The breaking strength of a real solid is not determined directly by the interatomic forces but by the properties of the imperfections that the solid contains. To be very strong a solid must resist both pltlstic yielding and brittle cracking. One material cannot normally resist both types of failure. Strong metals are designed to resist plastic yielding. Modern strong composite materials can be devised, in which brittle substances are used and a fibrous microstructure designed to arrest crack propagation.