Single theoretical strength

PLASTIC DEFORMATION. When a metal or other solid is plastically deformed it suffers a permanent change of shape. The theory of plastic deformation in crystalline solids such as metals is complicated but well advanced. Metals are unique among solids in their ability to undergo severe plastic deformation. The observed yield stresses of single crystals are often 10 4 times smaller than the theoretical strengths of perfect crystals. The fact that actual metal crystals are so easily deformed has been attributed to the presence of lattice defects inside the crystals. The most important type of defect is the dislocation. See also Creep (Metals) Crystal and Hot Working. 

Whisker. Tiny, whisker-like fiber (a few mm long, a few p.m in diameter) that is a single crystal and almost free of dislocations. Note that this term involves a material requirement. The small size and crystalline perfection make whiskers extremely strong, approaching the theoretical strength. 

For the theoretical strength, the stress required to cause decohesion in a perfect single crystal can be assessed by... 

This chapter is concerned with stiffness, that is with elastic behaviour in the small strain region. There are a number of reasons for this. One is that stiffness is a very important property in engineering design, which is often limited by a permitted deflection. Furthermore, as Vincent has pointed out one cannot expect to reach the theoretical strength before reaching the theoretical modulus. However, the most important reason is that it is much easier to relate stiffness to molecular structure than it is to relate the other more complicated mechanical properties. For that reason, stiffness is the most informative single mechanical measurement one can make on a material. 

A perfect single crystal undergoes only elastic deformations up to the failure. The experimental yield stress is inferior to the theoretical strength of materials. This can be explained only by the existence of the crystal defects and the model takes it into account by including i.s.v. 

---