We investigate a family of isotropic volumetric-isochoric decoupled strain energies
$$\begin{aligned} F\mapsto W_{\mathrm{eH}}(F):=\widehat{W}_{\mathrm{eH}}(U):=\left \{ \begin{array}{l@{\quad}l} \frac{\mu}{k} e^{k\|\operatorname {dev}_n\log{U}\|^2}+\frac{\kappa}{{2 {\widehat {k}}}} e^{\widehat{k}[\operatorname {tr}(\log U)]^2}&\text{if}\ \det F>0,\\ +\infty&\text{if}\ \det F\leq0, \end{array} \right . \end{aligned}$$
based on the Hencky-logarithmic (true, natural) strain tensor log
U, where
μ>0 is the infinitesimal shear modulus,
\(\kappa=\frac{2\mu+3\lambda}{3}>0\) is the infinitesimal bulk modulus with
λ the first Lamé constant,
\(k,\widehat{k}\) are additional dimensionless material parameters,
F=∇
φ is the gradient of deformation,
\(U=\sqrt{F^{T} F}\) is the right stretch tensor and
is the
n-dimensional deviatoric part of the strain tensor log
U. For small elastic strains,
W
eH approximates the classical quadratic Hencky strain energy
$$\begin{aligned} F\mapsto W_{\mathrm{H}}(F):=\widehat{W}_{\mathrm{H}}(U)&:={\mu}\| \operatorname {dev}_n\log U\|^2+\frac{\kappa}{2}\bigl[\operatorname{tr}( \log U)\bigr]^2, \end{aligned}$$
which is not everywhere rank-one convex. In plane elastostatics, i.e.,
n=2, we prove the everywhere rank-one convexity of the proposed family
W
eH, for
\(k\geq\frac{1}{4}\) and
\(\widehat{k}\geq\frac{1}{8}\). Moreover, we show that the corresponding Cauchy (true)-stress-true-strain relation is invertible for
n=2,3 and we show the monotonicity of the Cauchy (true) stress tensor as a function of the true strain tensor in a domain of bounded distortions. We also prove that the rank-one convexity of the energies belonging to the family
W
eH is not preserved in dimension
n=3 and that the energies
$$\begin{aligned} F\mapsto\frac{\mu}{k}e^{k\|\log U\|^2},\qquad F\mapsto\frac{\mu }{k}e^{\frac{k}{\mu} \bigl(\mu\|\operatorname {dev}_n\log U\|^2+\frac{\kappa}{2}[\operatorname {tr}(\log U)]^2 \bigr)}, \quad F \in\mathrm{GL}^+(n), n\in \mathbb {N}, n\geq2 \end{aligned}$$
are also not rank-one convex.