Abstract
In this work, a mathematical modeling of the elastic properties of cubic crystals with centrosymmetry at small scales by means of the Toupin–Mindlin anisotropic first strain gradient elasticity theory is presented. In this framework, two constitutive tensors are involved, a constitutive tensor of fourth-rank of the elastic constants and a constitutive tensor of sixth-rank of the gradient-elastic constants. First, 3+11 material parameters (3 elastic and 11 gradient-elastic constants), 3 characteristic lengths and 1+6 isotropy conditions are derived. The 11 gradient-elastic constants are given in terms of the 11 gradient-elastic constants in Voigt notation. Second, the numerical values of the obtained quantities are computed for four representative cubic materials, namely aluminum (Al), copper (Cu), iron (Fe) and tungsten (W) using an interatomic potential (MEAM). The positive definiteness of the strain energy density is examined leading to 3 necessary and sufficient conditions for the elastic constants and 7 ones for the gradient-elastic constants in Voigt notation. Moreover, 5 lattice relations as well as 8 generalized Cauchy relations for the gradient-elastic constants are derived. Furthermore, using the normalized Voigt notation, a tensor equivalent matrix representation of the two constitutive tensors is given. A generalization of the Voigt average toward the sixth-rank constitutive tensor of the gradient-elastic constants is given in order to determine isotropic gradient-elastic constants. In addition, Mindlin’s isotropic first strain gradient elasticity theory is also considered offering through comparisons a deeper understanding of the influence of the anisotropy in a crystal as well as the increased complexity of the mathematical modeling.
Highlights
Strain gradient elasticity theories are challenging generalized continuum theories to model crystals at small scales like Ångström-scale, where classical elasticity is not valid and leads to unphysical singularities
The basic framework of the considered theory is given answering among others the question: How many characteristic lengths can be defined in a natural way in anisotropic first strain gradient elasticity for cubic materials with centrosymmetry? Section 3.2 deals with the derivation of the involved 11 gradient-elastic constants
There are 3 characteristic lengths appearing in the 3 modified Helmholtz operators, which are part of the Mindlin operator, in the Toupin–Mindlin anisotropic first strain gradient elasticity for cubic materials with centrosymmetry
Summary
Strain gradient elasticity theories are challenging generalized continuum theories to model crystals at small scales like Ångström-scale, where classical elasticity is not valid and leads to unphysical singularities. 6, the normalized Voigt notation is used in order to derive a Mathematical modeling of the elastic properties of cubic crystals tensor equivalent matrix representation of the two constitutive tensors in first strain gradient elasticity. Based on this representation, the independent eigenvalues of the considered constitutive tensors for the cubic as well as for the isotropic case are derived. In “Appendix A”, the matrix representation in Voigt notation and the conditions of positive definiteness of the two involved constitutive tensors of fourth-rank and sixth-rank using the Sylvester criterion are given
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