In this paper, linear vibrations of axially moving systems which are modelled by a fractional derivative are considered. The approximate analytical solution is obtained by applying the method of multiple scales. Including stability analysis, the effects of variation in different parameters belonging to the application problems on the system are calculated numerically and depicted by graphs. It is determined that the external excitation force acting on the system has an effect on the stiffness of the system. Moreover, the general algorithm developed can be applied to many problems for linear vibrations of continuum.
Keywords:linear vibrations; dynamic analysis of continuum; fractional derivative; perturbation method
Fractional derivatives are useful for describing the occurrence of vibrations in engineering practice. The studies involving fractional calculus and its applications to mechanical problems appear widely in different studies . The advances in fractional calculus focus on modern examples in differential and integral equations, physics, signal processing, fluid mechanics, viscoelasticity, mathematical biology and electrochemistry .
The general solution procedure including all the problems instead of separately solving each problem is quite advantageous. Many different linear or nonlinear models addressing vibrations of continuum appear in the literature. Some of these works are as follows: Pakdemirli  developed a general operator technique to analyse the vibrations of a continuous system with an arbitrary number of coupled differential equations. Özhan and Pakdemirli [4-6] suggested the general solution procedure to investigate a more general class of continuous systems such as gyroscopic and viscoelastic systems. Ghayesh et al. considered a general solution procedure for the vibrations of systems with cubic nonlinearities subjected to nonlinear and time-dependent boundary conditions. Hence, a general solution is adapted to solve the dynamic problems constituting continuum.
In recent years, there has been a growing interest in the area of fractional variational calculus and its applications [8,9]. Fractional calculus, which is used successfully in various fields such as mathematics, science and engineering, is one of the generalisations of classical calculus. The merits of using a fractional differential operator lie in the fact that few parameters are needed to accurately describe the constitutive law of damping materials . Bagley and Calico  modelled the mechanical properties of damping materials by fractional order time derivatives. The mechanical scientific community recognised the significance of fractional calculus for modelling viscoelastic material behaviour thanks to Bagley et al.. Also, they studied longitudinal vibrations of rods and flexural vibrations of beams based on viscoelastic fractional derivative models . For solving dynamic problems with a fractional derivative, the analysis of free damped vibrations of various mechanical systems, whose behaviour is described by linear viscoelastic models with fractional derivatives, were studied by Rossikhin and Shitikova . Mainardi  considered the problems in continuum mechanics related to mathematical modelling of viscoelastic bodies. Cooke et al. investigated the response of a viscoelastic beam with a fractional derivative. Skaar et al. used a fractional standard linear solid model. French and Rogers  presented a small group of structural dynamics problems for which fractional calculus was adopted.
The general solution allows one to investigate the effects on a dynamic analysis of continuum whose damping term is modelled by a fractional derivative. An engineering problem which is a special application of the general model developed in this study was formerly considered in . In our previous study , the analysis of primary and parametric resonance for the external excitation term having ε-order was performed. As the forced term is obtained in one-order, sum or difference type of resonance also appears in the present model. The method of multiple scales is used in the analysis. Thus, the amplitude and phase modulation equations are produced in terms of operators. In addition, the variations of the curves with respect to the dimensionless parameters are presented. Finally, the effects of fractional damping on the linear vibrations of continuum are investigated in detail.
2 Equation of motion
Let us consider a non-homogeneous and dimensionless model as follows:
where represents the displacement, x and t are the spatial and time variables. ε is a small dimensionless parameter, F is the external excitation force, and are the internal and the external excitation frequencies, respectively. defines the fractional derivative of order α. The dot denotes differentiation with respect to time t; , and are self-adjoint operators involving only the spatial variable x, and are linear operators of boundary conditions. Here, the associated boundary conditions are linear, homogeneous and free from the time.
3 Method of multiple scales
The method is directly applied to the partial differential equation (1). Thus, we can write
where is the usual fast-time scales, is the slow-time scales. Time derivatives are expressed in terms of fast and slow time scales as follows :
In order to calculate the fractional derivative of the exponential function, we may use Riemann-Liouville derivatives 
At ε-order, the solution is
where is related to the non-secular terms and other parts of the solution are associated with the secular terms. For the approximate solution at ε-order, one substitutes Eq. (12) into Eq. (10). Thus, the resulting equation is obtained as
Then, five cases occur as follows.
4 Case studies
In this section, we assume that one dominant mode of vibrations exists. Depending on the numerical values of natural frequency, five different cases occur.
This case corresponds to absence of any resonances. Then Eq. (18) turns into
Thus, the solvability condition  requires
Finally, the amplitude is obtained as
and in the same sense, the displacement is calculated as
Principal parametric resonance occurs in this case. Thus, the internal excitation frequency is considered as
For the stability analysis, one introduces the transformation
Substituting (29) and (30) into Eq. (27), and separating real and imaginary parts, the representation of the system of equations with the coefficient matrix is given as
For the steady state condition, λ must be zero. Therefore, the stability boundaries are
Arranging Eq. (18), one obtains
Solving Eq. (36),
Substituting the polar form
into Eq. (39) and separating the equation into real and imaginary parts, we obtain
By the same mathematical manipulation, the stability boundaries are found as
4.5 Sum and difference type of resonance
Substituting Eq. (41) into Eq. (46) and separating into real and imaginary parts, we get
For steady-state solutions, the equations must be rearranged according to the condition (44). Then the stability boundaries are obtained as
5.1 The longitudinal vibrations of a tensioned rod
We will investigate longitudinal vibrations of an axial loaded rod with linear fractional damping for application. This problem is quite important in engineering applications. Also, the rods are used as a structural element in many civil and mechanical engineering problems. The governing equation motion of a tensioned rod with fractional damping is introduced as
where is the longitudinal displacement of the rod, ε is a small dimensionless parameter, m denotes the mass and defines the damping coefficient. It is assumed that the tension P is characterised as a small periodic perturbation on the steady-state tension
where is the frequency of the rod . Introducing the dimensionless quantities as
the new dimensionless parameters are
where L is the length of the rod. Thus, the dimensionless equation is presented as
For the simply supported beam, the boundary conditions are
Considering Eq. (55), the operators corresponding to the general model are
and the other terms are
Finally, the solution of eigenvalue-eigenfunction problem (62) is
In this problem, three different cases arise at ε-order as follows.
By the general solution (23), we may write
Thus, the displacement is obtained as
In Figure 1, it is observed that the damping and the natural frequency changed for different modes. The damping decreases and the natural frequency increases as the number of modes enlarges.
Figure 1. Displacement-time graph for different mode values (,,).
Thus, the amplitude and displacement are calculated respectively as follows:
Furthermore, the supplementary term of the natural frequency from a fractional derivative is
The variation of a supplementary term from a fractional derivative according to α-order is shown in Figure 2. The effects of the order of a fractional derivative on the displacement-time curves are seen readily in Figure 3. The damping accelerates acutely in the classic damping approach, namely .
Figure 2. Supplementary term versusαfor various mode values at(,,).
Figure 3. Displacement-time curves for the different values ofα(,,,,).
Figure 4 shows the effects of α-order on the critical value N and the variation of an unstable region with some different values of α. It is observed that the critical value N becomes zero for . The unstable region diminishes as α increases.
Figure 4. Stability boundaries for various fractional order for second mode ().
5.2 The dynamic analysis of an axially loaded viscoelastic beam resting on foundation
The fractional viscoelastic beam with axial load is resting on linear elastic foundation. This type of foundation is known as Winkler foundation. In the linear Winkler foundation model, denotes the soil coefficient. The beam is modelled by fractional Kelvin-Voight viscoelastic material. The governing equation is given by
and the new dimensionless parameters are
Thus, the operators corresponding to a general model are
and the other terms are
Then Eqs. (13) and (15) reduce to
Finally, the solutions of Eqs. (82) and (84) are
Using the general solution (23), we get
The amplitude and displacement are obtained
Thus, the supplementary term of the natural frequency due to a fractional derivative is
where the coefficient is
6 Conclusion and discussions
In this study, the general model subject to internal and external excitation is developed. The general model proposed for continuum is linear and one-dimensional. The effect of the damping term which is obtained from viscoelastic material properties is modelled with a fractional derivative. The dynamic analysis of the general model is examined by the method of multiple time scales. The approximate solutions are derived in terms of operators. The external force term is considered at order one. This consideration leads to sum and difference type of resonance in addition to primary and parametric resonance cases. The application of the general solution to two specific engineering problems is presented. The solvability boundaries are approximately obtained and numerically illustrated. It is shown that the order of the fractional derivative has an effect on natural frequencies and stability boundaries. It is shown that the stable region becomes smaller with increasing fractional order. And also, the coefficient of a fractional damping term has similar effects to fractional order.
The authors declare that they have no competing interests.
All authors read and approved the final manuscript.
Dedicated to Professor Hari M Srivastava.
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