# On the deformation of almost cylindrical elastic beams

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## Abstract

### Background

The Saint-Venant problem for porous elastic cylinders is of interest both from the technical and mathematical point of view. The intended applications of solution are to mechanics of bone and to some engineering structures.

### Method

This work investigates the Saint-Venant problem for almost prismatic bars made of an isotropic porous material. We express the solution in terms of the solutions of two problems concerning the deformation of a right cylinder.

### Results and Conclusion

We use the method to study the extension of an almost prismatic conical frustum. It is found that the displacement vector is a polynomial of two degree in the cartesian coordinates. The volume fraction field depends linearly on the axial coordinate. The solution contains terms characterizing the influence of the material porosity and the dependence on the lateral surface.

### Keywords

Elastic bars Porous bodies Almost cylindrical beams Elastic materials with voids## Background

A generalization of the classical theory of elasticity is the theory of elastic materials with voids established by Nunziato and Cowin (1979) and Cowin and Nunziato (1983). The intended application of the theory is to elastic bodies with pores which are distributed throughout the material. In the framework of the linear theory of isotropic elastic materials with voids, the deformation of the right cylinders has been the subject of various investigations. Cowin and Nunziato (1983) have studied the pure bending of a cylinder made of a homogeneous material. The problem of extension and bending for nonhomogeneous porous elastic bodies has been investigated by Ciarletta and Iesan (1993), Ieşan and Nappa (1994), Ieşan and Scalia (2007), and Ieşan and Scalia (2009). A treatment of Saint Venant’s problem for homogenous and isotropic porous elastic cylinders has been presented by Dell’Isola and Batra (1997), Ieşan and Quintanilla (1995), Ieşan (2009), and Ieşan (2011).

In the context of the classical elasticity, the mechanical behavior of the noncylindrical elastic bars has been studied in many papers (see, e.g., Dryden (2007), Zupan and Saje (2006), You et al. (2002), and the references therein). These bodies are of interest both from the technical and mathematical point of view. The present paper is concerned with the Saint-Venant problem for almost cylindrical bars made of porous elastic materials. In the classical elastostatics, the deformation of almost cylindrical bars has been studied in various papers (see, e.g., Bors (1973), Chirita (1983), Khatiashvili (1983b), Khatiashvili (1983a), and the references therein). First, we present the basic equations of the linear theory of isotropic porous elastic solids and the formulation of the problem. The next section is devoted to the solution of the Saint-Venant problem for almost cylindrical bars. The problem is reduced to the solving of a problem of Almansi type and to the Saint-Venant problem for a right cylinder. In the next section we use the method to solve the problem of extension of a conical frustum. The solution is expressed in terms of solutions of some problems associated with the deformation of a right circular cylinder.

## Formulation of the problem

*O*

*x*

_{ k }(

*k*=1,2,3) is used. We shall employ the usual summation and differentiation conventions: Greek subscripts are understood to range over the integers (1,2) whereas Latin subscripts to the range (1,2,3); summation over repeated subscripts is implied and subscripts preceded by a comma denote partial differentiation with respect to the corresponding cartesian coordinate. We denote by

*B*the interior of a noncilindrical bar of length

*l*, with the ends located at

*x*

_{3}=0 and

*x*

_{3}=

*l*, and with the lateral surface

*Π*defined by

Here, *k* is a constant small enough for squares and higher powers to be neglected. The bar *B* is called almost cylindrical. We assume that *B* is a bounded regular region (Gurtin (1972), Section 5). We denote by *Σ* _{1} the cross section located at *x* _{3}=0 and by *Σ* _{2} the cross section located at *x* _{3}=*l*. We call *∂* *B* the boundary of *B* and denote by *n* _{ i } the components of the outward unit normal of *∂* *B*.

*u*

_{ i }is the components of the displacement vector. Let

*t*

_{ ij }be the stress tensor and let

*h*

_{ i }be the equilibrated stress vector. The surface force and the equilibrated surface force at a regular point of

*∂*

*B*are given by

where *δ* _{ ij } is the Kronecker delta, *φ* is the volume distribution function, *g* is intrinsic equilibrated body force, and *λ*,*μ*,*b*,*α*, and *ξ* are constitutive constants.

**R**=(

*R*

_{1},

*R*

_{2},

*R*

_{3}) and

**M**=(

*M*

_{1},

*M*

_{2},

*M*

_{3}) be prescribed vectors representing the resultant force and resultant moment about

*O*of the tractions acting on

*Σ*

_{1}. On

*Σ*

_{2}, there are tractions applied so as to satisfy the equilibrium conditions of the body. On the end located at

*x*

_{3}=0, we have the conditions

*ε*

_{ ijk }is the alternating symbol. We note that there is no contribution of the equilibrated surface force in the resultant force and resultant moment (Ciarletta and Iesan 1993). The elastic potential corresponding to the considered continuum is

*W*is a positive definite quadratic form in the variables

*e*

_{ ij },

*φ*, and

*φ*

_{,j }. Then, following Cowin and Nunziato (1983)

## Methods

*B*is mapped into the right cylinder

*D*, of length

*l*, with the lateral surface

*S*given by

*f*is a function of point of class

*C*

^{1}, then

*φ*

^{(α)}, (

*α*=1,2), are unknown functions. We denote

on *D*.

*S*are (

*N*

_{1},

*N*

_{2},0), then, we have

*D*located at

*y*

_{3}=0. In view of Eqs. (10) and (18), the conditions (7) reduce to

We note that the functions \(u_{i}^{(1)}\) and *φ* ^{(1)} satisfy the equations and the boundary conditions in the Saint-Venant problem characterized by Eqs. (15), (19), and (21) on *D* and the boundary conditions (24) and (26). The functions \(u_{i}^{(1)}\) and *φ* ^{(1)} can be determined by using the method given by Ciarletta and Iesan (1993). To find the functions \(u_{i}^{(2)}\) and *φ* ^{(2)}, we have to solve Eqs. (15), (19), and (22) on *D* and the boundary conditions (25) and (27). In this problem, the body loads and the surface tractions on the lateral surface *S* are, in general, different from zero. The functions \(u_{i}^{(2)}\) and *φ* ^{(2)} satisfy a problem of Almansi type, in which the body loads and surface forces depend on the functions \(u_{i}^{(1)}\) and *φ* ^{(1)}. A general method to solve the Almansi problem has been presented by Ieşan and Scalia (2009).

## Results and discussion

*B*is the interior of a circular cone frustum, bounded by plane ends perpendicular to the axis of the cone (Fig. 1). We choose the rectangular cartesian coordinate frame such that

*x*

_{3}-axis can be the axis of the cone. We assume that the ends

*Σ*

_{1}and

*Σ*

_{2}are circles of radius

*r*

_{1}and

*r*

_{2}, respectively, and that

*r*

_{1}<

*r*

_{2}. The lateral surface

*Π*is defined by

where *θ* is the angle between the generator and the axis of the cone.

*k*is small enough for squares and high powers to be neglected. Then, Eq. (28) can be expressed as

*S*is described by

*D*is a right circular cylinder,

*R*

_{ α }=0,

*R*

_{3}≠0 and

*M*

_{ j }=0. In this case, the conditions (26) reduce to

*φ*

^{(α)}, (

*α*=1,2). We seek the functions \(u_{j}^{(1)}\) and

*φ*

^{(1)}in the form

*C*

_{ α }and

*a*

_{3}are unknown constants. It follows from Eqs. (34), (15), and (19) that

*b*

^{2}−

*ξ*(

*λ*+

*μ*) is different from zero. The conditions (32) are identically satisfied. From Eqs. (33) and (35), we find the constant

*a*

_{3},

where *A* is the area of the cross section \(\Sigma _{1}^{*}\).

*φ*

^{(1)}are determined. Let us study the Almansi problem characterized by Eqs. (15), (19), and (22) on

*D*and the boundary conditions (25) and (27). We note that in the case of extension, we have

*D*, where

*φ*

^{(2)}which satisfy Eqs. (15), (19), (43), and (44) on

*D*, and the boundary conditions (46) and (27). First, we determine the functions \(u_{i}^{*}\) and

*φ*

^{∗}which satisfy Eqs. (15), (19), (43), and (44) on

*D*and the boundary conditions on the lateral surface (46). We seek these functions in the form

*D*, where

*A*

_{ α }and

*B*

_{ k }are unknown constants, and

*Ψ*is an unknown function. The strain tensor \(e_{ij}^{*}\), corresponding to the displacements \(u_{j}^{*}\), is

*g*

^{∗}associated with the deformation described by Eq. (48) are given by

*Δ*

*U*=

*U*

_{;α α }. The third boundary condition of Eq. (46) becomes

*B*

_{ k }are determined by the system (50), (53), and (57). We obtain

We note that the relations (9) imply that \(\mathcal {D}\neq \) 0. The last condition from Eq. (46) is identically satisfied.

where *r* ^{2}=*y* _{ α } *y* _{ α }.

*φ*

^{(2)}in the form

*v*

_{ i }and

*ψ*must satisfy the equilibrium equations in the absence of the body loads and the conditions on the lateral surface

*S*, in the absence of superficial forces. Let

*s*

_{ ij }be the stress tensor corresponding to the displacements

*v*

_{ i }and volume distribution function

*ψ*. It follows from Eqs. (49), (61), and (62) that the conditions (27) reduce to

*v*

_{ i }and

*ψ*satisfy a problem of extension with the axial force \(R_{3}^{*}\). These functions are given by

*C*

_{ α }is defined by Eq. (38) and

**e**

_{1},

**e**

_{2},

**e**

_{3}) and the origin

*O*. Let

*R*

_{3}=−

*F*, where

*F*is a positive constant. In this case, the resultant force of the tractions acting on the end located at

*x*

_{3}=

*l*is

*F*

**e**

_{3}and the point

*O*is fixed. Let

*x*

_{ i }be the coordinates of the point

*X*

_{0}in the reference configuration, and let

*ξ*

_{ i }be the coordinates of the corresponding point

*X*in the deformed configuration. Then, we have

*ξ*

_{ i }=

*x*

_{ i }+

*u*

_{ i }, and from Eq. (67), we obtain

*Y*

_{0}which, prior to deformation, had the coordinates (0,0,

*l*) goes into point

*Y*from the deformed configuration. From Eq. (68), we find that the point

*Y*has the coordinates (0,0,

*l*

^{∗}) where

*l*

^{∗}for a homogeneous right cylinder is \((1+F/E \mathcal {A})\)

*l*, where

*E*is Young’s modulus and \(\mathcal {A}\) is the area of the cross section. Let us choose magnesium crystal as the hypothetical material for which the values of the constitutive coefficients are (Bachher, 2015)

*ρ*, located at the plane

*x*

_{3}=

*η*, where

*ρ*and

*η*are given constants. It follows from Eq. (68) that the image of \((\mathcal {C})\) in the deformed configuration is the circle

*ξ*

_{3}=(1+

*Ω*

_{1})

*η*+

*Ω*

_{2}

*η*

^{2}+

*k*

*Q*

*ρ*

^{2}. The relation (71) can be used to describe the deformation of the surface

*S*. Let us assume that

*r*

_{1}=10 mm. We consider the circle of radius

*r*

_{1}located at the plane

*x*

_{3}=

*c*. Let (

*Γ*) be the image of this circle in the deformed configuration. We denote by

*R*(

*c*) the radius of (

*Γ*). The variation of

*R*(

*c*) with respect to variable

*c*is presented in Fig. 2. The material parameters used are the same as in the previous example.

We note that in the case of the problem of extension of a right cylinder, the displacements and the volume fraction field depend on the coordinates *x* _{ i } at most linearly.

## Conclusions

- a)
We have studied the Saint-Venant problem for an almost cylindrical bar made of a porous elastic material. The problem was reduced to the solving of a problem of Saint-Venant type for a right cylinder

*D*and to the problem of Almansi for*D*. - b)
We have used the above method to investigate the behavior of a conical frustum subjected to extension. In this case, the problem reduces to a problem of Almansi type for a right circular cylindrical. The displacement vector field and the volume fraction field have been determined. The displacement vector is a polynomial of two degree in the cartesian coordinate. The volume fraction field depends linearly on the axial coordinate.

- c)
The salient feature of the solution of the problem of extension is that the displacement vector field and the stresses contain new terms characterizing the influence of the material porosity, and their values are therefore modified from the values predicted by the classical elasticity.

## Notes

### Competing interests

The author declares that he has no competing interests.

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