The internal energy of a system is subdivided into a work or potential function and a thermal or kinetic function, the former expressed in terms of the current electrostatic theory of intercrystalline bonding, and these functions then examined for variations of temperature, hydrostatic pressure, unidirectional stress and combined hydrostatic and unidirectional pressure. From these considerations a theory is evolved which not only seems satisfactorily to explain and correlate phenomena of deformation, creep or plastic flow, cold working, elastic after‐working, rupture, shear and certain other phenomena hitherto described as ``anomalous'' effects but has been corroborated experimentally in some of its predictions, in particular for the effect of hydrostatic pressure on deformation and compressive strength. The mechanism evolved consists of two processes—one an elastic deformation which is a function of the strain or potential energy of the system. Failure occurs here by ``brittle'' rupture wherein the maximum extension or maximum internal tension is the criterion. The other is a deformation by means of a two‐phase transfer mechanism and is a function of the thermodynamic potential relations of the system. This latter type is also a function of time and therefore a function of the rate of application of load. When both processes of this mechanism are operative failure occurs by shear; the criterion for this type of failure is given by a function of time, the strain or potential energy and the thermodynamic potential relations of the system. Expressions are derived for creep or plastic flow of polycrystalline substances from the thermodynamic potential relations which not only satisfy the well‐known phenomena of creep in metals but also express recent empirical creep data of some substances immersed in liquids in which they are somewhat soluble. An expression is also derived for the ``brittle'' potential type of rupture under combined thrust and hydrostatic pressure.
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