We discuss a completely forgotten work of the geologist G.F. Becker on the ideal isotropic nonlinear stress-strain function [8]. In doing this we provide the original paper from 1893 newly typeset in L A T E X and with corrections of typographical errors as well as an updated notation. Due to the fact that the mathematical modelling of elastic deformations has evolved greatly since the original publication we give a modern reinterpretation of Becker's work, combining his approach with the current framework of the theory of nonlinear elasticity.Interestingly, Becker introduces a multiaxial constitutive law incorporating the logarithmic strain tensor, more than 35 years before the quadratic Hencky strain energy was introduced by Heinrich Hencky in 1929. Becker's deduction is purely axiomatic in nature. He considers the finite strain response to applied shear stresses and spherical stresses, formulated in terms of the principal strains and stresses, and postulates a principle of superposition for principal forces which leads, in a straightforward way, to a unique invertible constitutive relation, which in today's notation can be written aswhere T Biot is the Biot stress tensor, log(U ) is the principal matrix logarithm of the right Biot stretch tensor U = √ F T F , tr X = 3 i=1 X i,i denotes the trace and dev 3 X = X − 1 3 tr(X) · 11 denotes the deviatoric part of a matrix X ∈ R 3×3 . Here, G is the shear modulus and K is the bulk modulus. For Poisson's number ν = 0 the formulation is hyperelastic and the corresponding strain energyBecker (U ) = 2 G [ < U, log(U ) − 11 > +3 ] has the form of the maximum entropy function.
B Notation 70
References 71The Finite Elastic Stress-Strain Function by G.F. Becker 76 3 1 Introduction
Some reflections on constitutive assumptions in nonlinear elasticityThe question of proper constitutive assumptions in nonlinear elasticity has puzzled many generations of researchers. The problem of finding simple enough constitutive assumptions which are sufficient to characterize a physically plausible behaviour of "completely elastic" materials was even called "das ungelöste Hauptproblem der endlichen Elastizitätstheorie" (the unsolved main problem of finite elasticity theory) by C. Truesdell [79]. While such assumptions can only lead to an idealized material behaviour, the merits of such an ideal model were already described by H. Hencky in his 1928 article On the form of the law of elasticity for ideally elastic materials [31,55]:
Like so many mathematical and geometric concepts, it is a useful ideal, because once its deducible properties are known it can be used as a comparative rule for assessing the actual elastic behaviour of physical bodies. [. . . ] While it is certainly a matter of empirical observation to determine how actual materials compare to the ideally elastic body, the law itself acts as a measuring instrument which is extended into the realm of the intellect, making it possible for the experimental researcher to make systematic observations.The range of applicabili...