Читать книгу Properties for Design of Composite Structures - Neil McCartney - Страница 23

The balance equations of mass, momentum and energy introduce a variety of physical variables, but they do not involve in any way the properties of materials. An equation of state must now be introduced, implying the existence of a variety of material properties, depending upon the complexity of the material. If elastic effects are to be included in models, then it is through an equation of state that they must be introduced. Before using strain as a thermodynamic state variable, it is useful first to introduce some key thermodynamic relationships that can be applied when the stress field is hydrostatic, i.e. shear stresses are absent, characterised by a pressure p, such that the stress tensor has the form σ=−pI where I is the second-order unit tensor.