to its original length.
As you can see, by comparing it with an un-stretched sample, its impossible to tell which is which
by just looking at which one is longer.
If we add enough tension, then this stretch stays put.
Its undergone something called plastic deformation.
As you can see by comparing it to an un-streatched sample, it’s become much longer.
You don’t really want that for things like elevator cables, but take note; if you did
this with the bones in your leg then you would actually get taller.
Before you go running off to find your nearest Tinus Olsen machine to add an extra few centimetres
of height, be careful of what happens at the next stage we add when we add in even more tension.
These massive Gothic towers make up the iconic western facade of the Notre Dame Cathedral.
Each of these towers weigh in at over 9 thousand tonnes, putting the limestone base under a
huge amount of compressive force.
Compression is the force that pushes on an object; squeezing it together.
To help understand stress we’re going to use this Instrom machine.
Just like the Tinus Olsen machine from earlier we put the sample between these two plates.
Unlike the machine from earlier, these plates are now going to push the sample together.
Its obvious that a thicker rod can support more force than a thinner one, but that doesn’t
tell us anything about the material itself.
A much better thing to use is stress.
To put all samples on the same playing field we take the force and divide it by the sample area.
The Notre Dame towers exert 19 million newtons of force onto an area of 250 metres squared.
This gives a stress of 600 kilopascals.
Lets see how the steel rod fairs.
As we add more and more force, it goes through some very similar regions as our tension test
from earlier.
First elastic deformation where the rod will go back to its original shape.
Then crumpling – essentially equivalent to plastic deformation.
And finally failure.
Since tension and compression are both measured using stress and strain we can plot them all
of the same graph.
This is whats known as a stress-strain curve.
Here is the tension bit of the curve, you can see the key regions.
Here we have linear, elastic deformation.
Next we have the curvy plastic, and finally this end point is where failure occurs.
We have some very similar structures for the compression section.
We’ve got our elastic deformation, barrelling and finally breakage.
Since, as we recall, stress is measured in force per unit area, and strain is percentage
change in length; we can use these graphs for wildly different projects.
Which is useful, because that’s what we’re doing next.
Today we’ll be testing the properties of steel, concrete and bone.
Steel and concrete are widley used in the construction industry.
If we ae to truly build ith bone, its got to be able to compete with materials that
are actually being used.
Unfortunately no one I talked to was going to let me borrow an actual human bone in
order to do these experiments.
But this cow knuckle bone is going to do just fine.
We know the theory, lets do some tests.
We now have a stress-strain curve from each of the tests.
Performing the best is steel.
It has excellent tensile strength and very good compressive strength.
Surprisingly concrete didn’t fare too well.
It’s pretty bad on tension and only slightly better in compression.
great in either tension or compression.
Finally we have bone.
It did well in the tension tests as well as in the compression.
Converting our results into more familiar units, in compression our bone can support
3.5 tonnes of weight. Thats the equivalent of almost 4 small cars!
That’s pretty amazing.
Can we build with bone? Yes.
Should we though? Perhaps not.
However, it is interesting to note that a group of researchers, by copying the internal structure
of bone were able to create a steel that was lighter, stronger and more resistant to wear
than the regular variety.
We may be building with bone sooner than you might have thought.
Happy Halloween and a huge thank you to everyone who helped make this video possible and, in particular,
to Bob, Diane and Tony for for lettung us use the lab.
We’re producing new episodes of Engineering Europe every two weeks.
Next episode we're off to Austria..
Until then this has been James Dingley from the Atomic Frontier.
Keep Looking Up.