The package provides information needed to set up and present a short practical demonstration in a lecture. This LDP (Lecture Demonstration Package) is centred on tin cry phenomenon, the characteristic sound heard when a bar of tin is bent. Variously described as a "screaming" or "crackling" sound, the effect is caused by the crystal twinning in the metal. The sound is not particularly loud, despite terms like "crying" and "screaming". Although the cry is most typical of tin, a similar effect occurs in other metals, such as niobium, indium, zinc and gallium.
Twinning is common in structures of low crystallographic symmetry. They tend to have fewer independent slip systems, making it less likely that an imposed strain can be fully accommodated by dislocation glide. For example, hexagonal metals are much more prone to deformation twinning than cubic metals. Similarly, deformation of tin (tetragonal) occurs predominantly by twinning, causing the so-called “tin cry” (acoustic waves generated by twin nucleation and growth).
Three points are illustrated:
• The nature of the stress field in the wall of a pressurised thin-walled tube
• Fast fracture under conditions such that crack propagation is energetically favoured - ie there is a net release of energy
• By inhibiting the relaxation of regions around a crack, the energy balance can be altered such that fact fracture does not occur (but stable crack growth does)