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Influence of reinforcement on the properties of filled epoxy composites.

1. INTRODUCTION

While the major aim of composites is replacing the metals in aircraft, special applications the electromagnetic properties of such materials are very important. As it is well known metals show high electric conductivities while the composites (excepting metal matrix, metal reinforced or metal-metal ones) show poor electric conductivity. It is obvious that the polymeric composites are showing extremely small values of electric conductivity since their matrix are traditionally used as insulators but their major advantage is the low specific weight. Polymers are also relatively sensitive to the action of aggressive media and show high values of water absorption. The most common solution to improve the polymers' properties is to fill them with various solid systems (such as small metallic spheres) or powders obtaining filled composites, according to (Jones 1999). The mechanical properties of composites are improved in the case of reinforcements (reinforced composites or fibrous composites). There are many studies regarding the design of the reinforcement in various geometries (2D, 3D, 4D) in order to obtain dedicated composite structures (Vinson & Sierakowski 1986) and regarding the mechanical analysis of laminate composites (Constantinescu & Alamoreanu 1995, Vasiliev & Morozov 2001). In the case of laminate composites, the use of reinforcements induces a high level of anisotropy while the use of fillers has the opposite effect.

[FIGURE 1 OMITTED]

Combining both alternatives of polymers' properties improvement it is possible to obtain better materials but the problem is to predict the final properties which generally are unknown. The design approach is based on the knowledge of components' properties and on the researchers' abilities to preview the major effects of their utilisation. It is generally accepted that the most important feature of the composites is the reinforcement/matrix or filler/matrix ratio (weight or volume) and this parameter is used in order to describe the macroscopic properties of finite material (Callister 1994, Morozov 2004).

2. MATERIALS

A set of 10 different materials were realized in order to evaluate the influence of filler over the electric and electromagnetic properties of the composite materials. The results are showed in (Circiumaru et al. 2007). In order to identify the effects of reinforcement over composite's electric and electromagnetic properties two types of samples were formed. Both types of samples have reinforcements of 13 sheets of simple type of fiber fabric. The mixed fabric is realized of alternate untwisted fascicles kevlar and carbon filaments. For each type of reinforcement four types of matrix were realized using filled epoxy resin in various setups. The samples were realized through layer-by-layer method. In this case the matrix was realized, each time, by using the same concentration of various fillers (CNT, Ferrite, Talc) but the filled resin was used in layers. In fact, the samples are named with four characters the first one denoting the type of reinforcement (K for Kevlar-carbon fiber fabric and C for carbon fiber fabric). The other three characters are denoting the epoxy's filler (C for CNT, F for ferrite, T for talc). Assuming the symmetry of reinforcement reported to medial plane there were used, for example, three layers of Ferrite filled epoxy (external layers), three layers of CNT filled epoxy (middle layers) and two of Talc filled epoxy (intermediate layers). So, the structure of the sample from the matrix point of view is 3F-2T-3C-2T-3F and for carbon fiber fabric reinforcement the sample is CFTC.

In the C-type samples there are alternate 0 degrees 45 degrees sheets of reinforcements wile in K-type of samples all sheets are placed such as fill and warp are parallel. In order to ensure the adhesion of epoxy to the two types of fibers the fabrics were covered with a thin film of PNB rubber.

[FIGURE 2 OMITTED]

[FIGURE 4 OMITTED]

3. MESUREMENTS AND RESULTS

Standard DIN EN ISO 10545-4 three point bending test was performed. Three types of samples were cut using a high pressure jet machine: p-type along the warp, a-type across the warp and fill at an angle of 45 degrees. The electric permittivity of each sample was evaluated using a measurement technique described in (Misra 1999, Heaney 1999). The measurement cell allows the four point measurement method and is respecting the imposed technical specifications. The same measurement geometry is used in order to measure the electric resistance and evaluate the electric conductivity. The apparent elastic modulus is evaluated and it is a measure of composite properties in the domain of linear deformations.

4. CONCLUSIONS

We might say that in the C-type samples the degree of anisotropy is decreased. In both types of samples can be easily noticed the effect of external talc filled layers. Analyzing the graphs in above page the main conclusion is the known one about the improving of mechanical properties by using various oriented sheets as reinforcement. Also we may conclude that the solution with more than one layer of filled matrix leads to weak results than the alternate layers one.

The carbon fiber fabric seems to be a better reinforcement solution in order to form strong composites. An impact analysis has to be performed in order to identify chock resistance of such materials. Also we intend to develop our research by forming kevlar-carbon fiber fabric based composites but alternating the orientations of the sheets. It is our concern that it can be found the better concentrations combination of two or more fillers, in the same polymer, in order to reach the intended properties of the final material.

Thermal and thermo-mechanical analysis might be performed in order to refine the solution about composition and the architecture of pseudo-laminate plate. We use the term pseudo-laminate because in our case we cannot say that we used laminae in order to obtain the final laminate, in fact our reinforcement sheets cannot be treated as laminae.

Analyzing the graphs the main conclusion is the known one about the improving of mechanical properties by using various oriented sheets as reinforcement. Also we may conclude that the solution with more than one layer of filled matrix leads to weak results than the alternate layers one. The carbon fiber fabric seems to be a better reinforcement solution in order to form strong composites. An impact analysis has to be performed in order to identify chock resistance of such materials. Also we intend to develop our research by forming kevlar-carbon fiber fabric based composites but alternating the orientations of the sheets. It is our concern that it can be found the better concentrations combination of two or more fillers, in the same polymer, in order to reach the intended properties of the final material. Also it might be possible a design having as result a composite structure into which the reinforcement plays the role of electric circuit. Using small amounts of clay we succeed in improving the nanosized particles dispersion into the polymeric matrix and identifying other possibilities to control the electric properties (Circiumaru et al. 2008).

[FIGURE 5 OMITTED]

Thermal and thermo-mechanical analysis might be performed in order to refine the solution about composition and the architecture of pseudo-laminate plate. We use the term pseudo-laminate because in our case we cannot say that we used laminae in order to obtain the final laminate, in fact our reinforcement sheets cannot be treated as laminae.

5. REFERENCES

Callister, W. D. (1994) Materials Science and Engineering, John Wiley & Sons, ISBN 0471736961.

Circiumaru, A., Andrei, G., Birsan, I.-G., Dima, D., (2007) Electric and electromagnetic Properties of Fiber Fabric Based Filled Epoxy Composites, The Annals of "Dunarea de Jos" University of Galati, Fascicle IX, Faculty of Metallurgy and Materials Science, XXV (XXX), May 2007, no. 1, pp. 97-102, ISSN 1453083X.

Circiumaru, A., Birsan, I.-G., Andrei, G. (2008) Electric Conductivity of Fabric Based Filled Epoxy Composites, ECTP--18th European Conferenece in Thermophysical Properties, Pau, France (Poster prezentation).

Constantinescu, D. M., Alamoreanu, Elena, (1995) Proiectarea placilor compozite laminate (Design of composite laminate plates), Editura Academiei Romane, Bucuresti.

Heaney, M. B. (1999) Electrical conductivity and resistivity in Webster, J. G. (ed), Measurements, Instrumentations, and Sensors, CRC Press, 49, ISBN 0849383471.

Jones, R. M., (1999) Mechanics of composite materials, Taylor & Francis.

Misra, D. K. (1999) Permittivity measurement, in Webster, J. G. (ed), Measurements, Instrumentations, and Sensors, CRC Press, 46, ISBN 0849383471

Morozov, E. V., Mechanics and Analysis of Fabric Composites and Structures, AUTEX, vol. 4, No. 2, june 2004, p. 60.

Vasiliev, V. V., Morozov, E. V. (2001) Mechanics and Analysis of Composite materials, Elsevier, ISBN 0080427022.

Vinson, J. R., Sierakowski, R. L., (1986) The behavior of structures composed of composite materials, Martinus Nijhoff Publishers, Dordrecht.
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Author:Circiumaru, Adrian; Birsan, Iulian Gabriel; Andrei, Gabriel; Bria, Vasile; Crudu, Liviu
Publication:Annals of DAAAM & Proceedings
Date:Jan 1, 2008
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