Classes of Ephemeral Continua

The qualifier `ephemeral' was proposed for continuous models of bodies, such as gases, for which the generally tacit axiom of permanence of material elements fails to apply. Consequently, to their scrutiny, a Eulerian (local) approach is mandatory, such as one adopted, e.g., in molecular dynamics. Within the scheme of ephemeral continua we discuss here three essential subclasses of bodies: (1) those undergoing energy-preserving processes (in this sense hyperelastic), (2) hypoelastic bodies inspired by a type proposed by C. Truesdell and (3) a number of minor ones. We re-examine the essential issues of the general format focusing on the proposal of appropriate concepts of strains and strainings.