Abstract
In the modeling of solids the free energy, the energy, and the entropy play a central role. We show that the free entropy, which is defined as the negative of the free energy divided by the temperature, is similarly important. The derivatives of the free energy are suitable thermodynamical driving forces for reversible (i.e. Hamiltonian) parts of the dynamics, while for the dissipative parts the derivatives of the free entropy are the correct driving forces. This difference does not matter for isothermal cases nor for local materials, but it is relevant in the non-isothermal case if the densities also depend on gradients, as is the case in gradient thermoplasticity. Using the total entropy as a driving functional, we develop gradient structures for quasistatic thermoplasticity, which again features the role of the free entropy. The big advantage of the gradient structure is the possibility of deriving time-incremental minimization procedures, where the entropy-production potential minus the total entropy is minimized with respect to the internal variables and the temperature. We also highlight that the usage of an auxiliary temperature as an integrating factor in [30] serves exactly the purpose to transform the reversible driving forces, obtained from the free energy, into the needed irreversible driving forces, which should have been derived from the free entropy. This reconfirms the fact that only the usage of the free entropy as driving functional for dissipative processes allows us to derive a proper variational formulation.
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Acknowledgments
The author is grateful to Michael Ortiz for many stimulating discussions on the interaction of mathematics and mechanics. Moreover, he thanks Ulisse Stefanelli for the fruitful collaboration concerning variational methods for evolutionary systems. The resarch was partially supported by the European Research Council via the grant ERC-2010-AdG no.267802 Analysis of Multiscale Systems Driven by Functionals.
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Mielke, A. (2016). Free Energy, Free Entropy, and a Gradient Structure for Thermoplasticity. In: Weinberg, K., Pandolfi, A. (eds) Innovative Numerical Approaches for Multi-Field and Multi-Scale Problems. Lecture Notes in Applied and Computational Mechanics, vol 81. Springer, Cham. https://doi.org/10.1007/978-3-319-39022-2_7
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DOI: https://doi.org/10.1007/978-3-319-39022-2_7
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