Isotropic Compressible Material Research Articles

Understanding the need of the compression branch to characterize hyperelastic materials

Soft biological tissues are frequently modeled as hyperelastic materials. Hyperelastic behavior is typically ensured by the assumption of a stored energy function with a pre-determined shape. This function depends on some material parameters which are obtained through an optimization algorithm in order to fit experimental data from different tests. For example, when obtaining the material parameters of isotropic, incompressible models, only the extension part of a uniaxial test is frequently taken into consideration. In contrast, spline-based models do not require material parameters to exactly fit the experimental data, but need the compression branch of the curve. This is not a disadvantage because as we explain herein, to properly characterize hyperelastic materials, the compression branch of the uniaxial tests (or valid alternative tests) is also needed, in general. Then, unless we know beforehand the tendency of the compression branch, a material model should not be characterized only with tensile tests. For simplicity, here we address isotropic, incompressible materials which use the Valanis-Landel decomposition. However, the concepts are also applicable to compressible isotropic materials and are specially relevant to compressible and incompressible anisotropic materials, because in biomechanics, materials are frequently characterized only by tensile tests.

WYPIWYG hyperelasticity for isotropic, compressible materials

Nowadays the most common approach to model elastic behavior at large strains is through hyperelasticity. Hyperelastic models usually specify the shape of the stored energy function. This shape is modulated by some material parameters that are computed so the predicted stresses best fit the experimental data. Many stored energy functions have been proposed in the literature for isotropic and anisotropic materials, either compressible or incompressible. What-You-Prescribe-Is-What-You-Get (WYPIWYG) formulations present a different approach which may be considered an extension of the infinitesimal framework. The shape of the stored energy is not given beforehand but computed numerically from experimental data solving the equilibrium equations. The models exactly fit the experimental data without any material parameter. WYPIWYG procedures have comparable efficiency in finite element procedures as classical hyperelasticity. In this work we present a WYPIWYG numerical procedure for compressible isotropic materials and we motivate the formulation through an equivalent infinitesimal model.

On constitutive models of finite elasticity with possible zero apparent Poisson’s ratio

The idea in this paper is to build a class of constitutive equations for highly compressible isotropic materials that, among others, are capable to describe a zero apparent Poisson’s ratio in the whole finite strain range, not only for moderate straining. This remarkable property is, for instance, observed in many soft materials with micro-structures such as sponges and polymeric foams with high porosities. It would then be suitable to describe their behavior within a macroscopic modeling framework. More specifically, herein by means of elementary considerations, we deduce adequate forms of strain-energy functions that are a priori decomposed into purely volumetric and volume-preserving parts. A class of compressible hyperelastic materials of the general Odgen type is obtained. It can consequently be specialized, for instance, to neo-Hookean, Mooney–Rivlin, and Varga’s model types as well. Furthermore, for the elastic parameters, a connection with the limiting case of linear elasticity is made whenever possible, in particular with the classical Poisson’s ratio, and with the bulk to shear moduli ratio.

Large displacement stability analysis of elastic–plastic unsymmetrical sandwich cylindrical shells

The paper presents stability analysis of elastic–plastic, free supported sandwich cylindrical shell with unsymmetrical faces, loaded by longitudinal forces, transversal pressure, and shear. The J 2 incremental Prandtl–Reuss plastic flow theory constitutive relations were used in the analysis and it was assumed that geometrical strain-displacement relations are nonlinear ones. It was also assumed that the shell faces have different thicknesses and these are made of different isotropic compressible materials with linear stress-hardening. Active loading processes are accepted in the analysis, the stability equations are derived using strain energy formulation. Ritz method is used to solve the equations and an iterative computational algorithm was elaborated to get numerical results.

Hyperelastic materials behavior modeling using consistent strain energy density functions

Hyperelastic materials have high deformability and nonlinearity in load–deformation behavior. Based on a phenomenological approach, these materials are treated as a continuum, and a strain energy density is considered to describe their hyperelastic behavior. In this paper, the mechanical behavior characterization of these materials is studied from the continuum viewpoint. For this purpose, the strain energy density is expressed as sum of independent functions of the mutual multiple of principal stretches. These functions are determined by applying the governing postulates on the form of the strain energy density. It is observed that a consistent strain energy density is expressible in terms of the mathematical functions of polynomial, power law, logarithmic and particularly exponential. The proposed strain energy density functions cover modeling both of compressible and incompressible materials. Moreover, the material parameters of these models are calculated based on the correlation between the values of the strain energy density (rather than the stresses) cast from the test data and the theory. In order to investigate the appropriateness of the proposed models, several experimental data for incompressible and compressible isotropic materials under homogeneous deformations are examined in which the predictions of the proposed models show a good agreement with experimental data.

Hyperelastic materials modelling using a strain measure consistent with the strain energy postulates

In this article, a strain energy density function of the Saint Venant—Kirchhoff type is expressed in terms of a Lagrangian deformation measure. Applying the governing postulates to the form of the strain energy density, the mathematical expression of this measure is determined. It is observed that this measure, which is consistent with the strain energy postulates, is a strain type with the characteristic function more rational than that of the Seth—Hill strain measures for hyperelastic materials modelling. In addition, the material parameters are calculated using a novel procedure that is based on the correlation between the values of the strain energy density (rather than the stresses) cast from the test data and the theory. In order to evaluate the performance of the proposed model of the strain energy density, some test data of pure homogeneous deformations are used. It is shown that there is a good agreement between the test data and predictions of the model for incompressible and compressible isotropic materials.

Propagation and interaction of non-linear elastic plane waves in soft solids

Using the perturbation method of weakly non-linear asymptotics we analyze the propagation and interaction of elastic plane waves in a model of a soft solid proposed by Hamilton et al. [Separation of compressibility and shear deformation in the elastic energy density, J. Acoust. Soc. Am. 116 (2004) 41–44]. We derive the evolution equations for the wave amplitudes and find analytical formulas for all interaction coefficients of quadratically non-linear interacting waves. We show that in spite of the assumption of almost incompressibility used in Hamilton et al. [Separation of compressibility and shear deformation in the elastic energy density, J. Acoust. Soc. Am. 116 (2004) 41–44], the model behaves essentially like that of a compressible isotropic material. Both the structure of the equations and the interaction patterns are similar. The models differ, however, in the elastic constants that characterize them, and hence the values of the coefficients in the evolution equations and the values of the interaction coefficients differ.

Homogeneous Equilibrium Configurations of a Hyperelastic Compressible Cube under Equitriaxial Dead-Load Tractions

Non-uniqueness, bifurcation and stability of homogeneous solutions to the equilibrium problem of a hyperelastic cube subject to equitriaxial dead-load tractions are investigated. Besides the basic and theoretical questions raised by the analysis, the study is motivated by the somewhat surprising feature of this nonlinear problem for which the symmetric load may give rise to asymmetric stable deformations. In reality, the equilibrium problem, formulated for general homogeneous compressible isotropic materials with polyconvex energy function, may exhibit primary and secondary bifurcations. A primary bifurcation occurs when there exist paths of equilibrium states that bifurcate from the primary path of three equal principal stretches. These bifurcation branches have two coinciding stretches and along them, through secondary bifurcations, other completely asymmetric bifurcation branches, which are characterized by all three stretches different, may risen. In this case, the cube transforms into an oblique parallelepiped. With increasing loads, they are also possible discontinuous paths of equilibria which evince prompt jumps in the deformation process. Of course, the set of asymmetric solutions admitted by the equilibrium problem depends on the specific form of the stored energy function adopted. In this paper, expressions governing the global development of asymmetric equilibrium branches are derived. In particular, conditions to have bifurcation points are individualized. For compressible neo-Hookean and Mooney-Rivlin materials a wide parametric analysis is carried out showing by means of graphs the most interesting branches. Finally, using the energy criterion, a detailed study is performed to assess the stability of the computed solutions.

On stability and non-uniqueness of the dilatation states of a compressible isotropic unit cube subjected to dead loading

This paper considers a unit elastic cube, made of compressible isotropic material, with its faces subjected to certain dead-load tractions that produce a possible equilibrium state of non-uniform dilatation. It is seen that, at the considered equilibrium state, the cube material acquires properties of pseudo-transverse isotropy. Conditions are obtained for the stability of such an equilibrium state with respect to superimposed pure homogeneous deformations having principal directions parallel to the cube edges. The problem of non-uniqueness of the cube dilatation states is also addressed, and non-uniqueness is illustrated in an example application dealing with an isotropic cube made of the Blatz–Ko material. The nature and the stability features of these equilibrium states are studied in depth.

Plane Strain Bending of Cylindrical Sectors of Admissible Compressible Hyperelastic Materials

In the theory of nonlinear elasticity of rubber-like materials, if a homogeneous isotropic compressible material is described by a strain–energy function that is a homogeneous function of the principal stretches, then the equations of equilibrium for axisymmetric deformations reduce to a separable first-order ordinary differential equation. For a particular class of such strain–energy functions, this property is used to obtain a general parametric solution to the equilibrium equation for plane strain bending of cylindrical sectors. Specification of the arbitrary function that appears in such strain–energy functions yields some parametric solutions. In some cases, the parameter can be eliminated to yield closed-form solutions in implicit or explicit form. Other possible forms for the arbitrary constitutive function that are likely to yield such solutions are also indicated.

Implications of Shield’s inverse deformation theorem for compressible finite elasticity

Some properties of the Shield transformation on elastic strain energy functions are established. It is reflexive, it preserves objectivity and material symmetry for isotropic materials, and it also preserves infinitesimal strain response, ellipticity and Hadamard stability, and the Baker–Ericksen condition. Two new classes of strain energies for compressible isotropic materials are introduced, one of them being the image under the Shield transformation of the class of harmonic strain energies. In view of Shield’s Inverse Deformation Theorem, these new classes of strain energies will allow solution in closed form of a variety of problems in finite elastostatics.

A Lie Group Analysis of the Axisymmetric Equations of Finite Elastostatics for Compressible Materials

Many boundary-value problems in nonlinear elastostatics for incompressible isotropic materials have been solved analytically. The situation for compressible materials is quite different. The main difficulty is the absence of the simplified kinematics arising from the incompressibility constraint. Considerable progress in the analytic solution of boundary-value problems for classes of compressible materials has however taken place in recent years and further developments in this direction are considered in this paper. A very special topic in the equilibrium theory for finite deformations of homogeneous isotropic compressible materials is investigated, namely that of spherically symmetric or cylindrically symmetric deformations. Since only axisymmetric deformations are considered, the equilibrium equations reduce to a single second-order nonlinear ordinary differential equation for the radial deformation field. In previous work, two main methods for obtaining analytic solutions for these equations have been developed, the first based on a change of variable that allows one to transform the second-order ODE to a pair of first-order ODEs that may be solved in parametric form. In some cases, the parameter can be eliminated to yield an explicit closed form expression for the deformation. The second method is based on the observation that the second-order ODE is invariant under a stretching transformation of the independent and dependent variables and thus may always be transformed to a first-order ODE. The purpose of this paper is to show how the powerful techniques of Lie group analysis (LGA) for ODEs can be used to unify both the above approaches and how LGA is an invaluable guide in the development of analytic solution methods. It is shown that both of the approaches can be derived from Lie algebra invariance criteria and that the second approach is, in fact, a special case of the first. Results from LGA are then applied to investigate the structure and solution set of the governing ODEs. A systematic procedure for solving the axisymmetric equations of compressible finite elastostatics is thereby proposed. The results are illustrated by application to several specific strain-energy density functions.

Inhomogeneous ?longitudinal? plane waves in a deformed elastic material

By definition, a homogeneous isotropic compressible Hadamard material has the property that an infinitesimal longitudinal homogeneous plane wave may propagate in every direction when the material is maintained in a state of arbitrary finite static homogeneous deformation. Here, as regards the wave, ‘homogeneous’ means that the direction of propagation of the wave is parallel to the direction of eventual attenuation; and ‘longitudinal’ means that the wave is linearly polarized in a direction parallel to the direction of propagation. In other words, the displacement is of the form u = ncos k(n · x − ct), where n is a real vector. It is seen that the Hadamard material is the most general one for which a ‘longitudinal’ inhomogeneous plane wave may also propagate in any direction of a predeformed body. Here, ‘inhomogeneous’ means that the wave is attenuated, in a direction distinct from the direction of propagation; and ‘longitudinal’ means that the wave is elliptically polarized in the plane containing these two directions, and that the ellipse of polarization is similar and similarly situated to the ellipse for which the real and imaginary parts of the complex wave vector are conjugate semi-diameters. In other words, the displacement is of the form u = ℜ{S exp iω(S · x − ct)}, where S is a complex vector (or bivector). Then a Generalized Hadamard material is introduced. It is the most general homogeneous isotropic compressible material which allows the propagation of infinitesimal ‘longitudinal’ inhomogeneous plane circularly polarized waves for all choices of the isotropic directional bivector. Finally, the most general forms of response functions are found for homogeneously deformed isotropic elastic materials in which ‘longitudinal’ inhomogeneous plane waves may propagate with a circular polarization in each of the two planes of central circular section of the \(\mathbb{B}\) n-ellipsoid, where \(\mathbb{B}\) is the left Cauchy-Green strain tensor corresponding to the primary pure homogeneous deformation.

Finite strain––isotropic hyperelasticity

This paper presents a strain energy density for isotropic hyperelastic materials. The strain energy density is decomposed into a compressible and incompressible component. The incompressible component is the same as the generalized Mooney expression while the compressible component is shown to be a function of the volume invariant J only. The strain energy density proposed is used to investigate problems involving incompressible isotropic materials such as rubber under homogeneous strain, compressible isotropic materials under high hydrostatic pressure and volume change under uniaxial tension. Comparison with experimental data is good. The formulation is also used to derive a strain energy density expression for compressible isotropic neo-Hookean materials. The constitutive relationship for the second Piola–Kirchhoff stress tensor and its physical counterpart, involves the contravariant Almansi strain tensor. The stress stretch relationship comprises of a component associated with volume constrained distortion and a hydrostatic pressure which results in volumetric dilation. An important property of this constitutive relationship is that the hydrostatic pressure component of the stress vector which is associated with volumetric dilation will have no shear component on any surface in any configuration. This same property is not true for a neo-Hookean Green’s strain–second Piola–Kirchhoff stress tensor formulation.

A Micromechanics Solution of a 3D Internal Instability Problem for a Fiber Series in an Infinite Matrix

A micromechanics solution of an internal instability problem for a fiber series in an infinite isotropic elastic matrix subjected to compression is presented. The theoretical solution permits application of transversely isotropic compressible material models to the reinforcement. Possible instability modes are discussed. Numerical solution procedure is demonstrated on a simple particular case.

Application of transversely isotropic materials to multi-layer shell elements undergoing finite rotations and large strains

Abstract The contribution deals with an extension of a classical Neo–Hookean model for compressible isotropic materials to transverse isotropy. With this enhancement for one preferred material direction there is a possibility to simulate large strains in volume changes of the isotropic basic continuum and supplementary in fiber direction. The integrity basis of polynomial invariants in case of transversely isotropic hyperelasticity consists of three principal invariants of the isotropic basic continuum and additionally of two principal invariants for the preferred material direction. The proposed stored energy function for transverse isotropy contains the classical theory near to the natural state and fulfills the restriction on polyconvexity and coerciveness. By numerical enforcement of the material model into shell kinematics without rotational variables a four-node isoparametric finite element is developed using special concepts to avoid locking. The capability of the algorithms proposed is demonstrated by a numerical example involving large strains as well as finite rotations.

Solutions for an infinite compressible nonlinearly elastic body under a line load

The axisymmetric deformation of a nonlinearly elastic isotropic compressible infinite elastic body subjected to a concentrated vertical line load is considered. We first derive the solution to this problem within the context of the linear theory of elasticity. We then obtain the governing equations for the nonlinear problem via the Principle of Stationary Potential Energy, and use these equations to obtain classes of compressible finite elasticity solutions for the line load problem. Finally, a comparison with finite anti-plane shear of compressible isotropic materials is made.

A note on the pure torsion of a circular cylinder for a compressible nonlinearly elastic material with nonconvex strain-energy

The large deformation torsion problem for an elastic circular cylinder subject to prescribed twisting moments at its ends is examined for a particular homogeneous isotropic compressible material, namely the Blatz-Ko material. For this material, the displacement equations of equilibrium in three-dimensional elastostatics can lose ellipticity at sufficiently large deformations. For the torsion problem, it is shown that this occurs when the prescribed torque reaches a critical value. For values of the twisting moment greater than this critical value, there is an axial core of the cylinder on which ellipticity holds, surrounded by an annular region where loss of ellipticity has occurred. The physical implications in terms of localized shear bands are briefly discussed.

On a method of investigating fibre stability in an elastic semi-infinite matrix near a free surface

The properties of an infinite characteristic determinant in the three-dimensional linearized problem /1/ of fibre stability in an elastic semi-infinite matrix near a free surface are investigated. As in the case of two and a number of doubly-periodic systems of fibres in an infinite matrix /2–4/ it is proved that the mentioned determinant is a determinant of normal type. Non-linearly elastic transversely isotropic compressible materials are examined within the framework of the theory of finite subcritical deformations without taking account of the specific form of the elastic potential. The results elucidated hold even for different modifications of the theory of small subcritical deformations. Results of an analysis of kindred questions of the theory of elastic wave diffraction are used /5/.

The finite deformation of a hollow cylinder subjected to internal or external pressure.

This paper is concerned with the plane train deformation of a hollow cylinder, within the theory of finite elastostatics for a particular homogeneous isotropic compressible material, the so-called Blatz-Ko material. The body is subjected to uniform pressure, either internal or external. In the case of internal pressure, it is found that there is a maximum pressure beyond which there does not exist a solution. Under that pressure there exist two sets of solutions. In the case of external pressure, for a sufficiently large value of pressure, the location of the maximum value of the compressive hoop stress departs from the inner surface ; there exists, however, a supremum of the location. If the hollow cylinder is thinner than the supremum, the maximum value of the compressive hoop stress occurs at the outer surface.