A numerical boundary integral equation analysis for standard linear viscoelastic media made of functionally graded materials
© Ashrafi et al.; licensee Springer 2014
Received: 13 April 2014
Accepted: 7 July 2014
Published: 3 October 2014
Although structures made of functionally graded materials have been studied by many researchers, no research may be found in literature on boundary element analysis of the functionally graded viscoelastic structures.
In the present paper, a 2D boundary element formulation capable of modeling time-dependent functionally graded materials (FGM) is presented. A numerical implementation of the Somigliana identity in terms of the displacements is developed to solve 2D problems of the exponentially graded viscoelasticity. The FGM concept can be applied to various materials, for structural and functional purposes. In this model, only Green functions of the nonhomogeneous elastostatic problems are needed with material properties that vary continuously along a given dimension.
Results and Conclusions
Results reveal that the boundary element approach can successfully be employed for the present complicated problem for arbitrary time histories of the applied loads and arbitrary boundary conditions, without the need to use relaxation functions or mathematical transformations.
KeywordsBoundary integral equation formulation Viscoelastic FGMs Nonhomogeneity Time-dependent materials
In recent years, the functionally graded materials (FGMs), as a category of the composite materials, have generated a great deal of attention. An FGM is an advanced material whose composition changes gradually and results in corresponding changes in its properties (Suresh and Mortensen 1998). Because of the special features of the FGMs with potential applications to lots of engineering fields, they have attracted attention of numerous scientists and engineers in broad areas of research. Moreover, a variety of nonhomogeneous engineering media made of polymers, plastics, metals and alloys at elevated temperatures, composites, concrete, etc., exhibit significant rate and history dependencies. Appropriate simulation of these types of structures requires using appropriate viscoelastic models.
Boundary element method (BEM) has recently found considerable applications in solving lots of engineering problems, such as viscoelasticity, contact mechanics, elastoplasticity, thermoelasticity, fracture mechanics, elastodynamics, etc. (Aliabadi 2002). The viscoelastic media can effectively and accurately be treated by the BEM (Ashrafi and Farid 2009; Ashrafi et al. 2012). The BEM just requires the boundary data as input, and there is no need for discretizing the domain under consideration into elements. Adaptation of the BEM to nonhomogeneous media is a hard task; because determination of the fundamental solutions corresponding to the concentrated loads is difficult for such materials. The fundamental solutions for heat transfer problems in the nonhomogeneous media have been presented using BEM algorithms by some researchers (Shaw and Makris 1992; Clements 1998; Clements and Budhi 1999; Gray et al. 2003; Sutradhar and Paulino 2004; Kuo and Chen 2005). Also, the fundamental solutions of the FGMs for 2D and 3D elasticity problems have been recently developed in some other works (Chan et al. 2004; Criado et al. 2007, 2008). Using a Fourier transform technique, these functions have been derived for exponentially graded media. Although extensive works have been developed to effectively model the constitutive behavior of the FG structures, most of these researches have been limited to elastic behavior analysis (Gao et al. 2008; Wang and Qin 2012; Ashrafi et al. 2013a, 2013b).
In this paper, the boundary element formulation is proposed based on the differential viscoelastic constitutive equations of nonhomogeneous SLS model. The resulting algorithm is capable of solving the quasistatic problems for exponentially graded viscoelastic materials with arbitrary boundary conditions and therefore, provides a reasonable model for certain realistic situations. Avoiding internal domain elements is one of the objectives of this paper, which results in numerical discretization of the boundary of the considered nonhomogeneous problem, only. Therefore, this work reduces the number of variables to be computed, which makes numerical treatment of the infinite and semi-infinite time-dependent problems easy.
Boundary integral equations
The domain integral of the body forces can easily be transformed into an equivalent boundary integral equation, which results in equation in terms of the boundary values only; however, for more simplicity, we neglect it. In a similar manner, the boundary integral equations of the stresses can be derived.
It should be noted in the, for solving the above time-dependent differential equation, it was necessary to approximate the displacement and the traction rates in the time domain by a time marching treatment. Finally, the presented algorithm has been cast into a unique program and then solved by the MATLAB software.
Results and discussion
For evaluating the accuracy and the efficiency of the proposed approach, a numerical viscoelastic problem wherein the material properties are assumed to be exponential functions of the Cartesian coordinate x, is considered.
The opposite end of the considered plate is fixed, while its other boundaries are free of tractions. It is assumed that the plate is sufficiently thin such that a plane stress condition holds. In this problem, Boltzmann's solid model is employed to formulate the differential constitutive equations of the heterogeneous viscoelastic structures.
This problem is solved using the proposed method, adopting the geometric and material parameters as a = 2 m, b = 1 m, c = 1 m, γ = 0.25, 1, 2 and 3, λ 0 = 5 GPa, μ 0 = 350 MPa, υ = 0.4 and β = 24. The diameter of the circular cutouts is d = 0.35 m. Due to using the present-graded boundary integral equation approach, only about 54 boundary elements have been used for the boundary discretization. Using the traditional finite element procedures necessitates using thousands of elements and nodal points.
In the present paper, a new numerical formulation is presented for accomplishment of the simplified viscoelastic analysis of the functionally graded media by the BEM. This approach avoids using relaxation functions or mathematical transformations, and it is capable of solving the quasistatic viscoelastic problems with arbitrary load time dependence and arbitrary boundary conditions. A numerical example, as an application, was provided to evaluate this boundary element formulation. Only the displacement and traction fundamental solutions of the FG elastostatic structures are needed in the present formulation. This computational system was easily solved by adopting a linear time approximation for the displacement and traction rates. Other advantage of the presented approach is that the mathematical integral representation needs only the boundary data. Numerical discretization was done without any domain approximations, and the integral equations were applied only on the boundaries.
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