The beginnings of solid state physics - Early work on imperfections in crystals, and forerunners of dislocation theory

In his 1913 contribution to the Encyclopädie der mathematischen Wissenschaften entitled ‘Physikalische Grundlagen der Festigkeitslehre’, written in collaboration with L. Föppl, T. v. Kármán(1) reviewed the processes resulting in the permanent deformation of crystalline materials, i.e. twinning (‘einfache Schiebung’) and glide (‘Translation’), from the molecular point of view (‘Raumgitterhypothese’). In the last chapter, v. Kármán treats the phenomena of mechanical hysteresis and mechanical aftereffects in crystals and crystalline aggregates. He states clearly th at in ‘homogeneous’ crystals (in modern language, in perfect crystals) dissipation of mechanical energy is possible only through the processes resulting in heat conduction but that the existence of ‘ Trennungsflachen ’ may lead to mechanical hysteresis and, in connection with thermal fluctuation, to aftereffects and hence to internal friction other than that associated with heat conductivity. This thinking, which from the modern viewpoint must be considered as essentially correct if we understand by ‘ Trennungsfla chen ’ not only grain boundaries but any extended perturbation of the ideal crystal structure,')' was mainly based on a molecular model proposed by Prandtl but not published until 1928.(2)