WATER AND SEAWATER EFFECTS ON THE MEMBERS MADE OF E-GLASS COMPOSITE MATERIALS

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1 SISOM 26, Bucharest May WATER AND SEAWATER EFFECTS ON THE MEMBERS MADE OF E-GLASS COMPOSITE MATERIALS Camelia CERBU Department of Strength of Materials and Vibrations, Transilvania University of Brasov, B-dul Eroilor 29, 536 Brasov, România, In this paper, the effects of the water and seawater on the motor boat hull made of E-glass reinforced polymers, are analysed. Some experimental results concerning the moisture effects on the mechanical properties of the specimens made of E-glass reinforced resins are shown. The changes of the rigidity after 92 hours water and natural seawater (from Black Sea), is analysed in case of three composite materials: E-glass / polyester Heliopol 8431 ATX, E-glass / polyester Polylite 44- M88 and E-glass / epoxy LY 554. Then, a numerical model (FEM) of the rear plate of the motor boat hull is proposed. Due to a good conservation of the strength and flexural modulus under the action of the moisture effects in case of the E-glass / polyester composites studied, the stiffness of the member analysed decreases a little. This is the reason for which the present paper finally recommends the E-glass / polyester composites to manufacture members that are mechanical loaded in the wet environments. Key words: composite material, moisture, water, seawater, stiffness. 1. INTRODUCTION Environmental moisture can penetrate the organic materials by a diffusion process. Typically, moisture works as a plasticizer for a polymer, i.e., properties as stiffness, strength and glass transition temperature decrease with the ingress of moisture in polymer. It is now well recognised that the problem of moisture absorption in polymeric matrix composites is a very important one. There are two fundamental effects that may be taken into account when designing components made of polymeric matrix composites (PMCs) namely, temperature and humidity. The combined effect of these two, that is, hygrothermal effects, can result in a considerable degradation in the mechanical characteristics of polymeric matrix composites. This is especially so in high-performance composites such as those used in the aerospace industry where dimensional tolerances are rather severe. Glass fibre reinforced polymers are used in a wide variety of industries: from sporting goods, civil constructions to aerospace. Tanks and vessels (pressure and non-pressure) in the chemical process industry, as well as process and effluent pipelines, are routinely made of glass fibre reinforced polyesters resin. For boat builders, the composite materials used must be stiffness, waterproof, sun-proof in addition to good impact strength. To use the full potential of the composite materials, their response to environmental effects (moisture, temperature, thermal cycle etc.) must be known. In the previous papers [2, 3, 4, 5] it was shown that the mechanical characteristics (tensile strength, flexural modulus E and flexural stress σ e to the elastic limit) decreases after 92 hours water and natural seawater (from Black Sea). Some experimental results published in that papers will be used to show the advantages of the E-glass / polyesters composites in case of the naval applications. The conservation of the stiffness under the action of the water and seawater is an additional criterion should be used to design members loaded in wet environments. The stiffness of the rear plate of a motor boat hull is analysed in case of the undamaged and damaged composite materials to show the effects of water and seawater on the members which works in these environments.

2 Camelia CERBU WORK METHOD In this paper, theoretical researches concerning the states of stress and strain inside the rear plate of a motor boat hull (figure 2) were considered to analyse the effects of the water and seawater absorption on the stiffness. We note that S.C. Compozite S.R.L. of Brasov (Romania) manufactures the motor boat (figure 1) analysed within this paper. Figure 1. Motor boat Figure 2. Motorboat hull The first of all, experimental investigations [2, 3, 4] concerning environmental effects on the mechanical behaviour of the randomly E-glass fibres reinforced polymeric resins (polyester Heliopol 8431 ATX, polyester Polylite 44-M88 and epoxy LY 554), were made. Flexural stress σ e [MPa] E-glass / polyester HELIOPOL 8431 A Unprotected - 96,6 % Protected - 12,5 % Unprotected - 86,4 % 138 Protected - 89,46 % Flexural modulus E [MPa] a. b. E-glass / poliester Heliopol 8431 ATX 8913 Unprotected - 94,97 % Protected - 98,18 % 9215 Unprotected - 87,81 % Protected - 87,55 % Figure 3. Experimental results of the flexural test in case of E-glass / Heliopol 8431 ATX composite Flexural stress σ e [MPa] E-glass / polyester Polylite 44-M Unprotected - 86,9 % Protected - 91,6 % Unprotected - 81,3 % 134 Protected - 83,8 % a. b. Flexural modulus E [MPa] E-glass / polyester Polylite 44-M Unprotected - 9,58 % Protected - 98,86 % Unprotected -89,9 % Protected - 91,45 % Figure 4. Experimental results of the flexural test in case of E-glass / Polylite 44-M88 composite

3 179 Water and seawater effects on the members made of e-glass composite materials Flexural stress σ e [MPa] E-glass / epoxy LY Unprotected - 69,4 % Protected - 68,7 % 1 98 Unprotected - 78,6 % Protected - 76,62 % Flexural modulus E [MPa] a. b Unprotected - 77,99 % E-glass / epoxy LY 554 Figure 5. Experimental results of the flexural test in case of E-glass / epoxy LY 554 composite Protected - 81,72 % Unprotected - 91,42 % Protected - 9,27 % Water and fresh natural seawater from Black Sea at room temperature were used as wet environments. The salinity of the natural seawater was approximately of 1.6 %. The water tanks were covered to minimise evaporation and it was changed every month to keep conditions constant. After 92 hours of immersion, the flexural specimens were subjected to a flexural test (three points method) according to [6]. The researches lead us to the conclusion that the flexural modulus E and flexural stress σ e at the elastic limit, decrease due to the moisture absorption (figures 3 5). It was found that the greatest decreasing of the mechanical characteristics (figure 5) occurred in case of the E-glass / epoxy composite, especially after immersion in water. In general a grater decreasing of the mechanical properties was observed in case of the composite materials immersed in water. The reason would be that the composites tested absorbed more water than seawater [1]. The sodium chloride molecules contained in the seawater (as well as sulphate) appear to be limiting the diffusion of water into the matrix material. Then, a numerical model of the rear plate of a motorboat hull (figure 6) was proposed by using the method of finite elements. Stresses and strains occurred due to the action of the drag force ( Fmax = 1589, 22 N ) developed by the motor of the boat, were analysed. To model the rear plate Shell43 elements are used because this member is made of randomly E-glass fibres composite materials. It is known that randomly reinforcing is used to obtain a composite material whose elastic characteristics are the same about any direction (isotropic material). Figure 6. Boundary conditions and loading

4 Camelia CERBU 18 This means that the element Shell43 is justifiably chosen. Solid45 elements are used for the holding plate of the motor whose drag force acts upon the rear plate of the motor boat hull. Finally, nodes and elements were obtained for the numerical model of the rear plate of the motor boat hull. Figure 6 shows boundary conditions and the external drag force developed by the motor. The first of all, the results concerning stresses and strains developed in the rear plate of the motor boat E = 981MPa. Herein, we show only the equivalent hull made of E-glass / polyester Polylite 44-M88 ( ) normal stress σ ech. by using Von Misses theory (figure 7), normal strains ε x (figure 8) and displacement u x (figure 9). We observe that the normal stresses do not exceed the elastic limit of the composite materials analysed. Figure 7. Equivalent stress σ eqv. (Von Misses) Figure 8. Normal strain ε x Figure 9. Displacement u x (E-glass / polyester Polylite 44- M88 - E = 981MPa ) Figure 1. Displacement u x after 92 hours water (E-glass / polyester Polylite 44-M88 - E = 8225MPa ) It follows that E-glass / polyester Polylite 44-M88 composite could be used to manufacture the motor boat hull. The problem that arises is how the degradation of the composite material due to the moisture absorption, acts on the stiffness of the rear plate. Because the greatest values of the displacement are about Ox direction, the changes of the stiffness was analysed by comparing the maximum displacement about that direction as will be shown in the next section.

5 181 Water and seawater effects on the members made of e-glass composite materials 3. RESULTS AND DISCUSSIONS Since the conservation of some mechanical characteristics of the composite materials tested, had already been analysed, it was easy to apply the experimental results to the numerical model. The results concerning the displacement u x of the rear plate in case of the E-glass / Polylite 444-M88 undamaged (figure 9) may easily compared with the results obtained in case of the damaged composite (figure 1) after 92 hours water. After 92 hours seawater After 92 hours water E-glass / polyester HELIOPOL 8431 ATX 2,26 2,198 Dry material 1,924,,5 1, 1,5 2, 2,5 Maximum value of the displacement u x [mm ] Figure 11. Maximum value of the deflection u x for the rear plate (E-glass / Heliopol 8431 ATX) After 92 hours seawater After 92 hours water E-glass / polyester POLYLITE 44-M88 2,26 2,189 Dry material 1,983,,5 1, 1,5 2, 2,5 Maximum value of the displacement u x [mm ] Figure 12. Maximum value of the deflection u x for the rear plate (E-glass / Polylite 44-M88) After 92 hours seawater After 92 hours water E-glass / epoxy LY 554 3,324 3,848 Dry material 3,1,,5 1, 1,5 2, 2,5 3, 3,5 4, 4,5 Maximum value of the displacement u x [mm ] Figure 13. Maximum value of the deflection u x for the rear plate (E-glass / epoxy LY 554)

6 Camelia CERBU 182 Since the modulus of elasticity decreases due to the action of the water and seawater (table 1), the stiffness of the rear plate also decreases. The maximum value of the displacement u x of the rear plate, was graphically analysed (figures 11-13) in case of all composite materials tested, before and after 92 hours ( 13 months) water and natural seawater (from Black Sea). Analysing the above graphics the following important remarks are noted: - The effects of the water and seawater on the stiffness of the rear plate analysed are different in case of all composite materials considered. - The greatest value of the maximum displacement u x = mm is observed in case of the E-glass / epoxy LY 554 composite damaged due to the water absorption. - The maximum displacement u x in case of the E-glass / Polylite 44-M88 and E-glass / Heliopol 8431 ATX composites undamaged is approximately equal to half of the maximum displacement u x in case of the E-glass / epoxy LY 554 damaged due to the water absorption. - The conservation of the stiffness of the motor boat hull is not good when E-glass / epoxy LY 554 composite is used to manufacture this member. 4. CONCLUSION Finally, some important conclusions of this paper are noted: Since the flexural modulus E of the composite materials analysed decreases, it follows that the stiffness of these composites decreases by the same ratio (between 5 % and 2 %). The E-glass / polyester composites analysed are recommended as composite material in case of water or seawater environment. In particular case, that material may be used to manufacture the hull of the motor boat analysed. From material point of view, the structural optimisation of the rear plate of the motorboat analysed, leads the author, to the conclusion that E-glass / Polylite 44-M88 and E-glass / Heliopol 8431 composites must be used to manufacturer this member. The principal reason was the conservation of the strength and stiffness under the action of the environmental effects in case of that composite material. This criterion was considered in addition to the high-strength criterion and high-stiffness criterion. E-glass / epoxy LY 554 composite materials is not recommended in wet environment due to the degradation of the stiffness under the action of water and seawater. From point of view of the reasons shown in this work, it is recommendable to take into account the results of this paper to design the structural members which works in wet environments. REFERENCES 1. CERBU, Camelia, CURTU, I., ROŞU, D., Moisture behaviour of the glass fibre mat reinforced polymer, In: Sympózium drevné kompozitné materiály (Composite wood materials), ISBN , Zvolen, pp , june of 24; 2. CERBU, Camelia, Aspects concerning the moisture effects on the stiffness of the glass-reinforced polymers, Proceedings of the 4th Youth Symposium on Experimental Solid Mechanics, Castracaro Terme, Italia, ISBN , pp , May 25; 3. CERBU, Camelia, CURTU, I., Aspects concerning environmental effects on the glass-reinforced polymers, Proceedings of the 9th International Research / Expert Conference Trends in the Development of Machinery and Associated Technology TMT 25, ISBN , Antalya, Turcia, pp , 26 3 September 25; 4. CERBU, Camelia, Researches concerning structural optimization of some members made of composite materials loaded under aggressive environmental effects, Doctoral thesis, University Transilvania of Brasov, Romania, 17 December 25; 5. SPRINGER, G. S., Environmental Effects on Composite Materials, Vol. 2, Technomic Publishing Inc., Lancaster, PA, 1984; 6. SR EN 63, Glass fibre reinforced plastics, Determination of flexural properties, Three point method, CEN, Bruxelles, 1998.