Constitutive modeling within peridynamic theory considers the collective deformation at each time of all the material within a δ-neighborhood of any point of a peridynamic body. The assignment of the parameter δ, called the horizon, is treated as a material property. The difference displacement quotient field in this neighborhood, rather than the extension scalar field, is used to generate a three-dimensional state-based linearly elastic peridynamic theory. This yields an enhanced interpretation of the kinematics between bonds that includes both length and relative angle changes. A free energy function for a linearly elastic isotropic peridynamic material that contains four material constants is proposed as a model, and it is used to obtain the force vector state and the associated modulus state for this material. These states are analogous to, respectively, the stress field and the fourth-order elasticity tensor in classical linear theory. In the limit of small horizon, we find that only three of the four peridynamic material constants are related to the classical elastic coefficients of an isotropic linear elastic material, with one of the three constants being arbitrary. The fourth peridynamic material constant, which accounts for the coupling effect of both bond length and relative angle change, has no effect on the limit, but remains a part of the peridynamic model. The determination of the two undetermined constants is the subject of future investigation. Peridynamic models proposed elsewhere in the literature depend on the deformation state through its dilatational and deviatoric parts and contain only two peridynamic material constants, in analogy to the classical linear elasticity theory. Observe from above that our model depends on both length and relative angle changes, as in classical linear theory, but, otherwise, is not limited to having only two material constants. In addition, our model corresponds to a nonordinary material, which represents a substantial break with classical models.

There are problems in the classical linear theory of elasticity whose closed form solutions, while satisfying the governing equations of equilibrium together with well-posed boundary conditions, predict the existence of regions, often quite small, inside the body where material overlaps. Of course, material overlapping is not physically realistic, and one possible way to prevent it combines linear theory with the requirement that the deformation field be injective. A formulation of minimization problems in classical linear elasticity proposed by Fosdick and Royer [3] imposes this requirement through a Lagrange multiplier technique. An existence theorem for minimizers of plane problems is also presented. In general, however, it is not certain that such minimizers exist. Here, the Euler-Lagrange equations corresponding to a family of three-dimensional problems is investigated. In classical linear elasticity, these problems do not have bounded solutions inside a body of anisotropic material for a range of material parameters. For another range of parameters, bounded solutions do exist but yield stresses that are infinite at a point inside the body. In addition, these solutions are not injective in a region surrounding this point, yielding unrealistic behavior such as overlapping of material. Applying the formulation of Fosdick and Royer on this family of problems, it is shown that both the displacements and the constitutive part of the stresses are bounded for all values of the material parameters and that the injectivity constraint is preserved. In addition, a penalty functional formulation of the constrained elastic problems is proposed, which allows to devise a numerical approach to compute the solutions of these problems. The approach consists of finding the displacement field that minimizes an augmented potential energy functional. This augmented functional is composed of the potential energy of linear elasticity theory and of a penalty functional divided by a penalty parameter. A sequence of solutions is then constructed, parameterized by the penalty parameter, that converges to a function that satisfies the first variation conditions for 100 J Elasticity (2006) 84: 99-129 a minimizer of the constrained minimization problem when this parameter tends to infinity. This approach has the advantages of being mathematically appealling and computationally simple to implement. Mathematics Subject Classifications (2000)74B05 · 74E10 · 74G65 · 74G70 · 74S05

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This work is an extension of previous investigation concerning a free energy function for an isotropic, linearly elastic peridynamic material that depends quadratically on infinitesimal normal and shear strain states. The free energy function contains four peridynamic material constants, from which three constants are related to the classical elasticity coefficients of an isotropic linear elastic material, with one of the three constants being arbitrary. To determine this arbitrary constant, the difference displacement quotient state at a point is decomposed in terms of radial and non-radial components. If the radial component is zero, the quadratic free energy function reduces to an integral expression that multiplies the arbitrary constant. This result together with a correspondence argument is used next to find a general expression for this constant. A simple experiment in mechanics is then used to evaluate this constant in terms of the classical shear modulus and the horizon δ. The correspondence argument can also be used to find a general expression for the fourth peridynamic constant that appears in the quadratic free energy function.

This erratum concerns a correction in the expression (42) of the original article. As a consequence, the text between this expression and the expression (47) in that article was revised and is presented below.Substituting both (40.a) and (42) into (37), we find thatwhere α is the peridynamic constant that appears in (22) and is given by (34.a). Substituting (43) into (33) and using (32) together with (25) and (20), we getNext, recall from above that (25) holds and substitute α 33 , given by (43), and both α 11 and α 12 , given by (44), into the expressions (14), (15), and (16), to obtainThe online version of the original article can be found under