Modelling and Simulation in Engineering

Volume 2016, Article ID 8464205, 7 pages

http://dx.doi.org/10.1155/2016/8464205

## On Torsion of Functionally Graded Elastic Beams

Department of Structures for Engineering and Architecture, University of Naples Federico II, via Claudio 21, 80125 Naples, Italy

Received 14 December 2015; Accepted 5 October 2016

Academic Editor: Theodoros C. Rousakis

Copyright © 2016 Marina Diaco. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The evaluation of tangential stress fields in linearly elastic orthotropic Saint-Venant beams under torsion is based on the solution of Neumann and Dirichlet boundary value problems for the cross-sectional warping and for Prandtl stress function, respectively. A skillful solution method has been recently proposed by Ecsedi for a class of inhomogeneous beams with shear moduli defined in terms of Prandtl stress function of corresponding homogeneous beams. An alternative reasoning is followed in the present paper for orthotropic functionally graded beams with shear moduli tensors defined in terms of the stress function and of the elasticity of reference inhomogeneous beams. An innovative result of invariance on twist centre is also contributed. Examples of functionally graded elliptic cross sections of orthotropic beams are developed, detecting thus new benchmarks for computational mechanics.

#### 1. Introduction

Analyses of composite media are a well-investigated research field in structural mechanics. Theoretical noteworthy results also in the nonlinear range have recently contributed to several engineering applications, such as beam and plate theories [1–3], fracture mechanics [4–8], hyperelastic media [9–11], concrete systems [12–16], nonlocal models [17–19], homogenization [20–22], thermoelasticity [23–25], nanostructures [26–30], and limit analysis [31–33]. In the context of the classical theory of elasticity, an innovative methodology for the analysis of beams was proposed by Saint-Venant [34, 35], with the assumption that the normal interactions between longitudinal fibres vanish [36]. Basic results about this model are collected in classical treatments [37–44] with a coordinate approach. Coordinate-free investigations can be found in [45–48]. Nevertheless, analytical solutions of beams subjected to torsion can be obtained only for special cross-sectional geometries and shear moduli distributions. Exact solutions of functionally graded structures can be found in [49]. However, finite element strategies are often adopted in order to get effective numerical results when exact solutions are not available; see, for example, [50–53]. Alternatively, experimental methods are employed; see, for example, [54]. Recently Ecsedi showed that, for functionally graded cross sections under torsion, with shear modulus defined by a positive function of the Prandtl stress function of a corresponding homogeneous cross-section, the warping is invariant and the stress function is expressed in terms of the one associated with the reference homogeneous cross section [55, 56]. Ecsedi’s treatment is based on an integral transformation proposed by Kirchhoff in nonlinear heat conduction [57]. An intrinsic reasoning is illustrated in the present paper, by performing a direct discussion of Neumann and Dirichlet boundary value problems for the cross-sectional warping and for the stress function of orthotropic composite beams under torsion. An invariance condition for the Cicala-Hodges centre is also assessed (see Section 3). Finally, new analytical solutions of functionally graded elliptic cross sections are constructed in Section 4. Basic results of Saint-Venant theory of linearly elastic orthotropic beams are collected in the next section.

#### 2. Composite Saint-Venant Beams under Torsion

Let be the simply or multiply connected cross section of an orthotropic and linearly elastic Saint-Venant composite beam under torsion. Position of a point in , with respect to the centre of the Young moduli of beam’s longitudinal fibers, is denoted by . The tensor is the rotation by counterclockwise in the cross-sectional plane . Hence and . Tangential stresses can be expressed in terms of the warping function [34] or of the stress function [58] by the coordinate-free formulae [59]where the scalar is the twist, is the elastic tangential strain, and is the positive definite symmetric Lamé tensor field, being the two-dimensional linear space of translations in The warping field is the solution of the following Neumann-like problem [60]:where is the unit outward normal to the domain . Prandtl stress function is the solution of the Dirichlet problemwhere is a multiply connected cross-section, with exterior boundary and boundary of the th hole, being and . The procedure for the evaluation of integration constants is illustrated in [59]. Note that the warping function has been introduced above by assuming tacitly that the cross section undergoes a rotation about the pole . Denoting by the warping function corresponding to a cross-sectional rotation with respect to a point , we get the formulawith position vector of and . Tangential stress fields are independent of the rotation centre [40]. The twist centre and a particular value of the constant were introduced in [61] by requiring that zeroth and first elastic moments of the scalar field are zeroThe position of the twist centre is given by the formulawith bending stiffness and tensor product. An equivalent definition of twist centre was proposed by Trefftz [62] in energetic terms. In [59] it was shown that the twist centre coincides with the shear centre of Timoshenko beams [63], evaluated by the composite and orthotropic Saint-Venant beam theory. Hereafter, the point will be named the Cicala-Hodges centre. The next section provides a family of composite beams, generated by a Lamé tensor field , for which the warping field and the Cicala-Hodges centre are invariant.

#### 3. Invariances

Let us consider a sequence of tensor fields generated by a Lamé tensor field and by a sequence of positive scalar functions ; that is, , according to the rule:with Prandtl stress function associated with the torsion tangential stress field involving Lamé tensor field . The sequence of Lamé fields induces a sequence of Neumann-like PDE problems for the warping field, defined bywith . The following results hold true.

Proposition 1. *Neumann-like PDE problems provide, to within an additive constant, the same solution .*

*Proof. *Let be the solution, to within a constant, of the problem . Since , problem takes the form:Resorting to the formulaand setting with , we get , so that the problem may be rewritten asRecalling the relation , we infer that and the problem collapses into the one . The result follows.

Proposition 2. *The relationship between Prandtl stress functions corresponding to Lamé tensor fields and is expressed by the formula , with antiderivative of such that is identically zero on the cross-sectional exterior boundary .*

*Proof. *Resorting to Proposition 1 we get . Then the equivalences holdwhence which gives , with antiderivative of such that is identically zero on the cross-sectional exterior boundary .

Proposition 3. *Let be the Euler moduli scalar field of orthotropic and composite beams whose Lamé fields are described by the sequence . For these beams, the location of the Cicala-Hodges centre is invariant.*

*Proof. *The result follows by the formula providing the twist centre position and by Proposition 1.

#### 4. Examples

Let us provide some analytical solutions of functionally graded orthotropic beams under torsion with elliptic cross sections. Inertia principal axes with origin in the centre of the Euler moduli field will be adopted in the sequel. Position vector and rotation are written aswhence . Torsional warping of elliptic composite beams with Lamé tensor field,is provided by the formula [40] , with and lengths of the ellipse semidiameters. Plots of the shear modulus and of the warping are provided in Figures 1 and 2.