Non-Equilibrium Thermal/Mechanical Behaviour

Summary

A solid undergoes nonequilibrium changes when disturbed, a phenomenon that occurs at both the microscopic and macroscopic scale level. These changes depend on the rate at which the process takes place and cannot be described in terms of physical parameters representing the system as a whole. Their description involves synchronizing the thermal fluctuation with mechanical motion of the material at each location and time.

The concept of an isoenergy density space is introduced such that the energy state in a volume element can be characterized by the time history of only one stress and one strain component regardless of whether the element is in a uniaxial or multiaxial state of stress. A unique correspondence between the uniaxial and multiaxial stress state can thus be established without loss in generality. Unlike the classical approaches in continuum mechanics, the response of an isoenergy density element requires the simultaneous specification of element size, time and temperature. Mutual interaction of size/time/temperature cannot be ignored for nonequilibrium states because temperature averaged over a macroelement within a given time interval can differ qualitatively and quantitatively from that over a microelement and different time interval. The same applies to mechanical motion.

As an example, data for the nonequilibrium thermal/mechanical response of an aluminum cylindrical bar specimen stretched at a displacement rate of 1.27 × 10^-4 cm/sec will be presented. A two-phase medium is considered such that the conditions at the solid/air interface are allowed to chanqe with time rather than specified or ignored as in the classical treatment of equilibrium processes. Thermal/mechanical disturbances are found to prevail in the surrounding of the specimen; they diminished at distances more than one-half the specimen length. The response is transitory in character for the entire load history and depends on the initial thermal environment next to the specimen as well as the rate of the applied external disturbance.