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- 1. 2nd International Conference on Engineering Optimization September 6-9, 2010, Lisbon, Portugal Mechanical Properties of Nanocomposite Laminated Structures by Modal Method Horacio V. Duarte, L´azaro V. Donadon, Antˆonio F. ´Avila Universidade Federal de Minas Gerais, Av. Antˆonio Carlos 6627, Belo Horizonte, Brasil Abstract The Finite Element model of the laminate plate is used to approximate the experimental modal results by a optimization procedure. From the modal properties the tensile and shear modulus and the Poisson coeﬃcient of a woven orthotropic composite plate were determined by this method. The plates used in the experiment are made of S2-glass/epoxy with 16 layers are manufactured by vacuum assisted wet lay-up. The matrix of the nanocomposite has been obtained by adding 0%, 1%, 2% nanoclays in weight into epoxy matrix. Remarks are made about the results, analysis methodology and its limitations. . Keywords:Nanocomposites, Laminated Composites, optimization, Modal analysis . 1. Introduction There is a numbers of researchers who studied the eﬀect of nanoparticles (organically modiﬁed montmo- rillonite - Cloisite 30B) into epoxy systems. One aspect of the nanocomposite structure is the damage reduction due to impact loadings. Among researchers who studied the eﬀect of nanoparticles, Yasmin et al. [1] and Isik et al. [2] found also an increase in the elastic moduli and toughness [2]. A more comprehensive study on clay-epoxy nanocomposites was performed by Haque et al. [3], since they evaluated both mechanical and thermal properties. Their main conclusions were that thermo-mechanical properties mostly increase at low clay loadings ( 1-2% in weight) but decrease at higher clay loadings ( 5% in weight). In addition, the uses of nanoclays also decrease the coeﬃcient of thermal expansion (CTE). They also observed a degradation of properties at higher clay loadings. This phenomenon can be due to the phase-separated structures and defects in cross-linked structures. The objective of this paper is to study the nanoparticle inﬂuence into the plate vibration behavior and to determine the elastic constants using the optimization procedure. To analyze only the nanoparticle inﬂuence all manufacturing parameters are kept ﬁxed but the nanoparticle concentration. The modal analysis was made with the objective to determine the structural mode shape to compare the frequencies and modal properties for diﬀerent nanoclay composite plates. A commercial code, Ansys, which oﬀers the function of design optimization, is employed to model the woven laminate a orthotropic model. The optimization allows to approximate the theoretical laminate model modal results to experimental ones and by this way determining the elastic properties. 2. Testing Procedures and Results The nanocomposite prepared for this work is a S2-glass/epoxy-clay. The epoxy formulation is based on two parts, part A (diglycidyl ether of bisphenol A) and part B - hardener aliphatic amine- (triethylenete- tramine). The nanoclay particles used in this study are organically modiﬁed montmorillonite in a platelet form, while the S-2glass ﬁber has a plain-weave woven fabric conﬁguration with density of 180 g/m2 from Texiglass. The S2-glass/epoxy-nanoclay composite is a laminate plate with 16 layers and 65% ﬁber vol- ume fraction. The nanocomposite synthesis followed the methodology proposed by ´Avila et al. [?] [5]. In order to be able to investigate the nanoparticles inﬂuence into vibration analysis, samples with 0%, 1%, 2% of nanoclay with respect to the matrix mass were employed. All plates were rectangular and had the same dimensions 136mm x 116mm x 2.4mm. The vibration analyses were performed to determine the shape modes and its natural frequency, shape mode amplitude and the damping coeﬃcient. The vibration testing were performed using a Laser Doppler Vibrometer, laser model OFV 303.8 and controller model OFV 3001 S from Politec, a Hewlett Packard data acquisition system model 35670A, a nini-shaker, a shaker power ampliﬁer and force transducer from PCB. The test plates were hanged by a ﬁne nylon wire and excited by a random signal (white noise). A piezoelectric force transducer was used as the reference for the force bonded to the plate and linked to the stinger/shaker exciter, which transform the ampliﬁed electrical signal in force. There was only one force excitation point at the same position for all plates. The velocity of the plate surface was measured 1
- 2. using a grid of 35 points by the laser Doppler vibrometer. The data acquisition system processed the signal response of the measurement point generating the Mobility (velocity/force) Function Response Frequency (FRF), for each point of the plate. A modal analysis program has done the mode shape identiﬁcation from the 35 FRF for each plate. This modal analysis program is based on polynomial interpolation and employs Chebycheﬀ Orthogonal Polynomials method, Arruda et al. [6]. Mode 1 180.6321 [Hz] Damping=0.0297 Mode 2 344.4358 [Hz] Damping=0.0166 Mode 3 423.6049 [Hz] Damping=0.0098 Mode 4 568.7344 [Hz] Damping=0.0112 Mode 5 692.8686 [Hz] Damping=0.0137 Mode 6 735.2092 [Hz] Damping=0.0131 Figure 1: Shape modes for epoxy resin composite plate. There is no diﬀerence between the modal shapes of the 1% nanoclay composite and those of the resin reference plate, so the 1% resin shape are not showed. Figure (2) plots the shape modes for the composite plate with 2% nanoclay weight. The latest shape mode, named 7th mode, is out of the increasing frequency order, and this mode is similar to 4th one. Table (1) brings a summary of the modal properties natural frequencies, the associated damping coeﬃ- cients and Mobility amplitude for each experimental mode. The 7th mode, for the 2% nanoclay plate, is detached and presented in the appearing sequence. 2
- 3. Mode 1 173.544 [Hz] Damping=0.023 Mode 2 362.2734 [Hz] Damping=0.0248 Mode 3 423.8709 [Hz] Damping=0.0116 Mode 4 578.2479 [Hz] Damping=0.0136 Mode 5 715.5016 [Hz] Damping=0.0134 Mode 6 781.9341 [Hz] Damping=0.0166 Mode 7 655.2706 [Hz] Damping=0.0052 Figure 2: Shape modes for 2% nanoclay composite plate. 3
- 4. Table 1: Matrix composite content and Modal Properties. Composite Mass Modal Shape Modes Matrix [gr] Properties 1th 2th 3th 4th 7th∗ 5th 6th Pure Nat. Frequency [Hz] 180.6 344.4 423.6 568.7 692.9 735.2 Epoxy 66.37 Damping Coef. 0.0297 0.0166 0.0098 0.0112 0.0137 0.0131 Resin Amplitude [m/s]/kgf 57.0 17.0 6.5 4.8 12.1 8.5 Epoxy Nat. Frequency[Hz] 184.9 347.4 424.6 550.1 683.4 728.2 Nanoclay 65.27 Damping Coef. 0.0209 0.0247 0.0088 0.0136 0.0137 0.0138 1% weight Amplitude [m/s]/kgf 79.9 14.0 24.1 2.6 17.9 9.6 Epoxy Nat. Frequency [Hz] 173.6 361.3 424.0 578.0 655.3 ∗ 716.1 783.0 Nanoclay 66.92 Damping Coef. 0.023 0.0248 0.0116 0.0173 0.0052∗ 0.0134 0.0166 2% weight Amplitude [m/s]/kgf 32.9 5.8 4.7 21.5 44.3 ∗ 15.0 8.1 3. Data Analysis To determine the elastic properties was used the inverse method. This method is based on optimization of a Finite Element Model and is very popular. The optimization is a zero-order approach method, oﬀered as a tool of the FE commercial code, and followed the Hu and Wang [8] procedure. The state variables ξn are related to diﬀerence between FE, fF E n , and experimental modal nth frequency fn: ξn = fF E n − fn fn 100 (1) The cost function F is deﬁned as: F(Exy, Gxy, νxy) = k n=1 ξ2 (2) Where the mechanical properties are Exy the Tensile Modulus, Gxy the Shear Modulus and νxy the Poisson’s ratio. The element used in the Finite Element Model was the shell181 element. This shell element has four nodes and six degrees of freedom in each node. This element has been formulated to thin to moderately thick plates. This element also accepts multi-layers shell allowing the construction of complex models of laminates. The FE model also employed the mass21 element, to consider the eﬀect of mass and rotary inertia of the force transducer bonded to the plate. The structure was modeled by 320 shell elements and by concentrated mass element. For the reference resin plate and 1% nanocomposite plate the results are obtained by direct application of the method. The 2% nanocomposite plate showed a singular situation. The 4t h and 7t h has identical shape modes at diﬀerent frequencies. This behavior was not observed in the previous cases. The ﬁnite element simulation showed a coupling mode between the plate and the force transdutor, this result has appeared for all plate simulations but at a low frequency, below the experimental frequency range. There was not used a complex FE model to deal wiht the dynamical coupling between the elastic plate the force trandutor, the elastic stinger and shaker. So the dynamical coupling appeared something like a rigid body mode. But this model oﬀered a answer to the similar modes, one of them were a coupling mode between the experimental apparatus and the elastic plate. The nanocomposite mass in the matrix changed the elastic properties of the plates and the coupling frequency mode move inside the range of the experimental plate natural frequencies. The problem is to determine what mode is a plate mode and what is not. The solution presented here is to ﬁnd the real mode by comparing the error between the experimental mode frequencies and the frequencies of the optimized or minimized model. 4
- 5. Table 2: Frequency mode error in each iteration. Frequency for the 4th mode 578.0 Hz iteration 1th mode 2th mode 3th mode 4th mode 5th mode 6th mode 1th 0.872E-02 0 0 0 0 0 0.01 2th 0.423E-01 0.389E-01 0 0 0 0 0.08 3th 0.985 2.08 2.22 0 0 0 5.28 4th 0.984 2.10 2.20 0.789 0 0 6.08 5th 1.03 2.02 2.28 0.670 2.30 0 8.29 6th 1.02 2.03 2.27 0.692 2.28 3.92 12.21 The Tab. (2) and Tab. (3) show the frequency error for each mode in the minimization procedure. The minimization was not done for all frequencies in a unique process. The procedure was to include one mode frequency at time and ﬁnd the elastic properties. The latest elastic propeties are used as initial value in a new minimization process, that includes the next frequency. This procedure will be called an iteration. The latest iteration is performed using a small tolerance for the cost function F, Eq. (2), for all cases F < |10−2 |. The Tab. (2) presents error for each mode frequency in each iteration, in this case, the fourth mode is supposed to be the plate mode. The Tab. (3) presents the same error evolution considering the seventh mode (655.3 Hz) as the plate mode. Table 3: Frequency mode error in each iteration. Frequency for the 4th mode 655.3 Hz iteration 1th mode 2th mode 3th mode 4th mode 5th mode 6th mode 1th 0.872E-02 0 0 0 0 0 0.01 2th 0.423E-01 0.389E-01 0 0 0 0 0.08 3th 0.985 2.08 2.22 0 0 0 5.28 4th 1.0087 7.5204 2.0591 2.7306 0 0 13.3 5th 1.0262 2.4241 1.5333 8.8377 1.6721 0 15.5 6th 0.98710 2.4631 2.0953 9.5305 2.1113 4.1373 21.3 The Tab. (2 shows the lowest errors for the fourth and the sum of error of each frequency mode. So it is used to determine the elastic properties showed on Tab(4). 4. Closing Remarks The inverse method used here minimizing a ﬁnite element model showed to be a powerful, fast and very ﬂexible method of analysis. Without the method it was necessary to repeat all the experimental procedure to ﬁnd the problem. In this work the sensor mass and mainly the rotatory inertia has made a strong inﬂuence on the results, this parameter is diﬃcult to determine with precision. The excitation of the system is still a problem and can introduce error in the results. The Tensile Modulus Exy increases with nanoclay mass increase in the matrix epoxy resin. The Tensile modulus for the 2% composite plate is 17% greater than the equivalent resin modulus. For the Shear Modulus, Gxy, the maximum value is at 1 % nanoclay in matrix resin. The Poisson’s coeﬃcient νxy showed a very low value. Part of the problem is that the Shear and Tensile modulus have highest values compared to Poisson’s coeﬃcient value. Probably the shape modes used do not capture its inﬂuence [7]. Table 4: Elastic Properties Composition Exy[GPa] Gxy[GPa] νxy Resin 100% 27.3 4.2 0.10425E-07 Nanoclay 1% weight 28.3 5.1 0.10212E-03 Nanoclay 2% weight 31.9 3.8 0.75339E-03 5
- 6. All nanocomposite plates had a highest damping coeﬃcient and this properties must be included as it can change the frequencies mainly the highest ones. The plates used in this work can not be considered thin and some shape modes are diﬃcult to obtain, the synclastic mode for instance. The behavior of the new materials can not be obtained by standard procedures. Acknowledgements This work was supported by the Funda¸c˜ao de Amparo a Pesquisa de Minas Gerais FAPEMIG, which support the authors are grateful. References [1] Yasmin, A. and Abot, J.L. and Daniel, I.M., Processing of clay-epoxy nanocomposites by shear mixing, Scripta Materialia, 49 (1), 81-86, 2003. [2] Isik, I. and Yilmazer, U. and Bayram, G., Impact modiﬁed epoxy/montmorillonite nanocomposites: synthesis and characterization, Polymer, 44 (25), 6371-6377, 2003. [3] Haque, A. and Shamsuzzoha, M., S2-glass/epoxy polymer nanocomposites: manufacturing, struc- tures, thermal and mechanical properties, Journal of Composite Materials, 3 (1), 1821-1837, 2003. [4] ´Avila, A. F. and Duarte, H. V. and Soares, M. I., The Nanoclay Inﬂuence on Impact Response of Laminate Plates, Latin American Journal of Solids and Structures, 37 (20), 3-20, 2006. [5] ´Avila, A. F. and Duarte, H. V.,Impact Behaviour of NanoComposites, Proceedings 47th AIAA, ASME, ASCE, AHS, ASC Structures, Structural Dynamic and Materials Conference, New Port, RI.,AIAA (Eds.), CDROM, 2006. [6] Arruda, J. R. F. and Rio, S. A. V. and Santos, L. A. S. B., A Space-Frequency Data Compres- sion Method for Spatially Dense Laser Doppler Vibrometer Measurements, Journal of Shock and Vibration (Wiley), 3 (2), 127-133, 1996. [7] Sol, H. and Bottiglieri M., Identiﬁcation of the Elastic Properties on Composite Materials as a Func- tion of Temperature,Proceedings of 11th Pan-American Congress of Applied Mechanics PACAM ABCM (Eds), Foz do Igua¸cu, Brasil, CDROM, 2010. [8] Hu H. and Wang J., Damage detection of a woven fabric composite laminate using modal strain energy method, ,Engineering Structures, 31, 1042-1055, 2009. 6