Published: 31 December 2016

Free vibration analysis of electric-magneto-elastic functionally graded plate with uncertainty

G. Q. Xie1
J. H. Tian2
1Civil Engineering College, Hunan University of Science and Technology, Xiangtan, 411201, China
2School of Mechatronic Engineering, Xi’an Technological University, Xi’an, 710032, China
Corresponding Author:
G. Q. Xie
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Abstract

Free vibration analysis of electric-magneto-elastic functionally graded plate with uncertain parameters is studied in this paper. The elastic, electric and magnetic parameters are regarded as interval variables. Interval analysis of electric-magneto-elastic functionally graded plate are carried out by introducing the interval algorithm into the dynamic equations. A simply supported electric-magneto-elastic functionally graded plate as a numerical example are provided to the validity of the method.

1. Introduction

Electric-magneto material is a new intelligent material transforming energy from one to the other (among magnetic, electric and mechanical energy). Applied in ultrasonic imaging devices, sensors, actuators, transducers and many other emerging components, they have attracted wide and increasing attention to their static and dynamic behaviors in recent decades [1-7]. However, parameters of the studied electric-magneto-elastic structures used to consider as deterministic ones [8-10]. Indeed, it is very difficult to determine accurately the magnetic and electric parameters because errors will arise when they are manufactured or measured, specially the magneto-electric coefficient of mutual induction.

There are a lot of researches on interval static and dynamic analysis of structure, Han and Jiang et al. [11] dealt with the wave propagation problems in composite-laminated plates subjected to uncertainty in load and material property combined the interval analysis method with the hybrid numerical method (HNM). Li, Luo and Sun [12] studied the reliability-based multiobjective optimization by using a new interval strategy to model uncertain parameters. Jiang and Han et.al [13] proposed a method to solve the nonlinear interval number programming problem with uncertain coefficients both in nonlinear objective function and nonlinear constraints. Based on an order relation of interval number, the uncertain objective function is transformed into two deterministic objective functions, in which the robustness of design is considered. Kang, Luo and Li [14] investigated the formulation and numerical solution of reliability-based optimization of structures exhibiting grouped uncertain-but-bounded variations. Wu, Zhao and Chen [15] proposed an improved interval analysis method for uncertain structures.

2. Dynamic equations

The coupling physic equations for anisotropic and linear electric-magneto-elastic solids are given by:

1
σij=Cijklεkl-eijmEm-qijnHn,Dm=eijmεij+gmnEn+αmnHn,Bn=qijnεij+αmnEm+μmnHm, i,j,m,n=1,2,3,

where σij denotes stress tensor, εkl strain tensor, Dm the electric displacement tensor and Bm the magnetic induction tensor, Cijkl, gmn and μmn are the elastic, dielectric and magnetic permeability coefficient tensors. qijn,eijm and αmn are piezoelectric, piezomagnetic and magneto-electric material coefficient tensors.

Tenor forms of the generalized geometric equations are given by:

2
εij=12ui,j+uj,i,i,j=1, 2, 3,Ei=-φ,i, i=1, 2, 3,Hi=-ψ,i, i=1, 2, 3,

where u is the elastic displacement, φ is the electric potential, ψ is the magnetic potential. The comma in the subscript denotes partial derivative.

The governing equations of electric-magneto-elastic functionally graded plate, absent of the body force, electric charge density, and electric current density, are given by:

3
σij,j=ρu¨i,Di,i=0,Bi,i=0.

The functionally graded material parameters are assumed to obey exponential law across the thickness direction (z axis):

4
Cijkl=Cijkl0eηz, gij=gij0eηz, μij=μij0eηz,
qijl=qijl0eηz, eijl=eijl0eηz, αij=αij0eηz, ρ=ρ0eηz,

where η is the exponential factor characterizing the degree of the material gradient in the z-direction, and the superscript 0 is attached to indicate the material coefficients for z= 0. It is obvious that η= 0 corresponds to the homogeneous material case.

For a special case, an orthotropic transverse isotropic solid, the material coefficients in Eq. (1) can be written as:

5
C=C1111C1122C1133000C2222C2233000symC3333000C232300C31310C1212, e=00e11300e22300e3330e2320e31100000,
q=00q11300q22300q3330q2320q31100000, g=g11000g22000g33, α=α11000α22000α33,
μ=μ11000μ22000μ33.

For a simply supported and FGM rectangular plates (a×b), we adopt the solution of the generalized displacement in the form:

6
u1x,y,z,tu2x,y,z,tu3x,y,z,tφx,y,z,tψx,y,z,t=e-ηz2eiωta1cospxsinqya2sinpxcosqya3sinpxsinqya4sinpxsinqya5sinpxsinqy,

where p=mπ/a, q=nπ/b, with n and m are two positive integers.

Substitution of Eq. (6) into Eq. (2), the constitutive Eq. (1), and finally into the governing Eq. (3), yields the following eigenequation:

7
-Α+η2R+RT+η2G-ω2MU=0,

where:

8
U=a1a2a3a4a5T,
R=00pC11330pe3130pq313000qC2233qe3230qq3230-pC31310-qC23230000-pe3110-qe2310000-pq3110-qq2310000, M=ρ000000ρ000000ρ0000000000000,
G=C313100000C23230000symC33330e3330q3330-g330-α330-μ330,
A=A1102×303×2A22, A11=-C11110p2+C12120q2-pqC11220+C12120-pqC11220+C12120-C12120p2+C22220q2,
A22=-C31310p2+C23230q2-e3110p2+e2320q2-q3110p2+q2320q2-e3110p2+e2320q2g110p2+g220q2α110p2+α220q2-q3110p2+q2320q2α110p2+α220q2μ110p2+μ220q2.

3. Interval analysis of free vibration

Let WI is an n-dimensional interval vector of uncertain material parameters or the load, if uncertainty of the density of the plate is not considered, namely, the matrix M is independent of WI, and Eq. (7) can be rewritten as follows:

9
-ΑWI+η2RWI+RTWI+η2GWI-ω2MUWI=0.

Interval variable WI can also be expressed as:

10
WI=W|W=Wc+-1,1 Wr,WRn,

where Wc is the interval midpoint value, Wr the interval radius:

11
Wc=WU+WL2,
12
Wr=WU-WL2,

where WL denotes the lower bound, and WU the upper bound.

The uncertainty level of the interval variable is defined as:

13
Wlevel=WrWc×100 %.

The modal vector UI can be expanded into the first-order Taylor series:

14
UI=UW I=UW c+j=1nUW cWjWjI-Wjc.

The midpoint and interval radius of UI can be obtained from Eq. (14):

15
Uc=UWjc,
16
Ur=j=1nUWcWjWjr,

where:

17
Uc=UL+UU2,
18
Ur=UL-UU2,

where UL denotes the lower bound, and UU the upper bound.

Vibration natural frequencies of the plate can also be expanded into the first-order Taylor series:

19
ωniI=ωniWc+j=1nωniWcWjWjI-Wjc.

The midpoint and interval radius of ωniI can be obtained from Eq. (19):

20
ωnic=ωniWc,
21
ωnir=j=1nωniWcWjWjr.

In order to obtain ωc/Wj, Partial derivative both sides of Eq. (9) with respect to Wj at the midpoint Wc, we have:

22
-ΑWcWj+η2RWcWj+RTWcWj+η2GWcWj-2ωcωcWjMUWc
+-ΑWc+η2RWc+RTWc+η2GWc-ωc2MUWcWj=0.

The two sides of Eq. (22) multiplied by UTWc has:

23
UTWc-ΑWcWj+η2RWcWj+RTWcWj+η2GWcWj-2ωcωcWjMUWc
+UTWc-ΑWc+η2RWc+RTWc+η2GWc-ωc2MUWcWj=0.

From Eq. (9), we have:

24
-ΑWc+η2RWc+RTWc+η2GWc-ωc2MUWc=0.

The transpose of Eq. (24) yields:

25
UTWc-ΑWc+η2RWc+RTWc+η2GWc-ωc2MT
=UTWc-ΑWc+η2RWc+RTWc+η2GWc-ωc2M=01×5.

Substitution of Eq. (25) into Eq. (23), yields the following equation:

26
UTWc-ΑWcWj+η2RWc+RTWcWj+η2GWcWj-2ωcωcWjMUWc
=0.

ωc/Wj can be obtained from Eq. (26):

27
ωcWj=UTWc-ΑWcWj+η2RWcWj+RTWcWj+η2GWcWjUWc2ωcUTWcMUWc.

UWc/Wj can be obtained by substitution of Eq. (27) into Eq. (22) and then solution of Eq. (23).

4. Numerical example and discussion

For all the subsequent numerical examples, the interval midpoint values of the uncertain material parameters are given in Appendix A. The length of the plate is a= 0.8 m, the width b= 0.5 m, and the thickness h= 0.05 m. The exponential factor η= –10, –5, 0.5, 10.

In the paper, the following dimensionless parameters are employed:

28
x-=xh, y-=yh, z-=zh, u-1=C1212u1f0h, u-2=C1212u2f0h, u-3=C1212u3f0h,
φ-=C1212gsf0hesφ, ψ-=C1212μsf0hqsψ, ω-=ωhC1212ρ0,

where gs= 10-10 As/Vm, es= C/m2, μs= 10-6 Vs/Am, es= Vs/m2, C1212= 14.4 GPa, ρ0= 7454 Kg/m3, f0= 1 Pa, h= 0.05 m.

Case I. The elastic parameters are considered as uncertain, the uncertainty level ±1 % off from the midpoints of the elastic parameters is investigated. The numerical results shows that the midpoint value and uncertainty level of the natural frequencies is symmetric about η= 0 for η= –10, –5, 0.5, 10.

Therefore, Uncertainty level of the natural frequencies (m= 1, n= 1) is only shown in Table 1 for η= 0.5, 10.

Table 1Uncertainty level of the dimensionless natural frequencies (m= 1, n= 1)

η= 0 midpoint (Hz) (level)
η= 5 midpoint (Hz) (level)
η= 10 midpoint (Hz) (level)
7.910 (0.50 %)
9.083 (1.19 %)
10.147 (3.41 %)
8.805 (0.08 %)
9.489 (0.73 %)
12.752 (1.57 %)
16.770 (0.07 %)
17.073 (0.33 %)
18.072 (1.31 %)

It can be seen from Table 1 that no matter how much the exponential factor is, uncertainty level of the dimensionless natural frequencies will decrease with increase of the dimensionless natural frequencies. For the same order dimensionless natural frequency, uncertainty level of the dimensionless natural frequencies will increase with the exponential factor.

Case II. The piezomagnetic coefficients are considered as uncertain, the uncertainty level ±1 % off from the midpoints of the piezomagnetic coefficients is investigated. The numerical results show that the midpoint value and uncertainty level of the dimensionless natural frequencies is symmetric about η= 0 for η= –10, –5, 0.5, 10. Therefore, Uncertainty level of the dimensionless natural frequencies (m= 1, n= 1) is only shown in Table 2 for η= 0.5, 10.

Table 2Uncertainty level of the dimensionless natural frequencies (m= 1, n= 1)

η= 0 midpoint (Hz) (level)
η= 5 midpoint (Hz) (level)
η= 10 midpoint (Hz) (level)
7.910 (1.32×10-7 %)
9.083 (1.90×10-7 %)
10.147 (8.89×10-8 %)
8.805 (0.00 %)
9.489 (4.63×10-7 %)
12.752 (1.20×10-6 %)
16.770 (0.00 %)
17.073 (2.89×10-8 %)
18.072 (8.13×10-8%)

It can be seen from Table 2 that no matter how much the exponential factor is, uncertainty level of the dimensionless natural frequencies is so small that it can be ignored. This can be explained that the piezomagnetic coefficients does not cause a change in the mass of the plate, and it has only a very small effect on the stiffness of the plate. Therefore, the dimensionless natural frequencies of the plate mainly determined by elastic parameters and mass of the plate.

Case III. The piezoelectric coefficients are considered as uncertain, the uncertainty level ±1 % off from the midpoints of the piezoelectric coefficients is investigated. The numerical results show that the midpoint value and uncertainty level of the dimensionless natural frequencies is symmetric about η= 0 for η= –10, –5, 0.5, 10. Therefore, Uncertainty level of the dimensionless natural frequencies (m= 1, n= 1) is only shown in Table 3 for η= 0,5, 10.

Table 3Uncertainty level of the dimensionless natural frequencies (m= 1, n= 1)

η= 0 midpoint (Hz) (level)
η= 5 midpoint (Hz) (level)
η= 10 midpoint (Hz) (level)
7.910 (1.77×10-9%)
9.083 (2.54×10-9 %)
10.147 (1.1.9×10-9 %)
8.805 (0.00 %)
9.489 (6.18×10-9 %)
12.752 (1.60×10-8 %)
16.770 (0.00 %)
17.073 (3.86×10-10 %)
18.072 (1.09×10-9%)

It can be seen from Table 3 that no matter how much the exponential factor is, uncertainty level of the dimensionless natural frequencies is so small that it can be ignored. There is the same explanation as the piezomagnetic coefficients.

It can also be seen from comparison between table 2 and table 3 that for the same uncertainty level off from the midpoints, the piezomagnetic coefficients can cause much larger uncertainty level of the dimensionless natural frequencies than the piezoelectric coefficients. This is the cause that the piezomagnetic has a greater influence on the stiffness of the plate than the piezoelectric coefficient.

5. Conclusions

The elastic, electric and magnetic parameters are regarded as interval variables. Interval analysis of electric-magneto-elastic functionally graded plate are carried out by introducing the interval algorithm into the dynamic equations.

The numerical result shows:

1) The elastic parameters are considered as uncertain, no matter how much the exponential factor is, uncertainty level of the dimensionless natural frequencies will decrease with increase of the dimensionless natural frequencies. For the same order, dimensionless natural frequency, uncertainty level of the dimensionless natural frequencies will increase with increase of the exponential factor.

2) The piezomagnetic coefficients are considered as uncertain, no matter how much the exponential factor is, uncertainty level of the dimensionless natural frequencies is so small that it can be ignored. This can be explained that the piezomagnetic coefficients does not cause a change in the mass of the plate, and it has only a very small effect on the stiffness of the plate.

3) The piezoelectric coefficients are considered as uncertain, no matter how much the exponential factor is, uncertainty level of the dimensionless natural frequencies is so small that it can be ignored.

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About this article

Received
27 October 2016
Accepted
28 October 2016
Published
31 December 2016
Keywords
free vibration
electric-magneto-elastic functionally graded plate
uncertainty
interval algorithm
Acknowledgements

The work is supported by Natural Science Foundation of China under the Grant Number 11372109 and 11302159.