Abstract
The strain energy Σ for Hadamard materials takes the form Σ = aII + bI + f(III). Here a and b are constants, f an arbitrary function, and I, II, III are the principal invariants of the left Cauchy strain tensor. It is shown that if the Ordered Forces condition and the Strong Ellipticity condition are to be satisfied then a = 0, b > 0,f′(III) ≥ 0 and (III½f′)′ ≥ 0.

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