The equations that govern the thermomechanics of a solid include the balance laws for mass, momentum, and energy. Kinematic equations and constitutive relations are needed to complete the system of equations. Physical restrictions on the form of the constitutive relations are imposed by an entropy inequality that expresses the second law of thermodynamics in mathematical form.
The balance laws express the idea that the rate of change of a quantity (mass, momentum, energy) in a volume must arise from three causes:
the physical quantity itself flows through the surface that bounds the volume,
there is a source of the physical quantity on the surface of the volume, or/and,
there is a source of the physical quantity inside the volume.
Let be the body (an open subset of Euclidean space) and let be its surface (the boundary of ).
Let the motion of material points in the body be described by the map
where is the position of a point in the initial configuration and is the location of the same point in the deformed configuration.
Recall that the deformation gradient () is given by
Let be a physical quantity that is flowing through the body. Let be sources on the surface of the body and let be sources inside the body. Let be the outward unit normal to the surface . Let be the velocity of the physical particles that carry the physical quantity that is flowing. Also, let the speed at which the bounding surface is moving be (in the direction ).
Then, balance laws can be expressed in the general form
Note that the functions , , and can be scalar valued, vector valued, or tensor valued - depending on the physical quantity that the balance equation deals with.
It can be shown that the balance laws of mass, momentum, and energy can be written as
In the above equations is the mass density (current), is the material time derivative of , is the particle velocity, is the material time derivative of , is the Cauchy stress tensor, is the body force density, is the internal energy per unit mass, is the material time derivative of , is the heat flux vector, and is an energy source per unit mass.
With respect to the reference configuration, the balance laws can be written as
The Clausius-Duhem inequality can be used to express the second law of thermodynamics for elastic-plastic materials. This inequality is a statement concerning the irreversibility of natural processes, especially when energy dissipation is involved.
Just like in the balance laws in the previous section, we assume that there is a flux of a quantity, a source of the quantity, and an internal density of the quantity per unit mass. The quantity of interest in this case is the entropy. Thus, we assume that there is an entropy flux, an entropy source, and an internal entropy density per unit mass () in the region of interest.
Let be such a region and let be its boundary. Then the second law of thermodynamics states that the rate of increase of in this region is greater than or equal to the sum of that supplied to (as a flux or from internal sources) and the change of the internal entropy density due to material flowing in and out of the region.
Let move with a velocity and let particles inside have velocities . Let be the unit outward normal to the surface . Let be the density of matter in the region, be the entropy flux at the surface, and be the entropy source per unit mass.
Then the entropy inequality may be written as
The scalar entropy flux can be related to the vector flux at the surface by the relation . Under the assumption of incrementally isothermal conditions, we have
where is the heat flux vector, is a energy source per unit mass, and is the absolute temperature of a material point at at time .
We then have the Clausius-Duhem inequality in integral form:
We can show that the entropy inequality may be written in differential form as
In terms of the Cauchy stress and the internal energy, the Clausius-Duhem inequality may be written as