A nonlocal strain gradient plasticity theory for finite deformations

Strain gradient plasticity for finite deformations is addressed within the framework of nonlocal continuum thermodynamics, featured by the concepts of (nonlocality) energy residual and globally simple material. The plastic strain gradient is assumed to be physically meaningful in the domain of particle isoclinic configurations (with the director vector triad constant both in space and time), whereas the objective notion of corotational gradient makes it possible to compute the plastic strain gradient in any domain of particle intermediate configurations. A phenomenological elastic–plastic constitutive model is presented, with mixed kinematic/isotropic hardening laws in the form of PDEs and related higher order boundary conditions (including those associated with the moving elastic/plastic boundary). Two fourth-order projection tensor operators, functions of the elastic and plastic strain states, are shown to relate the skew-symmetric parts of the Mandel stress and back stress to the related symmetric parts. Consistent with the thermodynamic restrictions therein derived, the flow laws for rate-independent associative plasticity are formulated in a six-dimensional tensor space in terms of symmetric parts of Mandel stresses and related work-conjugate generalized plastic strain rates. A simple shear problem application is presented for illustrative purposes.     

On the application of the additive decomposition of generalized strain measures in large strain plasticity

In the present paper two thermodynamically consistent large strain plasticity models are examined and compared in finite simple shear. The first model (A) is based on the multiplicative decomposition of the deformation gradient, while the second one (B) on the additive decomposition of generalized strain measures. Both models are applied to a rigid-plastic material described by the von Mises-type yield criterion. Since both models include neither hardening nor softening law, a constant shear stress response even for large amounts of shear is expected. Indeed, the model A exhibits the true constant shear stress behavior independent of the elastic material law. In contrast, the model B leads to a spurious shear stress increase or drop such that its applicability under finite shear deformations may be questioned.     

Correlation of finite strain measures

A numerical scheme for correlating any two finite strain measures is given, and situations in which such a scheme might be used are introduced. A check is established by comparison with the Reiner-Hanin solution of the Almansi-to-Hencky conversion. A numerical example is provided.     

On mean rotations in finite deformations

Summary Firstly, the classical interpretation of the mean rotation tensor of Cauchy and Novozhilov is revised. Indeed, two distinctive and defective features of this interpretation reflect a severe restriction on the class of admissible deformations. Secondly, an alternative measure of mean rotation is introduced and its explicit aspect for pure rotations, pure strains and additively pure rotations is determined.

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Sommario In questa nota viene riesaminata l'interpretazione classica del tensore di rotazione media di Cauchy e Novozhilov. Preliminarmente, si mostra come due tratti distintivi di questa interpretazione ne limitino l'ampiezza e riflettano una severa restrizione della classe delle deformazioni ammissibili. Successivamente, si perviene ad una misura alternativa di rotazione media, il cui calcolo esplicito è condotto nei casi di rotazioni pure, deformazioni pure e rotazioni additivamente pure.

     

THE MOIR GEOMETRY OF PLANE FINITE STRAIN AND ROTATION

It is justified in this paper that the foundation of mathematical theory (12) of finite deformation by the method of co-moving coordinate is identical to Moiré method in experimental mechanics. Hence, the important practical value of this theory is further ascertained.     

Isotropy postulate for finite deformations

We present a new formulation of the isotropy postulate for finite deformations that covers a wider class of deformation or loading trajectories, as distinct from its formulations in the literature [11–13], and provide an experimental foundation for this new formulation of the isotropy postulate. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Priknadnaya Mekhanika, Vol. 35, No. 1, pp. 17–27, January, 1999.     

A class of exact solutions for finite plane strain deformations of a particular elastic material

Summary In this paper we consider plane deformations of an incompressible elastic material and we show that by a suitable choice of strain energy function we can find the class of deformations with constant local rotation angle. Although the form for the strain energy function is chosen in the first place for mathematical convenience it does correspond to physically reasonable behaviour and such a theory may be regarded as a first order theory. The class of solutions obtained are expressed in a parametric form involving an arbitrary function, simple choices of which correspond to the well known exact solutions of finite elasticity.     

On the local measures of mean rotation in continuum mechanics

We implement Cauchy's concept of a rotation-angle function on an oriented plane, and characterize situations when a rotation-angle function exists, and hence when measuring mean rotations in the manner of Cauchy or Novozhilov makes sense. We also discuss in passing the role of the skew part of the deformation gradient in measuring the mean deformation.     

Application of corotational rates of the logarithmic strain in constitutive modeling of hardening materials at finite deformations

In this paper a finite deformation constitutive model for rigid plastic hardening materials based on the logarithmic strain tensor is introduced. The flow rule of this constitutive model relates the corotational rate of the logarithmic strain tensor to the difference of the deviatoric Cauchy stress and the back stress tensors. The evolution equation for the kinematic hardening of this model relates the corotational rate of the back stress tensor to the corotational rate of the logarithmic strain tensor. Using Jaumann, Green–Naghdi, Eulerian and logarithmic corotational rates in the proposed constitutive model, stress–strain responses and subsequent yield surfaces are determined for rigid plastic kinematic and isotropic hardening materials in the simple shear problem at finite deformations.     

On the failure of ellipticity of the equations for finite elastostatic plane strain

Summary In this paper we establish necessary and sufficient conditions, in terms of the local principal stretches, for ordinary and strong ellipticity of the equations governing finite plane equilibrium deformations of a compressible hyperelastic solid. The material under consideration is assumed to be homogeneous and isotropic, but its strain-energy density is otherwise unrestricted. We also determine the directions of the characteristic curves appropriate to plane elastostatic deformations that are accompanied by a failure of ellipticity.The results communicated in this paper were obtained in the course of an investigation supported by Contract N00014-75-C-0196 with the Office of Naval Research in Washington, D.C.     

On natural strain measures of the non-linear micropolar continuum

We discuss three different ways of defining the strain measures in the non-linear micropolar continuum: (a) by a direct geometric approach, (b) considering the strain measures as the fields required by the structure of local equilibrium conditions, and (c) requiring the strain energy density of the polar-elastic body to satisfy the principle of invariance under superposed rigid-body deformations. The geometric approach (a) generates several two-point deformation measures as well as some Lagrangian and Eulerian strain measures. The ways (b) and (c) allow one to choose those Lagrangian strain measures which satisfy the additional mechanical requirements. These uniquely selected relative strain measures are called the natural ones. All the strain measures discussed here are formulated in the general coordinate-free form. They are valid for unrestricted translations, stretches and changes of orientations of the micropolar body, and are required to identically vanish in the absence of deformation. The relation of the Lagrangian stretch and wryness tensors derived here to the ones proposed in the literature is thoroughly discussed.     

On discontinuous strain fields in finite elastostatics

A general method for the study of piece-wise homogeneous strain fields in finite elasticity is proposed. Critical homogeneous deformations, supporting strain jumping, are defined for any anisotropic elastic material under constant Piola–Kirchhoff stress field in three-dimensional elasticity. Since Maxwell’s sets appear in the neighborhood of singularities higher than the fold, the existence of a cusp singularity is a sufficient condition for the emergence of piece-wise constant strain fields. General formulae are derived for the study of any problem without restrictions or fictitious stress–strain laws. The theory is implemented in a simple shearing plane strain problem. Nevertheless, the procedure is valid for any anisotropic material and three-dimensional problems.