Real time integrity monitoring of composite laminates

Real time integrity monitoring of composite laminates with magnetostrictive sensory layer Anand Kumar1 Assistant Professor, Department of Mechanical Engineering, Harcourt Butler Technological Institute, Kanpur, India & Bishakh Bhattacharya2 Associate Professor, Department of Mechanical Engineering, Indian Institute of Technology, Kanpur, India ABSTRACT Fundamental research and development in smart materials and structures have shown great potential for enhancing the functionality, serviceability and increased life span of civil and mechanical infrastructure systems. Researchers from diverse disciplines have been drawn into vigorous efforts to develop smart and intelligent structures that can monitor their own conditions, detect impending failure, control damage and adapt to changing environments. Smart structures are generally created through synthesis by combining sensing, processing and actuating elements integrated with conventional structural materials. The conventional non-destructive evaluation techniques are not very effective in monitoring the structural integrity of composite structures due to their micro-mechanical complexities. With the commercial availability of the magnetostrictive (MS) material Terfenol-D in particulate form, it is now feasible to develop particulate sensors to detect damage with minimum effect on structural integrity. In present investigation, the electromagnetic response in the MS layer at the onset of delamination in one of the weakest ply of the composite laminate has been analyzed. For the numerical analysis symmetric and asymmetric carbon epoxy laminates with one of its layers embedded with Terfenol-D particles have been taken. Terfenol-D layer experiences a change in stress due to onset of delamination causing a change in its magnetic state, which can be sensed as induced open circuit voltage in the sensing coil enclosing the laminate beam. The effect of material properties, lamination schemes and placement of MS layer on the sensing capabilities has been analyzed. KEYWORDS Real time monitoring, magnetostriction, delamination, Terfenol-D, smart sensing, composite laminate, stacking sequence.

1. INTRODUCTION Fundamental research and development in smart materials and structures have shown great potential for enhancing the functionality, serviceability and increased life span of civil and mechanical infrastructure systems. Researchers from diverse disciplines have been drawn into vigorous efforts to develop smart and intelligent structures that can monitor their own conditions, detect impending failure, control damage and adapt to changing environments. The potential applications of such smart materials and systems are abundant--ranging from design of smart aircraft skin embedded with smart sensors to detect structural flaws and bridges with sensing and actuating elements to counter violent vibrations to name a few. Smart structures are generally created through synthesis by combining sensing, processing and actuating elements integrated with conventional structural materials such as steel, concrete, or composites. Composite structures are now gaining attention due to overriding advantages over the conventional metallic structures. The 1 2

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Smart Structures, Devices, and Systems IV, edited by Said Fares Al-Sarawi, Vijay K. Varadan, Neil Weste, Kourosh Kalantar-Zadeh, Proc. of SPIE Vol. 7268, 72680N © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.810614 Proc. of SPIE Vol. 7268 72680N-1 2008 SPIE Digital Library -- Subscriber Archive Copy

conventional non-destructive evaluation techniques are not very effective in monitoring the structural integrity of composite structures due to their micro-mechanical complexities. Various types of smart patches e.g. PZT patches and PVDF films have been used as smart alternatives to sense and detect damage in composite structures. With the commercial availability of MS material Terfenol-D in particulate form, it is now feasible to develop particulate sensors to detect damage with minimum effect on structural integrity. These smart sensors provide real time sensing by exploiting their functional properties. They are light in weight, consume less power and have better reliability. In present work, the electromagnetic response in the MS layer at the onset of delamination in one of the weakest ply of the composite laminate has been analyzed. The effect of material properties, lamination scheme and placement of MS layer on the sensing capabilities has been analyzed. Classical laminate theory has been used.

2. STATE OF THE ART MS materials, like piezoelectric materials can be used both as sensors and actuators. With the commercial availability of Terfenol-D in particulate form, it is now feasible to developed MS particulate sensors to detect damages such as delamination with minimum effect on structural integrity on real time basis. 2.1 Magnetostriction Under the influence of an external magnetic field, the grains of certain materials that consist of numerous small randomly oriented magnetic domains align according to the applied magnetic field. The change in magnetic orientation brings about an internal magnetic field. This is known as magnetostriction. There has been a resurgence of interest in MS materials during the last decade. This is primarily due to the commercial availability of the rare earth-iron compounds capable of producing large quasistatic strains of over 1600x10-6 in response to moderate magnetic fields of less than 160 kA/m. The most technologically advanced of these compounds is the pseudo binary alloy Terfenol-D, Tb0.3Dy0.7Fe1.9-1.95, which has become the primary MS material for transducer applications. Terfenol-D exhibits a combination of high single crystal magnetostriction and low magneto-crystalline anisotropy. Since magnetostriction is an inherent material property, it does not degrade over time. Terfenol-D layers can easily be embedded into laminates made of modern day composites without significantly affecting the structural integrity. Thus, Terfenol-D is finding increased use in actuator and sensor applications in which high energy densities and sustained reliability is required. Most of the studies reported in the literature on the topic are related mainly to material characterization [1-5] and to search for right kind of combination of the elements to improve upon magnetostriction properties for better sensors and actuators. The studies of interaction between magnetostrictive layers and the composite laminates and their use in active vibration control and damping are also available [6-11] but only limited studies have been reported regarding the use of magnetostrictive materials for damage detection and sensing of integrity of the structures. Krishnamurthy et al, 1998[12] considered the possibility of delamination prediction of smart composite laminate using MS materials. Giurgiutiu et al, 2001[13] presented theoretical and experimental analysis of MS tagged woven fiber reinforced composites subjected to bending. Chen and Anjanappa, 2006[14] presented a non-contact sensing module for detecting delamination in a composite structure embedded with MS particulate sensors. 2.2. Delamination Delamination is one of the predominant forms of failure in laminated composites due to lack of reinforcement in the thickness direction. Fiber composites often consist of layers that are bonded together as part of the curing process. The strength of this bond is limited by the matrix strength. Delamination is separation of these layers, which results in severe loss of bending stiffness and strength of the composite structure. One method to design against delamination failure is to calculate the interlaminar stresses and compare these with interlaminar stress allowables. Another way of assessing the reliability of laminates with respect to delaminnation is to adopt fracture mechanics approach by assuming the presence of cracklike flaw as initial delamination and to determine the stress that will lead to propagation of this initial delamination crack. Though the structure is not usually designed with an initial delaminaton, it can arise from loadings

Proc. of SPIE Vol. 7268 72680N-2

such as accidental impact or from initial imperfections in the structure. Shah et al, 1994[15] suggested that delamination due to interlaminar stresses cause separation of plies from one another causing reduced stiffness and degradation of the structural response of laminated composites. Rybicki et al, 1997[16] demonstrated that delamination was a stable mode of crack propagation and caused mainly due to the presence of tensile stresses. Krishnamurthy et al, 1998[12] considered the possibility of delamination prediction of smart composite laminate using MS material and presented a theoretical basis for it. In this paper, an effort has been made to formulate a model to sense delamination in fibre composite laminates using MS layer of Terfenol-D, thus to establish magnetostriction as an effective method of real time integrity monitoring of composite structures.

3. ANALYSIS OF MAGNETOSTRICTIVE COMPOSITES Although the MS materials show nonlinear relationship, the behaviour of most of these materials can well be described by using a linear theory as the active materials could easily be biased. With biasing, the material behaves in quasi-linear manner and follows the piezomagnetic law

ε = SH σ + d H

(1)

B = d σ + µσ H

(2)

H

where S is the compliance at constant magnetic field intensity, piezo-magnetic coefficient, B the flux density and ε the strain.

H , µ the permeability at constant stress σ , d the

A laminated beam with one of its layer having Terfenol-D particles is shown in Figure.1. In such a laminate, the Terfenol-D layer will experience a change in stress due to the onset of delamination, which can be sensed as an induced open circuit voltage in sensing coil enclosing the beam. The Terfenol-D composite carries an initial or bias mechanical stress and magnetic field intensity. A carrier current of the form

I1 = I b + I 0 sin ωt 

(3)

is applied along with an appropriate bias field through the actuator coil resulting in stresses in the magnetostrictive layer in a localized portion only over a width equal to the width of coil. Where I b is the bias current, I 0 the amplitude and ω the frequency of the applied current.

delamination

mu-:.

Yv Z,w

X,u

magnetostrictive layer actuator and sensing coils

Figure.1 Smart composite laminate beam with delamination

In the present work, fiber composite laminates with one of its layer embedded with Terfenol-D have been analyzed. Symmetric laminate with the stacking sequence of (0/90/0/45/m/45/0/90/0) has Terfenol-D powder 2.24 percent by volume fraction in the middle layer. In the second case Terfenol-D particulates are embedded in the seventh layer of the


laminate making it asymmetric beam while bending. With the application of AC current of the form as in Equation 3, a compressive stress is generated in the MS layer. To balance the beam statically, there will be an equivalent and opposite tensile stress in the rest of the composite laminate (refer Figure 2a). With increased current rating, the stress in MS layer goes on increasing causing stresses in all the layers depending upon their stacking sequence. This results in delamination at the weakest interface. When the beam is symmetric, only axial forces are acting over it. If the same composite laminate beam is subjected to a pointed load at the centre of a simply supported beam it causes bending of the beam (refer Figure 2b). This brings curvature of the beam in calculation of stress and strain. The stress in the laminate results in the change in the magnetic state of the MS layer whish can be sensed as open circuit voltage in the sensing coil enclosing the beam. The induced voltage obtained in the sensing coil is direct measure of the stresses in the MS layer. Using classical laminate theory, the stress and strain profile in all other layers can be easily calculated. The weakest layer, dependent upon the elastic properties of that particular layer and its orientation, may be determined taking an appropriate failure theory. In this numerical analysis Tsai-Wu failure criterion of composite laminates has been used. The elastic properties of laminates, properties of Terfenol-D and other relevant numerical details are listed in Table. 1. 3.1 Constitutive relations for composite laminate Strain at the top and bottom of kth lamina

ε top = ε 0 + z k λ k

(4)

ε bot = ε 0 + z k +1λ k

Stresses in top and bottom interface

σ topk = [Q] k ε topk

(5)

σ botk = [Q]k ε botk 3.2 Tsai and Wu criterion

This failure criterion has been suggested for direct application to composites. The stress polynomial as suggeted by Tsai and Wu [17] is given in the quadrate form as

F1σ1 + F11σ12 + F2σ2 + F22σ22 + 2F12σ1σ2 + F66τ122 = 1 Where the

(6)

F terms are standard material constants and stresses are the in-plane ply stresses.

Delamination will be initiated when stress in the weakest ply increases beyond the allowable stress. Forces and moment in the laminates can be calculated as

⎧N ⎫ ⎡ A ⎨ ⎬=⎢ ⎩M ⎭ ⎣ B

B⎤ D ⎥⎦

⎧ε 0 ⎫ ⎨ ⎬ ⎩λ ⎭

h/2

N=

∫σ

−h / 2

(7)

h/2

dz

M =

∫σ

zdz

(8)

−h / 2

A, B & D are extensional, coupling and bending stiffness matrices, ε 0 is mid plane strain, λ mid plane curvature, N and M are force and moment per unit length. (Refer Agrawal and Broutmn [18])


n

⎡ − ⎤ [ Ai j ] = ∑ ⎢Qi j ⎥ [hk − hk −1 ] ⎦k k =1 ⎣ n ⎛1⎞ ⎡ − ⎤ [ Bi j ] = ⎜ ⎟∑ ⎢Qij ⎥ hk2 − hk2−1 ⎝ 2 ⎠ k =1 ⎣ ⎦ k ⎛1⎞ n ⎡ − ⎤ 3 [ Di j ] = ⎜ ⎟∑ ⎢Qi j ⎥ hk − hk3−1 ⎝ 3 ⎠ k =1 ⎣ ⎦ k

[

]

[

]

(9)

3.3 Symmetric laminate

P

Ni N

N2

A

-

Ni'

4

C

MS Layer

N

4

MS Layer

N2

a.

N N2

b.

Figure.2 a) Force balance in symmetric laminate, b) Force and moment balance in asymmetric laminate

Net force per unit length in MS layer (compressive in nature)

N = σ hm Forces in upper and lower segments

N1 = A1ε

&

N 2 = A2ε

If no other external force is applied

ε = N /( A1 + A2 )

Strain in MS layer and composite host are same. Comparing with Equation 1 and solving for

σ

σ =α I α=

where

dN ⎡ hm ⎤ − SH ⎥ ⎢ + A A 2 ⎣ 1 ⎦

From Equation 2, flux density B

B = (dα + µN ) I Open circuit voltage in the sensing coil

dB dt = nla r (dα + µN ) I 0ω cos ωt

V = nla r

where

(10)

n is the number of turns per unit length, l the total length and ar the area of the cross section of the conductor.


3.4 Asymmetric laminate Compressive stress in MS layer results in moment causing bending in the beam. Hence, axial strain in MS layer including the bending effect

ε ' = ε − zk λ where zk is the distance from mid plane in thickness direction. Mid plane curvature λ

=

M − Bε D Table.1. Numerical details used in the analysis

Number of plies including MS layer Symmetric laminate stacking Asymmetric laminate stacking Composite used Thickness of composite lamina Thickness of MS layer Elastic modulus of carbon fibers Elastic modulus of epoxy matrix Elastic modulus of Terfenol- D Volume fraction of fiber Volume fraction of Terfenol-D Poisson's ratio of carbon fiber Poisson's ratio of epoxy matrix Poisson's ratio for Terfenol-D Number of turns in the coil per meter length of beam Carrier frequency Carrier current Piezomagnetic coefficient Permeability Coupling coefficient of Terfenol-D Tensile strength of Terfenol-D Compressive strength of Terfenol-D Fracture toughness of MS layer Size of crack at delamination Length of beam Width of over all beam structure

9 [0/90/0/45/m/45/0/90/0] [0/90/0/45/0/90/m/90/0] carbon - epoxy 0.4 mm 0.4 mm 350 GPa 3.50 GPa 30 GPa 0.16 0.0224 0.3 0.4 0.25 1000 1000Hz 0.4 A 1.5 e-8 m/A 14.13e-7 0.75 28 MPa 700 MPa 30 MPa-m1/2 2 mm 100 mm 20 mm

and moment on beam due to stresses induced in MS layer

M = N1a + N 2 c − Nb where a ,

b and c are the distances from top where the forces are assumed to be acting.

Stress in MS layer including bending effect

σ '=

1 [ε '−dnI 0 sin ωt ] S

Voltage can be obtained as

V ' = nla r (dα '+ µN )I 0ω cos ωt

(11)


3.5 Composite beam subjected to mechanical loading

P1 sin ω t is assumed to be acting as pointed load at the center of the simply supported beam. Assuming mid plane strain and curvature as ε 0 'sin ωt and λ 'sin ω t

A force of the form of

Strain in MS layer

ε ' ' = ε 0 ' sin ωt + z k λ ' sin ωt (12)

σ ' ' = ⎡⎢Q ⎤⎥ [ε 0 '+ z k λ ] sin ωt −

⎣ ⎦

Voltage response can be obtained as

⎡ ⎡−⎤ ⎤ V ' ' = nlar ⎢d ⎢Q⎥(ε 0 '+ zk λ') + µnI0 ⎥ ω cosωt ⎣ ⎣ ⎦ ⎦

(13)

4. RESULTS AND DISCUSSION In the first case, the symmetric laminate beam is subjected to only actuating AC current causing magntostriction to develop in the MS layer leading to change in the stress in the layer. Variation of stress and strain in various interfaces and in the MS layer and voltage induced in MS layer are shown in Figures 3, 4 and 5. The MS layer is subjected to compressive stress while the remaining plies have tensile stresses induced in them to statically balance the laminate beam. Due to the symmetric stacking sequence, a number of interfaces can attain the delaminating stress simultaneously (1, 2, 3, 6, 7 and 8) and have theoretically equal chances of delamination. A peak open circuit voltage of around 57 mV is obtained in the sensing coil at the delaminating stress of approximately 13 MPa. The laminate is now subjected to increasing mechanical load along with the actuator AC current to induce MS effect till delamination in one of the weakest interface is produced. Stresses and strains in various interfaces and voltage induced in MS layer at the time of delamination in the weakest ply are shown in Figures 6,7and 8. Delamination will occur in 1st interface from the top due to increased stresses caused due to bending. Since the fracture toughness of MS layer is assumed to be greater, it will not be failing even if the stresses are a bit higher. Peak open circuit voltage in the sensing coil at delamination comes down to around 20 mV due to bending effect. Peak voltage at the time of delamination with mechanical loading is fairly lower than the voltage without mechanical loading and the kind of bending i.e. sagging or hogging is going to decide the open circuit voltage sensed in the sensing coil. Similarly, stress, strain and voltage profile of composite laminate with 7th layer from the top having Terfenol-D has been presented in Figures 9, 10 and 11. Figure 12 suggest that MS layer away from the mid plane is more effective as it helps in getting higher induced voltage (67 mV) in the sensing coil due to bending effect. Further, increased thickness of MS layer results in better sensing as it can sustain higher stresses which results in increased sensing voltage (Figure 13). Figure.14 suggests that higher current rating is required for increased sensing voltage. Analyzing other stacking arrangements, it has been seen that the effect of orientation of plies is more prominent than the bending effect, especially in this case where the difference in elastic properties of fiber and matrix is greater and the matrix governs the transverse behavior of the composite laminate. The model presented in this work has been validated on analytical and experimental results reported earlier. [11, 12,13]

5. CONCLUSIONS This numerical analysis suggest that a clear relationship can be established between the state of stresses in the composite laminate and its electromagnetic response induced in MS layer which is sensed through the sensing coil enclosing the beam. The response has been analyzed for composite laminates with actuator current and along with varying mechanical load acting at the center of a simply supported beam. It has been observed that the location of MS layer away from the


stacking sequence (0190101451m1451019010)


15

15

U)1 -

U)1

0.5

- 0.5

E0

E0

y

-05 U)

0

MS Lay&

-05

-1

U)

0

-1.5

-1

-1.5

U)

-20

-40

-60

20

0

-200

stress (MPa)

200

0

strain (micron)

Figure.3 Stress variation at the time of delamination at various interfaces and in MS layer when the laminate is subjected to actuator current only.

Figure.4 Strain variation at the time of delamination at various interfaces and in MS layer when the laminate is subjected to actuator current only.



V

60

E E

10 U)

D)

a

U)l

20

V

a 0.5

00 > =

1.5

E0

MC L1

-05

-20

0

U)

0

-40

0

60

a 0

20

10

5

time (ms)

-1.5 I

-200

-100

0

100

200

stress (MPa)

Figure.5 Open circuit voltage in MS layer at the time of delamination when the laminate is subjected to actuator current only.

Figure.6 Stress variation at the time of delamination at various interfaces and in MS layer when the laminate is subjected to mechanical loading along with actuator current.


stacking sequence (0190101451m1451019010) E E

7

7

I

1.5

U)1 U)

O.5

EO o5 U)

0 -

-1.5

-4000

-2000

0

2000

4000

strain (micron)

Figure.7 Strian variation at the time of delamination at various interfaces and in MS layer when the laminate is subjected to mechanical loading along with actuator current.

Figure.8 Open circuit voltage in MS layer when the laminate is subjected to mechanical loading along with actuator current.




/

E 15 E

1.5 -

E E a)

m

05

0.5 -

EQ

0

E

o5

E

-0.5 -

a)

-1

0

ol

MS Layer C.)

-15 I

-80

I

I

I

-60

-40

-20

/

MS Layer

-1.5-

II 0

20

-500

40

500

0

strain (micron)

stress (M Pa)

Figure.9 Stress variation at various interfaces and in the MS layer when the laminate is subjected to actuator current only.

Figure.10 Strain variation at various interfaces and in the MS layer when the laminate is subjected to actuator current only.

stacking sequence (O101010101010101O)


70 68 66

U) 64 D)

62 60

0 0

58 56 54

U)

52 50 2

1

3

4

5

7

6

position of MS layer

Figure.11 Open circuit voltage at the time of delamination when the laminate is subjected to actuator current only.

Figure.12 Open circuit voltage with change in the position of MS layer for unidirectional laminate.

stacking sequence (O101010101010101O) 100

105

-S- MS layer on top

stacking sequence (019010145101451019010)

100

-.-MS layer at center 90

95

-'MS layer on top

-u-MS layer at center

90

D)

80

85

0 80

7o .

60

0

75 70

U) 65 50

0 60 55 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.5

MS layer thickness (mm)

1

1.5

2

2.5

3

3.5

4

4.5

5

actuating current (amp)

Figure.13 Stress in the MS layer due to magnetostriction with change of MS layer thickness for unidirectional laminate.

Figure.14 Open circuit voltage in the sensing coil with increase in current rating in actuator coil.


mid plane and use of a thicker MS layer is better for sensing. Role of mechanical loading is more significant in comparison of actuator current in causing delamination. Use of MS layers in composites for in-service NDE offers great potential in comparison to the conventional strain measuring methods as it presents distributed sensory properties with easier fabrication. Further, the possibility of noncontact damage detection system of large structures will facilitate appropriate life cycle decisions related to the structures under consideration.

ACKNOWLEDGEMENT The author1 acknowledges the financial assistance from AICTE, New Delhi under its Research Promotion Scheme.

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