ARTICLE IN PRESS

International Journal of Mechanical Sciences 47 (2005) 82–93 www.elsevier.com/locate/ijmecsci

Anti-plane shear crack in a functionally gradient piezoelectric layer bonded to dissimilar half spaces K.Q. Hua, Z. Zhongb,, B. Jinb a

Department of Building and Structural Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, PR China b Key Laboratory of Solid Mechanics of MOE, School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, PR China Received 17 July 2003; received in revised form 30 November 2004; accepted 4 December 2004 Available online 21 January 2005

Abstract In this paper, the problem of a crack located in a functionally gradient piezoelectric interlayer between two dissimilar homogeneous piezoelectric half-planes being subjected to an anti-plane mechanical loading and an in-plane electric loading is considered. The material properties of the interlayer, such as the elastic stiffness, piezoelectric constant and dielectric constant, are assumed to vary continuously along the thickness of the interlayer, and the crack surface condition is assumed to be impermeable or permeable. By using the Fourier transform, the problem is first reduced to two pairs of dual integral equations and then into a Fredholm integral equation of the second kind. Numerical calculations are carried out, and the effects of crack geometric parameters on the stress intensity factor and the energy release rate are shown graphically. r 2004 Elsevier Ltd. All rights reserved. Keywords: Functionally gradient piezoelectric interlayer; Crack; Impermeable; Permeable; Stress intensity factor

Corresponding author. Fax: +86 21 65981138.

E-mail address: [email protected] (Z. Zhong). 0020-7403/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmecsci.2004.12.002

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1. Introduction Piezoelectric sensors and actuators have attracted attention in the active noise and vibration control area [1]. In particular, control of laminated structures including piezoelectric devices have been extensively studied by Lee and Jiang [2], Batra and Liang [3], Heyliger [4], and Narita and Shindo [5,6]. In some practical structures, one major concern has been the mechanical failure of the piezoelectric ceramic layers. Usually, the discrepancies in mechanical and piezoelectric properties between the component materials cause the cracking of the interface and therefore influence the strength of the whole structure. To meet the demand of advanced piezoelectric materials in lifetime and reliability and with the help of the development in modern material processing technology, the concept of functionally gradient materials has recently been extended into the piezoelectric materials (see Refs. [7–9]). These new kind of materials with continuously varying properties are called functionally gradient piezoelectric materials (FGPM). By using FGPM as a transit layer instead of the bonding agent, there are no discernible internal seams or boundaries that usually cause internal stress peaks when voltage is applied, so the failure from internal debonding or from stress peaks can be avoided [10–12]. Many piezoelectric devices comprise both piezoelectric and elastic layers, and an understanding of the fracture process of piezoelectric structural systems is of great importance in order to ensure the structural integrity of piezoelectric devices [13–15]. In this study, we consider the anti-plane shear problem for a cracked FGPM layer bonded to two piezoelectric half spaces. The two piezoelectric half spaces have dissimilar properties and the piezoelectric laminate is subjected to combined mechanical and electrical loads. How to impose the electrical boundary conditions on the crack surface for piezoelectric fracture analysis remains a controversial problem (see Refs. [16,17]). Here, we consider a crack with the impermeable surface condition as [16,18]  Dþ n ¼ Dn ¼ 0

and a crack with the permeable surface condition as [19]  Dþ n ¼ Dn ;

fþ ¼ f ;

where Dn is the electrical displacement in the direction normal to the crack surface, and f denotes the electric potential. Fourier transforms are used to reduce the problem to the solution of a Fredholm integral equation of the second kind. Numerical solutions are obtained for the stress intensity factors and the energy release rate, and the results show that the effect of the electroelastic interactions on the stress intensity factor is significant.

2. Problem formulation Consider a functionally gradient piezoelectric layer that is sandwiched between two piezoelectric half planes whose elastic stiffness constants, dielectric constants, and piezoelectric II II constants are C I44 ; lI11 ; eI15 and C II 44 ; l11 ; e15 ; respectively. The functionally gradient piezoelectric layer of thickness a þ b contains a crack of length 2c that is parallel to the interfaces, as shown in Fig. 1. A set of Cartesian coordinates (x; y; z) is set at the center of the crack for reference. We assume that a uniform shear traction, szy ¼ p0 ; and a uniform electric displacement, Dy ¼ D0 ; are

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K.Q. Hu et al. / International Journal of Mechanical Sciences 47 (2005) 82–93 y

b

I

x

a

2c

II

Fig. 1. Crack geometry of a functionally gradient piezoelectric material interlayer between two dissimilar homogeneous piezoelectric half planes.

applied at infinity. Applying a uniform electric displacement in the y-direction is equivalent to distributing a uniform surface charge on the boundary whose normal is in the y-direction. Quantities defined in the two half spaces will subsequently be designated by superscripts I and II; while quantities for the interlayer are denoted without superscripts. The piezoelectric boundary value problem is simplified considerably if we consider only the outof-plane displacement and the in-plane electric fields such that ux ¼ uy ¼ 0;

uz ¼ uz ðx; yÞ;

E x ¼ E x ðx; yÞ; uix ¼ uiy ¼ 0;

E y ¼ E y ðx; yÞ; uiz ¼ uiz ðx; yÞ

E ix ¼ E ix ðx; yÞ;

(1) E z ¼ 0;

(2)

ði ¼ I; IIÞ;

E iy ¼ E iy ðx; yÞ;

E iz ¼ 0

(3) ði ¼ I; IIÞ;

(4)

where (ux ; uy ; uz ) and (E x ; E y ; E z ) are the components of displacement and electric field vectors, respectively. The constitutive equations can be written as sxz ¼ C 44 uz;x  e15 E x ;

syz ¼ C 44 uz;y  e15 E y ;

Dx ¼ e15 uz;x þ l11 E x ;

Dy ¼ e15 uz;y þ l11 E y ;

ð5Þ

where sxz ;syz and Dx ;Dy are the components of stress tensor and electric displacement vector, respectively, C 44 is the elastic stiffness constant measured in a constant electric field, l11 is the dielectric constant measured at constant strain field, e15 is the piezoelectric constant, and the subscript comma denotes a partial derivative with respect to the coordinates. The electric field components are related to the electric potential f by the equation E x ¼ f;x ;

E y ¼ f;y :

(6)

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We suppose that the piezoelectric material possesses the following nonhomogeneous properties [11,20–22]: C 44 ðyÞ ¼ C 044 exp ðbyÞ; C 044 ;

e015 ;

e15 ðyÞ ¼ e015 exp ðbyÞ;

l11 ðyÞ ¼ l011 exp ðbyÞ ðapypbÞ;

(7)

l011

are material constants, and b is material property gradient. The similar where mechanical properties (7) have been used by Delale and Erdogan [20]. The parameter b can be determined by the values of the material properties at the y ¼ 0 plane II II ðC 044 ; e015 ; l011 Þ; y ¼ b plane (C I44 ; lI11 ; eI15 ) and y ¼ a plane (C II 44 ; l11 ; e15 ), i.e. b¼

1 1 1 ðln C I44  ln C II ðln eI15  ln eII ðln lI11  ln lII 15 Þ ¼ 44 Þ ¼ 11 Þ aþb aþb aþb

(8)

and b=ðaþbÞ C 044 ¼ ðC I44 Þa=ðaþbÞ ðC II ; 44 Þ

b=ðaþbÞ e015 ¼ ðeI15 Þa=ðaþbÞ ðeII ; 15 Þ

b=ðaþbÞ l011 ¼ ðlI11 Þa=ðaþbÞ ðlII : 11 Þ

(9) The governing equations and the boundary conditions have the following forms (see Refs. [21,22]): r2 uz þ b

quz ¼ 0; qy

r2 f þ b

r2 uiz ¼ 0; r2 fi ¼ 0

qf ¼0 qy

ðapypbÞ;

ðy4b or yo  aÞ ði ¼ I; IIÞ;

(10) (11)

where r2 ¼ q2 =qx2 þ q2 =qy2 is the Laplace operator in two dimensions. The boundary conditions of the crack problem are szy ¼ p0 ; szy ¼ 0

ðx2 þ y2 ! 1Þ

Dy ¼ D0 ;

(12)

ðjxjoc; y ¼ 0Þ

(13)

uz ðx; 0þ Þ ¼ uz ðx; 0 Þ; fðx; 0þ Þ ¼ fðx; 0 Þ; uIz ðx; bÞ ¼ uz ðx; bÞ;

uII z ðx; aÞ ¼ uz ðx; aÞ;

fI ðx; bÞ ¼ fðx; bÞ;

fII ðx; aÞ ¼ fðx; aÞ;

ðjxjXcÞ

sIzy ðx; bÞ ¼ szy ðx; bÞ;

sII zy ðx; aÞ ¼ szy ðx; aÞ;

DIy ðx; bÞ ¼ Dy ðx; bÞ;

DII y ðx; aÞ ¼ Dy ðx; aÞ

(14)

ð15Þ

ð16Þ

and the electric boundary conditions along the crack plane can be written as Dy ¼ 0

ðjxjoc; y ¼ 0Þ

(17)

for the impermeable case, and Dy ðx; 0þ Þ ¼ Dy ðx; 0 Þ; for the permeable case.

fðx; 0þ Þ ¼ fðx; 0 Þ

ðjxjoc; y ¼ 0Þ

(18)

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3. Solution procedure By applying the Fourier transform to (10,11), and using conditions (12–18), we have  Z 1 x  p1 uz ðx; yÞ ¼ 2 exp½2pðy  bÞ  x  p2 0 0

D ðxÞ expðp2 yÞ cosðxxÞ dx þ b1 ½expðbyÞ  1  Z 1 x  p1 fðx; yÞ ¼ 2 exp½2pðy  bÞ  x  p2 0 0

A ðxÞ expðp2 yÞ cosðxxÞ dx þ b2 ½expðbyÞ  1 ð0oypbÞ; Z

1

uz ðx; yÞ ¼ 2

0

F ðx; yÞD ðxÞ expðp1 yÞ cosðxxÞ dx þ b3 ½expðbyÞ  1 ; Z0 1

0

F ðx; yÞA ðxÞ expðp1 yÞ cosðxxÞ dx þ b4 ½expðbyÞ  1

fðx; yÞ ¼ 2

ðapyp0Þ

ð19Þ

0 0

0

where D ðxÞ and A ðxÞ are unknown functions, p1 ; p2 ; p and bi ði ¼ 1; 2; 3; 4Þ are defined, respectively, as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 b b b b b2 p1 ¼  x2 þ ; p2 ¼ þ x2 þ ; p ¼ x2 þ ; 2 2 4 4 4

b1 ¼

b  expðbbÞðl011 p0 þ e015 D0 Þ ; ½C 044 l011 þ ðe015 Þ2 ½expðbbÞ  1

b3 ¼

a  expðbaÞðl011 p0 þ e015 D0 Þ ; ½C 044 l011 þ ðe015 Þ2 ½1  expðbaÞ

b2 ¼

b4 ¼

b  expðbbÞ½e015 p0  C 044 D0 ½C 044 l011 þ ðe015 Þ2 ½expðbbÞ  1

a  expðbaÞðe015 p0  C 044 D0 Þ ½C 044 l011 þ ðe015 Þ2 ½1  expðbaÞ

(20)

and F ðx; yÞ is defined as Fðx; yÞ ¼

½ðx þ p1 Þ expð2pyÞ  ðx þ p2 Þ expð2paÞ f½p2  p1 expð2pbÞ þ x½1  expð2pbÞ g : ½p1 expð2paÞ  p2 þ x½1  expð2paÞ (21)

We can reduce the problem to simultaneous dual integral equations with new unknowns DðxÞ and AðxÞ Z 1 DðxÞ cosðxxÞ dx ¼ 0 ðx4cÞ; Z0 1 xf ðxÞDðxÞ cosðxxÞ dx ¼ f 1 ð0pxpcÞ ð22Þ 0

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Z

87

1

AðxÞ cosðxxÞ dx ¼ 0 ðx4cÞ; Z

0 1

xf ðxÞAðxÞ cosðxxÞ dx ¼ f 2

ð0pxpcÞ;

ð23Þ

0

where 8 l011 p0 þ e015 D0 > >  > < C 0 l0 þ ðe0 Þ2 44 11 15 f1 ¼ p > 0 >  0 > : C 44 8 0 0 > < C 44 D0  e15 p0 f 2 ¼ C 044 l011 þ ðe015 Þ2 > : 0 f ðxÞ ¼ 1 þ gðxÞ ¼ 1 þ

ðfor impermeable caseÞ; ðfor permeable caseÞ;

ðfor impermeable caseÞ; ðfor permeable caseÞ;

2ða1 x2 þ a2 x þ a3 Þ xða4 x2 þ a5 x þ a2 Þ

(24)

and ai ði ¼ 1; 2; 3; 4; 5Þ are defined, respectively, as a1 ¼ ½p1 expð2pbÞ  p2 ½p1  p2 expð2paÞ ; a2 ¼ 2px2 f1  exp½2pða þ bÞ g; a3 ¼ x4 ½1  expð2pbÞ ½1  expð2paÞ ; a4 ¼ 2pf1  exp½2pða þ bÞ g; a5 ¼ 4p2 f1 þ exp½2pða þ bÞ g: Introduce a function FðtÞ by Z 1 2 DðxÞ ¼ f 1 c tFðtÞJ 0 ðxtÞ dt;

(25)

2

Z

AðxÞ ¼ f 2 c

0

1

tFðtÞJ 0 ðxtÞ dt

(26)

0

where J0( ) represents the Bessel function of the first kind of order zero. The substitution of Eq. (26) into Eqs. (22) and (23) lead to the following Fredholm integral equation of the second kind for FðtÞ: Z 1 Kðx; tÞFðtÞ dt ¼ 1 (27) FðxÞ þ 0

whose kernel Kðx; tÞ is Z 1 Kðx; tÞ ¼ t xgðx=cÞJ 0 ðxxÞJ 0 ðxtÞ dx: 0

(28)

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By substituting Eqs. (19)–(22) into Eqs. (5)–(6) and considering the term that becomes unbounded as r ! 0 in Eq. (26), we can get the singular parts of the stresses, the strains, the electric field intensities and the electric displacements in the neighborhood of the crack tip:



pffiffiffi pffiffiffi ðC 0 f þ e0 f Þ cFð1Þ y ðC 0 f þ e0 f Þ cFð1Þ y szy ¼  44 1 p15ffiffiffiffiffi2 cos ; szx ¼ 44 1 p15ffiffiffiffiffi2 sin ; (29) 2 2 2r 2r



pffiffiffi pffiffiffi f 1 cFð1Þ y f 1 cFð1Þ y pffiffiffiffiffi zy ¼ ; zx ¼ pffiffiffiffiffi sin ; (30) cos 2 2 2r 2r



pffiffiffi pffiffiffi ðl0 f  e0 f Þ cFð1Þ y ðe0 f  l0 f Þ cFð1Þ y Dy ¼ 11 2 p15ffiffiffiffiffi1 cos ; Dx ¼ 15 1 p11ffiffiffiffiffi2 sin ; (31) 2 2 2r 2r



pffiffiffi pffiffiffi f 2 cFð1Þ y f 2 cFð1Þ y E y ¼ pffiffiffiffiffi cos ; E x ¼  pffiffiffiffiffi sin ; (32) 2 2 2r 2r where the polar coordinates r and y are defined as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

y  : (33) r ¼ ðx  cÞ2 þ y2 ; y ¼ arctan xc Extending the traditional concept of stress intensity factors to other field variables yields



KT y KT y ; szx ¼  pffiffiffiffiffi sin ; (34) szy ¼ pffiffiffiffiffi cos 2 2 2r 2r



K y K  y zy ¼ pffiffiffiffiffi cos ; zx ¼ pffiffiffiffiffi sin ; (35) 2 2 2r 2r



KD y KD y Dy ¼ pffiffiffiffiffi cos ; Dx ¼  pffiffiffiffiffi sin ; (36) 2 2 2r 2r



KE y KE y E y ¼ pffiffiffiffiffi cos ; E x ¼  pffiffiffiffiffi sin ; (37) 2 2 2r 2r where K T is the stress intensity factor, K  is the strain intensity factor, K D is the electric displacement factor and K E is the electric field intensity factor. Comparing Eqs. (34)–(37) with (29–32), these field intensity factors can be defined as hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i pffiffiffi 2ðx  cÞszy ðx; 0Þ ¼ p0 cFð1Þ; K T ¼ limþ x!c hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i  K ¼ limþ 2ðx  cÞzy ðx; 0Þ x!c 8 0 pffiffiffi ðl11 p0 þ e015 D0 Þ cFð1Þ > > ðfor impermeable caseÞ; > > < C 044 l011 þ ðe015 Þ2 ¼ pffiffiffi > p0 cFð1Þ > > ðfor permeable caseÞ; > : C 044

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hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i 2ðx  cÞDy ðx; 0Þ K D ¼ limþ x!c 8 pffiffiffi D0 cFð1Þ ðfor impermeable caseÞ; > > < p ffiffi ffi ¼ e015 p0 cFð1Þ > ðfor permeable caseÞ; > : C 044 hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i 2ðx  cÞE y ðx; 0Þ K E ¼ limþ x!c 8 pffiffiffi > ðC 044 D0  e015 p0 Þ cFð1Þ > < C 044 l011 þ ðe015 Þ2 ¼ > > : 0

ðfor impermeable caseÞ ð38Þ ðfor permeable caseÞ:

We find that the stress intensity factors for pffiffiffi the two crack boundary conditions are the same; the T =p normalized stress intensity factor K 0 c and the normalized electric displacement intensity pffiffiffi factor K D =D0 c are given by K¼

KT KD pffiffiffi ¼ pffiffiffi ¼ Fð1Þ p0 c D0 c

(39)

for the impermeable case, while for the permeable case, we find that KD ¼

e015 T K : C 044

(40)

To obtain the energy release rate for the mode III fracture problem of FGPM under consideration, we use the expression given by Pak [16] as follows: G¼

K T K   K DK E 2

(41)

Substituting Eq. (38) into Eq. (41), we arrive at " # 8 > cK 2 l011 p20 þ 2e015 D0 p0  C 044 D20 > > ðfor impermeable caseÞ; > < 2 C 044 l011 þ ðe015 Þ2 G¼ > 1 > > K 2III ðfor permeable caseÞ: > : 0 2C

(42)

44

Examining Eq. (42) one can see that the energy release rate can have negative values depending on the direction, and the magnitude of the electrical load in the impermeable case; but for the permeable case, the energy release rate would never be negative.

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Solving the roots of the equation in (42), the crack extension force can be shown to be positive in the impermeable case when qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e015  ðe015 Þ2 þ C 044 l011 D0 e015 þ ðe015 Þ2 þ C 044 l011 o o : (43) p0 C 044 C 044 The negative ratio implies the case in which the direction of the load is reversed. This shows that, at certain ratios of the electrical load to the mechanical load, crack arrest can be observed. It is also interesting to note that in the absence of the mechanical load, i.e., when p0 is zero, the energy release rate is always negative, indicating that a crack would not propagate under this condition. These observations agree with those of Pak [16] and Deeg [23] for homogeneous piezoelectric body.

4. Numerical results and discussion As an example, the material properties at y ¼ 0 are given as C 044 ¼ 3:53 1010 N=m2 ; e015 ¼ 17:0 C=m2 ; l011 ¼ 151 1010 C=Vm; G cr ¼ 5:0 N=m

(44)

which are the same as that of lead zirconate titante (PZT-5 H) piezoelectric given by Deeg [23]. Here N is the force in Newtons, C is the charge in coulombs, V is the electric potential in volts, m is the length in meters, and G cr is the critical crack extension force. We solve the integral Eq. (27) numerically for the values of FðxÞ; and these values are used to Eqs. obtain numerical values of the stress intensity factor K T and the energy release rate G fromp ffiffiffi D (38) and (42), respectively. Since the normalized electric displacement intensity factor K =D0 c is p ffiffi ffi equal to the normalized stress intensity factor K T =p0 c in the impermeable case, and the electric displacement intensity factor is proportional pffiffiffi to the stress intensity factor in the permeable case, we shall illustrate the variation of K T =p0 c only in what follows. 1.035 c/a=c/b=0 c/a=c/b=0.2 c/a=c/b=0.5 c/a=c/b=1.0

1.03

KT/p0c1/2

1.025 1.02 1.015 1.01 1.005 1 0.995 1

1.5

2

2.5

3

3.5

4

4.5

5

C44I /C44 II

Fig. 2. Variations of the normalized stress intensity factor with the ratio of C I44 =C II 44 in case of a ¼ b:

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pffiffiffi Fig. 2 displays the variations of the normalized stress intensity factor K T =p0 c with the ratio C I44 =C II 44 at different crack geometries in cases of a ¼ b: It is evident that the normalized stress intensity factor increases with the increase of C I44 =C II 44 in this case. This means that the stress intensity factor and the electric displacement intensity factor can be reduced by decreasing the material property gradient of functionally gradient piezoelectric materials. Fig. 2 also shows that the influence of the material property gradient C I44 =C II 44 tends to increase with decreasing the thickness of the strips a and b: In addition, when the material property gradient C I44 =C II 44 approaches to 1, the results of the stress and electric displacement intensity factors are the same as those for the case of homogeneous piezoelectric material (see Ref. [16]). The normalized energy release rate for the functionally gradient piezoelectric strip with a crack of length 2c ¼ 0:02 m are plotted in Fig. 3 as a function of the applied electrical displacement D0 ; with the mechanical loads fixed such that G ¼ Gcr at zero electrical loads. As can be seen from Fig. 3, for the impermeable crack conditions, the maximum energy release rate can be found by differentiating the energy release rate G with respect to the electrical loads. For this particular problem, they occur at D0 ¼

e015 p0 ¼ 2:0 103 C=m2 : C 044

(45)

The energy release rate can be made negative by making the electrical loads such that they are outside the ranges, 1:38 103 C=m2 oD0 o5:41 103 C=m2 :

(46)

As the magnitude of electrical load is increased from zero, the energy release rate G can be made either to increase or to decrease depending on the direction of the load. But once the maximum G is reached, further increase in the electrical load will monotonously decrease G: From Fig. 3, we can also know that the magnitude of the normalized energy release rate increases with increasing the ratio c=a; in other words, the greater the thickness of the strip, the smaller the energy release rate. 2.5 2 1.5 1

G/Gcr

0.5 0 −0.5

c/a=c/b=0 c/a=c/b=0.2 c/a=c/b=0.5 c/a=c/b=1.0

−1 −1.5 −2 −2.5

−3

−2

−1

0

1

2

D0 (C/m2)

3

4

5

6

7 x 10

-3

Fig. 3. Effect of the electric displacement on the normalized energy release rate in case of C I44 =C II 44 ¼ 10; a ¼ b and p0 ¼ 4:2 106 N=m2 ; (for impermeable case).

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3

G/Gcr

2.5 2 1.5 1 0.5 0

−5

−4

−3

−2

−1

0

p0 (N/m2)

1

2

3

4

5

x 10

6

Fig. 4. Effect of the applied stress load on the normalized energy release rate in case of C I44 =C II 44 ¼ 10 and a ¼ b (for permeable case).

Fig. 4 shows the effect of the applied stress load on the normalized energy release rate for the case of permeable crack conditions. As the magnitude of stress load is increased from zero, G will monotonously increase from zero. When the ratio C I44 =C II 44 is fixed, the normalized energy release rate tends to increase with decreasing the thickness of the strips a and b:

5. Conclusions The electroelastic problem of a crack located in a functionally gradient piezoelectric interlayer between two dissimilar homogeneous piezoelectric half planes being subjected to an anti-plane mechanical loading and an in-plane electric loading is analyzed. The closed forms of the local electroelastic field near the crack tip are obtained, and the results are expressed in terms of the stress intensity factor which may increase with increasing the material property gradients, while the stress intensity factor can be made to decrease by increasing the layer thickness to crack–length ratio. By computing the energy release rate, it was found that the energy release rate could be made either to increase or decrease depending on the direction of the electric load.

Acknowledgements This work has been supported by the National Natural Science Foundation of China (No. 10432030 and No. 10209) and the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, PRC.

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