Free Vibration Analysis of a Rotating Non-Uniform Blade with Multiple Open Cracks Using DQEM

In this paper, differential quadrature element method (DQEM) is used to analyze the free transverse vibration of a multiple cracked non-uniform Timoshenko blade rotating with a constant angular velocity. The blade’s deflection is assumed to be small so that the nonlinear terms can be neglected. Governing equations, compatibility conditions at the damaged cross-sections and boundary conditions are derived and formulated by the differential quadrature rules. First, the precision of the proposed method is confirmed by the exact solutions available in the literature. Then, the effect of angular velocity value as well as the location, depth and numbers of the cracks are investigated on the natural frequencies of the blade. The advantages of the proposed method are applicable for blades with any arbitrary variable section and less time-consuming for cases with several cracks.


Introduction
Existence of cracks causes to abrupt failure in structures and sometimes is difficult to be detected. Depended on the position and extension of the cracks, eigen parameters of the structure would be affected and it can be used as a non-destructive method to detection of the damages in the structure. Using vibration based detection methods has been one of the most important challenges confronting active research in the last decade. Several authors have proposed techniques to estimate the effects of damage on the eigen parameters of a structure (usually known as the direct problem), while others have dealt with the problem of detecting, locating and quantifying the extent of damage (well-known as the inverse problem). A few authors have addressed aspects concerning the identification of the cracks, leading to explicit expressions for the solution of the inverse problem [1]. Also, variation of the natural frequencies with the depth and position of a concentrated crack has been explained in the references such as Morassi [2,3] and Narkis [4]. During the recent years, more attention has been dedicated to the solution of the direct analysis problem of vibrating beams in the presence of multiple cracks. Some research works have introduced the concentrated cracks on the beams by considering a local reduction of the flexural stiffness. According to this model, a crack can be represented as an elastic link connecting the two adjacent beam segments. In other words, it is modeled as a mass-less rotational spring whose stiffness is dependent on the extent of the damage. The first studies were based on the division of the beam into sub-beams between two consequent cracks [5,6].
In most of the studies, the beam is considered as a slender one which can be analyzed by Euler-Bernoulli beam theory. Thus, shear deformation and rotary inertia of the beam are neglected. Recently, some authors have focused on vibration analysis of the cracked short beams which are modeled by Timoshenko beam theory. Zheng and Fan [7] used modified Fourier series to investigate vibration of the cracked Timoshenko beams with different boundary conditions. Lele and Maiti [8] proposed a new method based on the Timoshenko beam theory considering shear effects. In this method, the characteristic equation to obtain the natural frequencies for the cantilever cracked beam is expressed as an eighth-order determinant equated to zero. Lin [9] presented a closed-form solution for prediction of natural frequencies of a cracked simply supported Timoshenko beam. Using transfer matrix technique, Khaji et al. [10] introduced a closed-form solution for crack detection problem of Timoshenko beams for the various boundary conditions.
In above-mentioned studies, geometrical compatibility conditions at the crack position are only considered as a discontinuity in the slope due to the bending and vertical displacement. Also, some researchers modeled the cracked beam as two segments connected by two mass-less springs (extensional and rotational). This model promotes the discontinuities in rotations due to the bending and vertical displacement which are proportional to bending moment and shear force transmitted by the cracked section, respectively. Krawczuk et al. [11] used spectral element method to focus Free Vibration Analysis of a Rotating Non-Uniform Blade with Multiple Open Cracks Using DQEM on modal and elastic wave propagation analysis of a cracked Timoshenko beam. Loya et al. [12] obtained natural frequencies for bending vibrations of a simply supported cracked Timoshenko beam with simple expressions for the natural frequencies.
The DQM is a numerical approach for solving the differential equations which was initially introduced by Bellman et al. [13], and further was developed by Bellman and Roth [14]. Bert and his co-workers [15][16][17][18][19][20], contributed to the development of the method and used the DQM for the analysis of the structural problems. This method can be applied only for continuous problems. Hence, DQM was promoted to use for problems with local discontinuities. Recently, differential quadrature element method (DQEM) has appeared as a numerical technique for analyzing the structures with some local discontinuities in loading, material properties and geometry. Thus, this method is applied to solve many problems especially in the vibration analysis. Chen [21][22][23][24] solved different vibration problems using DQEM. E.g., he studied about the vibration of non-uniform shear deformable axisymmetric orthotropic circular plates [21], vibration analysis of non-prismatic shear deformable beams resting on elastic foundations [22], in-plane vibration of curved beam structures [23], and out-of-plane vibration of non-prismatic curved beam structures regarding the effect of shear deformation [24]. Malekzadeh et al. [25] presented a semi-analytical solution for the free vibration analysis of thick plates with the two opposite edges simply supported using DQEM. DQEM was hired by Torabi et al. [26] for free vibration analysis of a non-uniform cantilever Timoshenko beam with multiple concentrated masses. Torabi et al. [27] employed DQEM to analyze the free transverse vibration of multiple cracked non-uniform Timoshenko beams with general boundary conditions.
In this paper, DQEM is used to analyze free transverse vibration of a damaged non-uniform Timoshenko blade. Therefore, the accuracy of the proposed DQEM is first confirmed by the exact solutions available in the literature for the uniform Timoshenko beam. Then, the effect of location, depth and numbers of the cracks as well as the angular velocity of blade are investigated on the natural frequencies.

Differential Quadrature Method
In the differential quadrature method (DQM), all derivatives of a function are approximated by means of the weighted linear sum of the functions values at a pre-selected grid of discrete points as where N is the number of grid points in the x-direction. In Eq.
(1), A (r) is the weighting coefficient associated with the rth order derivative. This matrix is given for the first-order derivative as [18] (1) For higher-order derivatives, this matrix can be obtained from a recursive relation as A (r) =A (1) A (r-1) . In addition to number of grid points, distribution of them is an important factor in convergence of solution. A well-accepted set of the grid points is the Gauss-Lobatto-Chebyshev points given for interval [0,1] by 1 ( 1) 1 cos[ ] , ( 1, 2,3,..., ).

Governing Equations
As depicted in Fig. 1, a rotating non-uniform Timoshenko blade with multiple open cracks is considered. The governing equations for free vibration of an intact beam rotating with constant angular velocity can be written as [28] where w(x, t), ψ(x, t), A(x), and I(x) are vertical displacement, rotation due to bending, cross-section area and second moment of inertia about the neutral axis, respectively. Also, ρ, E, G, Ω and k are mass density, modulus of elasticity, shear modulus, angular velocity of spin and shear correction factor, respectively. Also T(x) is the centrifugal force provided due to the rotation of the blade that varies along the span wise direction of the blade as Applying separation of variables method, the displacement w(x,t) and rotation due to bending ψ(x,t) can be assumed as where ω is angular natural frequency of vibration. In order to obtain the dimensionless form of cross-sectional area and the second moment of inertia, these values are divided into the cross-sectional area and second moment of inertia of the beam at the clamped edge.
Substituting Eqs. (5)-(7) into the Eq. (4), the following set of differential equations obtain: For the ith sub-beam, Eq. (8) can be written as By introducing the following dimensionless parameters: Eq. (9) can be rewritten in the dimensionless form as 104 Free Vibration Analysis of a Rotating Non-Uniform Blade with Multiple Open Cracks Using DQEM By considering the equal number of grid points for all sub-beams, it can be clearly expressed that hence, Eqs. (11) can be simplified as It should be stated that in this paper, in order to simplify the DQ analogue, a modified form of the weighting coefficients of ith sub-beam are defined as Hence, the DQ analogue of the governing set of equations of element "i" becomes In the last equation, As mentioned in Refs. [29][30][31], in order to eliminate the redundant equations, the equations of motion should be written just for the domain points. Therefore, Eq. (15) should be represented as . . (1) (1) As mentioned in Ref. [32], Eq. (19) may be rearranged in order to separate the boundary, domain, and adjacent displacement and rotation components as

Compatibility Conditions
Compatibility conditions at common nodes of two adjacent sub-beam in the vicinity of each crack (x=x c ), are continuity in natural parameters and discontinuity in the geometrical parameters as where h is height of the beam, Ɵ(α) and q(α) are functions of the ratio of the crack depth to the beam height (α) and depend on the beam cross-section geometry. Also, M and V are bending moment and shear force, respectively and are (1) (2) ( ) (1) Free Vibration Analysis of a Rotating Non-Uniform Blade with Multiple Open Cracks Using DQEM presented for ith sub-beam as [28] ( ) Which can be rewritten using Eqs. (5) and (10) as For a rectangular cross-section, the value of θ(α) and q(α) have been presented respectively, by Tada Using Eqs. (17), (25) and (26) where dimensionless parameters in Eq. (28) are defined as: (1) (1) .

Boundary Conditions
Since a rotating blade acts as a cantilever beam, boundary conditions can be defined as In Eq. (45), I and [0] are identity and zeros matrices, respectively. Using (44), natural frequencies and corresponding modes will be obtained. Meanwhile, the corresponding modes will be completed by applying the Eqs. (33) and (42). It should be stated that in this paper, number of grid points will be chosen to satisfy the following relation for convergence of first m frequencies:

Numerical Results and Discussions
In order to validate the proposed method, a non-rotating uniform beam with a single crack in the middle of its length is considered. Material properties are adopted as: Young's modulus E=210 GPa, mass density ρ=7860 kg/m 3 , and Poisson's ratio υ=0.3. The geometric parameters of the beam are selected as: depth, h=25 mm, thickness, b=12.5 mm, and length, L=100 mm. Also, the value of the shear correction factor (k) for the rectangular cross-section is taken as 5/6 [33]. Table 1 shows the first four frequencies for two dimensionless depth value of the crack (α=0.2, 0.35) and corresponding modes are depicted in Fig. 2. Comparison of the proposed method and exact solution by Khaji et al. [10] show the accuracy and convergence of the method presented in this study.      Table 2 for different values of the angular velocity of spin. The versatility of the proposed technique is confirmed by comparison of obtained results with the results of exact solution presented by Kaya [28].
Also, a conical blade with r=0.03, s=0.05 is considered that diameter varies as d=d 0 (1-0.5ξ). Fig. 3 shows the values of the lowest four frequencies versus the value of the angular velocity for different values of the hub radius. As shown in this figure, values of the all frequencies increase as the value of the angular velocity and hub radius increase. This is due to increasing of centrifugal force for higher values of angular velocity and hub radius.
Consider a uniform blade (r=0.03, s=0.05, h/L=0. 1, δ=0.4, γ=1) with equally spaced similar open cracks. Fig. 4 shows the values of the first three dimensionless frequencies versus intensity of crack for various numbers of open cracks. As shown in this figure, because of the reduction in the stiffness of the beam, as numbers and depth of the cracks increase, all frequencies decrease.  03, s=0.05, δ=0.05, γ=1) is considered with the width and depth of b=b 0 (1-0.1ξ) and h=h 0 (1-0.1ξ) where h 0 =0.5L, respectively. Consider a single crack with a constant depth as a c =ηh 0 in a desired position on the blade. Fig. 5 shows the variation of the first four frequencies versus the position of the crack for various values of η. It can be concluded that the effect of the crack has its highest effect on a natural frequency, when the crack is located at a point which has maximum value of curvature in the corresponding normal mode; for example clamped end for first mode. Reversely, it has little effect on a natural frequency when the crack is located at the inflection point of the corresponding normal mode like the free end for all modes. It should be noticed that as this figure reveals, numbers of these points increase in higher modes.

Conclusions
DQEM analysis of free transverse vibration of a rotating non-uniform Timoshenko blade with multiple open cracks was presented. Comparison of the proposed method with the exact solutions available in the literature revealed the excellent accuracy of this method. Also, effects of location, depth, and numbers of cracks as well as the angular velocity of the spin are investigated on the natural frequency of the blade. The results showed that existence of cracks in the blade decrease the frequencies. Also, increasing the numbers of the cracks and the crack depth reduce the natural frequencies. It can be concluded that the damage has the highest effect on a natural frequency when the crack is located at a point which has maximum value of curvature in the corresponding normal mode. Reversely, it has little effect on a natural frequency when the crack is located at inflection point of the corresponding normal mode. Numerical results also showed that all natural frequencies increase when angular velocity of spin increases.