Research Article  Open Access
Haar Wavelet Method for Nonlinear Vibration of Functionally Graded CNTReinforced Composite Beams Resting on Nonlinear Elastic Foundations in Thermal Environment
Abstract
This paper presents a simple and effective approach based on the Haar wavelet discretization method (HWDM) for the nonlinear vibration analysis of carbon nanotubereinforced composite (CNTRC) beams resting on a nonlinear elastic foundation in a thermal environment. Material properties are assumed to be functionally graded (FG) in the thickness direction and temperaturedependent and are evaluated through the extended rule of mixture. Based on the firstorder shear deformation beam theory in conjunction with the von Kármán nonlinearity, the nonlinear governing equations of CNTRC beams on nonlinear elastic foundations are derived as well as the related boundary conditions. Moreover, the initial thermal stress due to uniform temperature rise is considered in this study. To evaluate the nonlinear natural frequencies, the obtained equations are discretized into a set of nonlinear algebraic equations through HWDM and then the direct iteration technique is employed to solve the resulting algebraic equations. The convergence and comparison studies are carried out, and the results indicate that the numerical rate of convergence of the proposed method is in agreement with the convergence theorem, and good accuracy of the present results is observed. Studies on the effects of different parameters such as CNT volume fraction, distribution type of CNTs, foundation stiffness coefficients, boundary condition, slenderness ratio, temperature rise, and initial thermal stress on the linear frequencies and the nonlinear frequency ratios are also reported. This study offers an insight into the vibration behaviors of CNTRC beams resting on nonlinear elastic foundation subjected to the uniform temperature rise.
1. Introduction
Owing to their extraordinary mechanical, thermal, and electrical properties, carbon nanotubes (CNTs) have been considered to be an ideal reinforcement material for high performance composites. Structural applications of carbon nanotubereinforced composites (CNTRCs) may include aviation, automotive, military, and spacerelated parts, in which the material with high strength, stiffness, and light weight is required. In the past decades, extensive numerical and experimental studies have focused on the mechanical properties of CNTRCs. An analytical study on axial Young’s modulus of singlewalled CNT arrays with diameters ranging from nanometer to meter scales was conducted by Sun et al. [1]. Their results showed that the mechanical properties of CNTs outperform those of carbon fibers. Gojny et al. [2] presented the effects of different types of CNT fillers on mechanical properties of epoxybased nanocomposites. They found that the strength and stiffness of the resulting CNT/epoxy composite can be remarkably enhanced. Omidi et al. [3] showed that only a 3 wt.% addition of multiwalled CNTs increases Young’s modulus and the tensile strength of epoxybased composites up to 43.1% and 55.2%, respectively. The above researches have proved that the addition of CNTs into the matrix leads to significant improvements in the strength and stiffness of the composite.
Functionally graded materials (FGMs) are a class of novel materials with properties that vary spatially according to a given nonuniform distribution of the reinforcement phase. Inspired by the concept of FGMs, Shen [4] first proposed CNTbased FGMs, namely, functionally graded carbon nanotubereinforced composites (FGCNTRCs). In his study, CNTs were distributed linearly along the plate’s thickness directions into an isotropic matrix and the results showed that the nonlinear bending behavior of CNTRC plates can be considerably improved through the FG distribution of CNTs. Recently, using a powder metallurgy route, Kwon et al. [5] successfully fabricated functionally graded CNTreinforced aluminum matrix composites, which proved the feasibility of the concept of FGCNTRCs. After the pioneering work of Shen, extensive investigations about FGCNTRC beams, plates, and shells were carried out to explore their mechanical properties, such as static bending behaviors [6], elastic buckling and postbuckling characteristics [7–10], and linear and nonlinear free vibration features [11–14]. For more details, a comprehensive review on the mechanical analysis of FGCNTRC structures has been presented by Liew et al. [15].
Composite beams are extensively used in many engineering applications where higher strength to weight ratio is desired, such as aircraft structures and space vehicles. In recent years, researches on the mechanical properties of FGCNTRC beams have been increasing rapidly. Ke et al. studied the nonlinear free nonlinear [16] and dynamic stability [17] of FGCNTRC beams and found that the natural and excitation frequencies of FGCNTRC beams with symmetrical distribution of CNTs are higher than those of beams with uniform or unsymmetrical distribution of CNTs. Wu et al. investigated extensively nonlinear vibration [18], mechanical, and thermal postbuckling [19, 20] behaviors of FGCNTRC beams with various geometric imperfections. Their results indicated that geometric imperfections have a considerable effect on the mechanical behaviors of the FGCNTRC beams. Ansari et al. [21] examined nonlinear forced vibration behaviors of FGCNTRC Timoshenko beams using the GDQM and pseudoarc length technique. Beams resting on an elastic foundation often come across in engineering practice. So far, some studies have been conducted on the mechanical behaviors of FGCNTRC beams on the elastic foundation. Using a twostep perturbation technique, Shen and Xiang [22] dealt with the linear and nonlinear free vibration, nonlinear bending, thermal buckling, and thermal postbuckling of simply supported FGCNTRC beams resting on a linear elastic foundation. Free vibrations and buckling of FGCNTRC beams resting on the linear elastic foundation were carried out by Yas and Samadi [23] using the generalized differential quadrature method (GDQM). Their results found that both the natural frequency and the critical buckling load increase with the use of an elastic foundation.
From the above literature review, it is noted that the aforementioned researches only considered the mechanical behaviors of FGCNTRC beams resting on linear elastic foundations. However, with the increase in the applied external load, the beam’s transverse deflection will influence the axial force and the resulting governing equations become coupled and nonlinear. In this case, the linear elastic subgrade model is inadequate to describe the real behavior of the foundation and hence the use of a more sophisticated nonlinear one becomes inevitable. However, to the best knowledge of the authors, no research has examined the mechanical behaviors of FGCNTRC beams interacting with nonlinear elastic foundations. Moreover, composite structures made of FGMs are often operated in a thermal environment and the initial thermal stresses due to the temperature change will occur, which tends to affect the mechanical behaviors of structures [24]. However, from the aforementioned studies, only the temperature dependence of material properties was considered and the influence of initial thermal stresses was neglected.
Wavelet methods have been developed as a new powerful tool for mathematical analysis and engineering computation in recent years. Different types of wavelets have been employed in numerical approximations, such as Chebyshev [25], Legendre [26], Daubechies [27], and Haar wavelets [28]. Among them, the Haar wavelet has gained extensive attention due to its unique properties such as simple applicability, orthogonality, and compact support. Hence, Haar wavelet methods have been widely applied for solving differential, integral, and integrodifferential equations [28–32]. Moreover, the Haar wavelet discretization method (HWDM) introduced in Ref. [34] was adopted successfully for analysis of wide class of solid mechanics problems. For instance, free vibration of composite laminated cylindrical, conical, and annular plate structures was studied by Xie et al. by means of the HWDM [33, 34]. Hein and Feklistova [35] utilized the HWDM to analyze vibration behaviors of axially FG beams with various boundary conditions. Majak et al. [36] investigated the accuracy issues of the HWDM for solving the transverse vibration problem of the FG beam, and they found that the numerical order of convergence of the HWDM is equal to two and is in good agreement with the theoretical value. Recently, Majak et al. developed a new higher order Haar wavelet method (HOHWM) for FGM structures, and their results showed that using the proposed method allows to improve the order of convergence of the HWDM from two to four and to reduce the absolute error by several orders of magnitude [37]. It can be seen from the aforementioned literature review that most of researches on the application of the Haar wavelet approach for the analysis of composite structures are concerned with linear problems, such as free vibration. However, lots of scientific problems in solid mechanics problems are inherently nonlinear. Hence, study on the use of the Haar wavelet method to the nonlinear problem in composite mechanics is extremely necessary.
In the present work, an attempt is made to employ the Haar wavelet method to analyze the nonlinear vibration behavior of FGCNTRC beams in a thermal environment. The considered beam is fixed on a threeparameter nonlinear elastic foundation with cubic nonlinearity and a shearing layer and meanwhile subjected to a uniform temperature rise. The temperaturedependent material properties and the initial thermal stress due to the uniform temperature rise are both taken into consideration. Based on the firstorder shear deformation theory (FSDT) in conjunction with the von Kármán nonlinearity, the nonlinear governing equations of CNTRC beams are derived via Hamilton’s principle. The resultant governing equations and corresponding boundary conditions are first discretized into nonlinear algebraic equations using the HWDW and then solved by a direct iteration technique to obtain the linear and nonlinear frequencies of FGCNTRC beams. To validate the convergence and accuracy of the proposed method, some numerical examples are conducted to compare the results obtained from the proposed method with those available in the open literature. A detailed parametric study is carried out to examine the effects of the CNT volume fraction, distribution type of CNTs, foundation stiffness coefficients, boundary condition, and slenderness ratio on the nonlinear vibration behavior of FGCNTRC beams. Moreover, the influences of the temperature and initial thermal stress are also investigated.
2. Theoretical Formulations
2.1. System Description
Consider an FGCNTRC beam resting on a threeparameter nonlinear elastic foundation with cubic nonlinearity and a shearing layer, as shown in Figure 1. The geometric sizes of the beam are the length and the thickness . The reference surface of the nanocomposite beam is taken to be at its midplane where an orthogonal coordinate system with the origin at one end of the beam is fixed and the x and zaxes are taken along the length and thickness directions, respectively. The nonlinear elastic foundation is assumed to be made up of massless springs with shear interaction among the beam and them, and no part of the beam lifts off the foundation during its large amplitude vibration. The loaddisplacement relationship of the mechanical model of the foundation considered here is given as [38]where denotes the reaction force of the nonlinear elastic foundation; (N/m^{3}) and (N/m) indicate linear and nonlinear coefficients of the Winkler elastic foundation, respectively; (N/m) represents the coefficient of the shear stiffness of the elastic foundation; and is the displacement component in the z direction.
The CNTRC beam adopted here is assumed to be made of a mixture of uniaxially aligned SWCNTs and isotropic matrices. The CNTs are distributed into the matrix material in a functionally graded (FG) or uniform form along the beam thickness direction. In this study, two types of FG distributions are considered, FGX and FGO. Meanwhile, as a counterpart, the uniform distribution (UD) is also treated. Various CNT distribution types in the composite are depicted graphically in Figure 2, and the volume fraction of CNTs for each distribution pattern can be expressed as [23]where denotes the total CNT volume fraction given bywhere denotes the mass fraction, and represent the mass densities of the CNT and matrix, respectively. It should be noted that the total mass fractions in FGX, FGO, and UD CNTRC beams are the same.
(a)
(b)
(c)
The effective material properties of CNTRCs vary continuously in the beam thickness direction and are calculated using the extended rule of mixtures as [4]where , , and are Young’s modulus and shear modulus of SWCNTs, respectively, and and indicate the corresponding properties of the isotropic matrix. The coefficients , , and are the efficiency parameters to explain the scaledependent material properties and are evaluated by matching the effective properties of CNTRC obtained from the MD simulations with those from the rule of mixtures [39]. Moreover, in Equation (4), and indicate the volume fractions of CNTs and matrix phases, respectively, which satisfy the condition . It can be seen from Equation (4) that the effective material properties of CNTRC beams are position dependent.
The mass density, effective Poisson’s ratio, and thermal expansion coefficient of the CNTRC composite can also be obtained according to the rule of mixtures as follows [22]:in which and are the Poisson ratios of SWCNTs and the matrix, respectively, and and denote the corresponding thermal expansion coefficients.
2.2. Governing Relations and Equations
Based on the firstorder shear deformation beam theory, the displacement field of a CNTRC beam is expressed aswhere indicate the displacement components of an arbitrary point in the beam; denote the displacement components of the point on the midplane; denotes the rotation of the beam cross section; and is the time. The von Kármántype nonlinear straindisplacement relationships associated with the thermal effect can be written as [40]where is the temperature change with respect to a reference temperature. It is assumed that the considered beam is initially stressfree at the reference temperature and then subjected to a uniform temperature rise , where is the current temperature.
The force and moment resultants of the CNTRC beam based on the linear constitutive relationship are given by [40]where , , and are the axial force, bending moment, and shear force, respectively. Here, is the shear correction factor and is taken as in this study. The stiffness components are defined aswhere and . Here, and are the thermal force and moment resultants,
By means of Hamilton’s principle, the governing differential equations of a FGCNTRC beam resting on a nonlinear elastic foundation in a thermal environment can be derived as follows [20, 23]:where are the inertia terms.
Substituting Equations (8)–(10) into Equation (11), the nonlinear governing equations of the considered beam in terms of displacements can be obtained as
In the present study, the boundary conditions at the two ends of the beam can be considered as either a clamped end or a hinged end, which are given by
In the harmonic vibration analysis, the displacements can be expressed aswhere is the natural frequency and ; , , and denote the unknown displacement functions to be determined. Substituting Equation (14) into Equation (12) and introducing the following normalized variable , we can obtain the nonlinear governing equations in terms of a single nondimensional variable as follows:where the , , and are the constant coefficients defined in Appendix A. It should be noted that the variable satisfies so that the Haar wavelet method can be directly used to solve the given equations.
3. Solution Methodology
In this section, a simple and effective numerical approach based on the Haar wavelet discretization method (HWDM) will be adopted to solve the nonlinear governing equations of the CNTRC beam. The HWDM is an effective approach for solving differential, integral, and integrodifferential equations [28, 41] and will be used to discretize the derivatives in the governing equations and the boundary conditions.
3.1. The Haar Wavelet Functions and Their Integrals
The Haar wavelet is known as one of the simplest orthonormal wavelets with a compact support. Mathematically, the Haar wavelet family is defined as a group of square waves in with magnitude in given intervals and zero elsewhere, which is expressed as [35, 42]where , , and . The integer indicates the factor of scale, where is the maximal level of resolution. The parameter denotes the translation parameter (). The index is defined as , and the maximal value is (); the minimal value is reached when and . It should be noted that case corresponds to the scaling function .
Let us assume that is a square integrable and finite function in the interval and it can be expanded into the Haar wavelet series with infinite terms. In practice, the series need to be truncated, and only the finite terms of the Haar wavelet series are considered. Hence, the function can be expressed as a finiteterm series, that is,where () denote unknown wavelet coefficients.
To solve differential equations using the HWDM, the integrals of the Haar wavelet functions (16) are required which can be calculated analytically as follows [36]:where indicates the integral order of the Haar wavelet function. In the case , the function will be obtained.
Let us divide the interval into 2M subintervals of equal length, ; the coordinates of the collocation points are given as
Substitution of the collocation points in Equation (19) into Equations (16) and (18) leads to the Haar coefficient matrix and the integral coefficient matrices as follows:where and are matrices and will be used to discretize the given governing equations in the next section.
3.2. Solution Procedure
Firstly, according to the HWDM [34], the highestorder derivatives of the unknown displacement functions with respect to the normalized variable in Eq. (15) can be expanded by the truncated Haar wavelet series. Hence, it is assumed that these functions can be expressed aswhere , , and are the unknown wavelet coefficients and indicates the Haar wavelet function defined in Equation (16). Integrating Equation (21) and substituting Equation (19) leads to the following expressions:where and are the integrating constants to be determined.
The evaluation of Equations (21) and (22) at the given collocation points can be written in matrix form aswhere , , , , , , and , .
Substituting Equation (23) into Equation (20) with some simple manipulation, the governing equations of the CNTRC beam can be converted into the following nonlinear algebraic equations which are expressed in the matrix form aswhere and . The matrices , , , , and are constant matrices, but and are dependent on the displacement component , which are given in Appendix B.
The discretization of the boundary conditions can be carried out in the same way. For example, for a clamped beam in both ends, the boundary conditions can be expressed using HWDM as
Equation (25) can also be written in the matrix form aswhere , , , and are listed in Appendix B. It should be noted that the matrices , , , and are varied for the different boundary conditions.
Assembling of Equations (24) and (26) leads to the global equations of the nonlinear vibration of the FGCNTRC beam resting on the elastic foundation in a thermal environment, which are written aswherewhere is the unknown displacement vector. , , , and denote the linear and nonlinear stiffness matrices, the stiffness matrix due to the uniform temperature rise, and the mass matrix, respectively. It should be noted that the coefficient matrices above are all dependent on the current temperature . Moreover, it can be seen that the solution of Equation (27) is essentially a nonlinear eigenvalue problem.
An direct iterative method adopted by Mirzaei and Kiani [43] is used to solve the system of nonlinear eigenvalues in Equation (27). First, to solve this equation, the nonlinear terms are set to zero and the resulting linear eigenvalue problem is solved to obtain the linear nature frequencies and the corresponding linear eigenvectors . Then, the linear eigenvectors are used to evaluate the nonlinear coefficient matrix . The eigenvalue problem is solved again to obtain the nonlinear nature frequencies and eigenvectors , and subsequently the nonlinear coefficient matrix is reevaluated using the obtained nonlinear eigenvectors . The iterative procedure continues until the frequency values from the two subsequent iterations “r” and “r + 1” satisfy the prescribed convergence criteriawhere is a smallvalue number and, in the present study, it is taken to be 0. 01%.
4. Results and Discussion
In this section, firstly, the convergence and accuracy of the present method are demonstrated by comparing the obtained results with those available in the open literature. Then, parametric studies are conducted to show the influences of the CNT volume fraction, CNT distribution type, slenderness ratio, boundary condition, and elastic foundation coefficients on the nonlinear vibration behavior of the FGCNTRC beams resting on a nonlinear elastic foundation. In addition, the effects of the temperature rise and the initial thermal stress are also discussed.
In what follows, otherwise specified, the CNTRC beam considered here is made of the PMMA matrix and (10, 10) armchair SWCNT reinforcements. The material properties of the PMMA matrix are assumed to be temperaturedependent and are given bywhere and is the room temperature, which is assumed to be and those of SWCNTs at the given temperatures are listed in Table 1. To establish a temperaturedependent relationship, a thirdorder interpolation is used to estimate the material properties of SWCNTs as a function of temperature. For , the variations of the material properties of SWCNTs with respect to the temperature are as follows:

For three different CNT volume fractions, the CNT efficiency parameters ( = 1, 2, 3) are as follows: = 0.137, = 1.022, and = 0.715 for = 0.12; = 0.142, = 1.626, and = 1.138 for = 0.17; = 0.141, = 1.585, and = 1.109 for = 0.28.
For simplicity, the letters C and H are used to denote clamped and hinged boundary conditions, respectively. For instance, CH represents the beam with one end clamped and the other end hinged. The dimensionless stiffness coefficients of the elastic foundation are defined as follows:where , where is the value of at room temperature.
4.1. Convergence and Comparison Studies
In this subsection, the convergence of the proposed method for the vibration analysis of FGCNTRC beams is first examined to establish the maximal level of resolution of the truncated Haar wavelet series to achieve accurate results. Table 2 presents the linear fundamental frequency parameters of CC FGCNTRC beams with varying maximal level of resolution . The linear fundamental frequency parameters are defined as . The geometrical sizes are = 0.1 m and = 25. The material properties used are defined in Equations (30) and (31) at room temperature. As a comparison, the results available in Ref. [18] are also included in Table 2. It can be seen from this table that, by increasing the maximal level of resolution , the numerical results converge stably and the converged values are in close agreement with those of Ref. [18]. In addition, based on the obtained frequency results for three successive values of , the numerical rate of convergence can be estimated as follows [36]:where is the obtained result in the case of . Here, the estimated rates of convergence are also listed in Table 2 and it is observed that the numerical rates of convergence tend to two for all the cases, which is in agreement with the theoretical result given in Ref. [44]. Moreover, it can be noted that the results obtained are nearly unchanged with the increase in when it reaches a certain value of . Thus, unless otherwise stated, the maximal level of resolution is chosen as in the following examples.

A comparison study is presented in the next example for HH FGCNTRC beams resting on linear elastic foundations with various elastic foundation coefficients. The first three linear frequency parameters ( = 1, 2, 3) are evaluated and compared with the available results in the open literature, as shown in Table 3. The material properties used are taken from Ref. [23], and the geometrical parameters are = 0.1 m and = 15. It can be seen that the obtained frequency parameters are in good agreement with the available results in the open literature [23].

In the next example, the nonlinear frequency ratios at different dimensionless vibration amplitude are given in Table 4 for UDCNTRC beams with different boundary conditions and compared with those given by the open literature. The dimensionless vibration amplitude is defined as , where denotes the vibration amplitude. The material properties used are the same with those given by Ke et al. [16]. As can be seen, our results are in excellent agreement with those obtained by Ke et al. [16] and Lin et al. [45].

The convergence and comparison studies show the excellent convergence and accuracy of the proposed Haar wavelet method for the nonlinear vibration analysis of FGCNTRC beams.
4.2. Parametric Studies
In this subsection, parametric studies will be presented to examine the nonlinear vibration behaviors of FGCNTRC beams resting on a nonlinear elastic foundation. In the rest of the manuscript, the material properties of CNTRC are taken as the values in Equations (30) and (31).
As a first case, the effects of the distribution and volume fraction of CNTs together with the boundary condition on the nonlinear vibration behaviors of FGCNTRC beams are examined. Table 5 presents the linear fundamental frequency parameters and nonlinear frequency ratios at different dimensionless vibration amplitude with various distribution types of CNTs, various CNT volume fractions, and different boundary conditions. Three kinds of boundary conditions (CC, CH, and HH) are considered. Tabulated results are given for CNTRC beams with = 0.1 m and = 25 resting on a nonlinear elastic foundation at room temperature. The stiffness coefficients of the foundation are , and . It is seen from the table that the nonlinear frequency ratio increases with an increase in vibration amplitude, showing the wellknown “hard spring” vibration behavior. Among the CNT distributions considered here, FGX leads to the highest linear fundamental frequency, followed by UD and FGO, but for the nonlinear frequency ratios , FGO yields the highest value, followed by UD and FGX. This can be explained by the fact that the CNT reinforcements distributed close to the top and bottom are more efficient than those distributed near the midplane for increasing the stiffness of CNTRC beams. Besides, for all the boundary conditions considered, the CC beam has the highest linear fundamental frequency but lowest nonlinear frequency ratios, followed by CH and then HH beams. It can also be seen that the linear fundamental frequency increases with an increase in the CNT volume fraction, but it is unexpected that the CNTRC beams with = 0.17, rather than = 0.12, have the lowest frequency ratio. This phenomenon was also found by Wu et al. [18].

The influences of the slenderness ratio on the linear fundamental frequency parameters and nonlinear frequency ratios of the FGCNTRC beams with and without elastic foundation are investigated in Table 6. The results are provided for the FGX CNTRC beam with = 0.17 and = 0.1 m subjected to the HH boundary condition at room temperature. It is found that the linear fundamental frequency parameter decreases with an increase in the slenderness ratio. For the beam without the elastic foundation, the increase in the slenderness ratio yields a decrease in the nonlinear frequency ratio, but, for the beam resting on the nonlinear elastic foundation, the nonlinear frequency ratio firstly decreases and then increases with an increase in the slenderness ratio.

Next, the effects of the stiffness coefficients of the nonlinear elastic foundation on the nonlinear vibration behaviors of FGCNTRC are studied. The variations of the nonlinear frequency ratio of FGX CNTRC beams against the dimensionless vibration amplitude are displayed in Figure 3 for different stiffness coefficients. The corresponding dimensionless linear fundamental frequencies are also shown in Figure 3. The obtained results are given for the HH beam with = 0.1 m and = 25 resting on a nonlinear elastic foundation at room temperature. The volume fraction of CNTs is taken as = 0.28. It can be seen from Figure 3(a) that the linear fundamental frequency increases and the nonlinear frequency ratio decreases with an increase in the linear stiffness coefficient . The same trend is also observed from Figure 3(c) for the shear stiffness coefficient . It can be noted that, different from the above two linear elastic coefficients, an increase in the nonlinear stiffness coefficient leads to a larger nonlinear frequency ratio, but the linear fundamental frequency remains constant.
(a)
(b)
(c)
In the following, the effects of the temperature rise and initial thermal stress on the nonlinear vibration behavior of FGCNTRC beams on nonlinear elastic foundations are discussed.
In Figure 4, the variations of the dimensionless linear fundamental frequency against the temperature rise with and without considering the initial thermal stress are compared. Three kinds of elastic foundations are considered. It can be seen from Figure 4 that as the temperature rise increases, the linear fundamental frequency parameter decreases monotonically to zero. In the case considering the initial thermal stress, the decrease in the linear fundamental frequency parameter is much rapider than that without considering the initial thermal stress. The zero values of the dimensionless linear fundamental frequency represent the onset of instability or buckling of the beams, and the corresponding temperature is the critical temperature at which the beam begins to be instable or buckled. Besides, it is found that, by increasing the linear and shear stiffness coefficients, both the linear fundamental frequency and the critical temperature increase.
Figure 5 presents the variations of the nonlinear frequency ratio of FGCNTRC beams against the dimensionless vibration amplitude under different temperature rises. Both the case with and without the initial thermal stress are taken into consideration. It is found that the nonlinear frequency ratio increases with an increase in the temperature rise. In addition, when the initial thermal stress is considered, the nonlinear frequency ratio increases.
As can be seen from Figures 4 and 5, the initial thermal stress has remarkable effects on the nonlinear vibration behavior of FGCNTRC beams and hence cannot be neglected when CNTRCs are operating in a thermal environment.
5. Conclusion
The nonlinear vibration behavior of FGCNTRC beams resting on a nonlinear elastic foundation in a thermal environment was investigated in this paper using the HWDM. The FSDT together with the von Kármán nonlinearity was adopted to model the kinematic relationships. Both the initial thermal stress and the temperature dependence of the material properties were considered in the theoretical modeling. The nonlinear governing equations of CNTRC beams on nonlinear elastic foundations were derived and then solved using the HWDM in conjunction with the direct iteration technique to obtain the linear and nonlinear natural frequencies. The convergence and accuracy studies were conducted, and the results revealed that the numerically estimated rate of convergence of the proposed method tends to two, which is consistent with the convergence theorem, and good accuracy of the proposed method is confirmed by comparing the present results with those available in the literature. Finally, parametrical studies were carried out to explore the influences of the volume fraction of CNTs, distribution type of CNTs, slenderness ratio, boundary conditions, foundation stiffness coefficients, temperature rise, and initial thermal stress on the nonlinear vibration behavior of FGCNTRC beams. The following conclusions can be carried out:(a)The beams with FGX CNT distribution have the highest linear fundamental frequency but lowest nonlinear frequency ratios, followed by UD and then FGO beams(b)The CC beam has the highest linear fundamental frequency but lowest nonlinear frequency ratios, followed by CH and then HH beams(c)The linear fundamental frequency increases with an increase in the CNT volume fraction or a decrease in the slenderness ratio(d)The linear fundamental frequency increases and the nonlinear frequency ratio decreases with an increase in the linear or shear stiffness coefficients of the foundation; the increase in the nonlinear stiffness coefficient of the foundation yields larger nonlinear frequency ratio, but the linear fundamental frequency remains constant(e)By increasing the temperature rise, the linear fundamental frequency decreases, yet the nonlinear frequency ratio increases(f)The initial thermal stresses have significant effects on the nonlinear vibration behavior of CNTRC beams and cannot be neglected
This study offers an insight into the nonlinear vibration behaviors of CNTRC beams resting on nonlinear elastic foundations in a thermal environment.
Appendix
A. Detailed Expressions of the Constant Coefficients in the Governing Equations
The coefficients in Equation (15) are given by
B. Detailed Expressions of the Submatrices in the Mass and Stiffness Matrices
The coefficient matrices defined in Equation (24) are given bywhere stands for the product of a vector and a matrix and is defined as follows:
The coefficient matrices of a clamped beam at both ends defined in Equation (26) can be expressed aswhere
Data Availability
All data used to support the findings of this study are included within this published article. The source code that supports the findings of this study is available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that there are no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (Grant nos. 51575419 and 51705387), the 111 Project (Grant no. B14042), the Joint Funds of Education of China (Grant no. 6141A02022107), and the Fundamental Research Funds for the Central Universities (Grant no. SPSY101405).
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Copyright © 2018 Jianyu Fan and Jin Huang. 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.