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Dynamics and fragmentation of fragile bent rods (the spaghetti enigma) joint work with Basile Audoly
If you take a spaghetti stick and try to break it, it will most of the time break in more than two pieces. See video here (RealMedia, 414Ko). Why not just two pieces ?
In the following experiment, a fragile elastic rod is held in a simply supported way and bent with compressive forces. The curvature is maximal at the middle of the rod, and that is where we anticipate the rod to break.
See a video of a typical classical experiment here (animated GIF, 300ko).
Most of the time, the rod undergoes several fractures. It takes a micro-second (10E-6 sec) for the fracture to travel across the section of the rod. Hence the probability that all the fractures happen in the same micro-second is very low. This leads us to make the hypothesis that a first facture happens (at the point of maximum curvature (middle point in the classical experiment)) and is then followed by another one, that will induce a third one, etc.
In order to check the above hypothesis, we focus on what happens to the right part of the rod just after time t=tA. This boils down to studying the dynamics of a rod that is (strongly) bent and then released: a catapult.
The fact that the rod breaks after it is set free implies that the curvature is increased, at least locally and temporarily. See a video of a typical catapult experiment here (animated GIF, 200ko).
A Spaghetti stick is a wonderful experimental model of a brittle elastic rod. We could have used teflon bars or fiber glass rods but it would have been much more expensive to do the experiments (we broke few hundreds of spaghetti).
A lot of different structures in civil, aerospatial or bio engineering are subject to fracture (concrete or brickwork buildings, wings of aeoroplane, femur bone, etc.). After an impact or due to excess loading, they will all vibrate and break. What our work says is that the first facture will most probabily be followed by secondary (induced) fractures, leading to the fragmentation of the structure.
This work has been published.
Visit our other spaghetti page (more movies and explanations).
Andrew Belmonte & Emmanuel Villermaux experiments: here
Falk Wittel & Hans Herrmann egg shell fragmentation computations and experiments: here
Pole vault breaking: here
Brian Skelcher video on the catapult experiment: here