8.2.2: When do atomic bonds break?

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Fracture is the separation of atoms. We normally do this by applying a force to the body. How big a force should we need?

We want to know how the energy, U, changes with distance, r, between two atoms. A commonly used expression (known as the Lennard-Jones potential is

\[U(r)=4 \varepsilon\left[\left(\frac{\beta^{12}}{r^{12}}\right)-\left(\frac{\beta^{6}}{r^{6}}\right)\right]\]

Other potentials exist, but the argument is essentially the same.

This contains a short range repulsive term, and a long range attractive term. The parameter ε is a measure of the depth of the potential well, and β is the non-infinite distance where the interparticle potential equals zero.

Use the animation to see how the energy changes as the distance between two atoms is varied.

The force between a pair of atoms is calculated by taking the derivative of the energy function, giving:

\[\boldsymbol{F}=\frac{\mathrm{d} U(\mathrm{r})}{\mathrm{d} r}=24 \varepsilon\left[-2\left(\frac{\beta^{12}}{r^{13}}\right)+\left(\frac{\beta^{6}}{r^{7}}\right)\right]\]

Now see how the force between the atoms changes with distance. What is the force needed to separate the atoms in a crystal?

Because we are interested in the material’s properties we normally think of failure stresses, so in a unit area of crystal we need to know how many bonds in an area of crystal normal to the applied force are carrying the load, say 6.25 × 10⁻²² m².

From the animation it is clear that materials should have breaking stresses of the order of 10 GPa, but a piece of glass normally breaks at a stress of 70 MPa.

Where have we gone wrong?

What have we just estimated? It is the stress required to break the bonds on a given plane simultaneously, what becomes the fracture surface. So do all the bonds on a given plane break at once?

Anybody who has opened a packet of crisps or torn a sheet of paper knows the answer is ‘no’ – the bonds break a few at a time – at the tip of a growing crack.

So the analysis above does not describe what actually happens when a material breaks, which is a relief as the predictions were no good anyway.