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Hooke's law, which is the foundation of the mathematical theory of elasticity, is unfit to describe satisfactorily the phenomena in elastic bodies even if we limit the scope of our research to ideal elastic deformations. We call a deformation ideally elastic, if the deformations disappear completely and if the energy stored up in the body is given back without loss, when the load is removed.
H. Hencky (Wed,) studied this question.