Effect of UD Carbon on the Specific Mechanical Properties of Glass Mat Composites for Marine Applications

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1 Effect of UD Carbon on the Specific Mechanical Properties of Glass Mat Composites for Marine Applications A. ALENZA AND. FIORE* Dipartimento di Ingegneria Chimica dei Processi e dei Materiali University of Palermo, iale delle Scienze, 9128 Palermo, Italy G. DI BELLA Dipartimento di Tecnologia Meccanica Produzione e Ingegneria Gestionale University of Palermo, iale delle Scienze, 9128 Palermo, Italy ABSTRACT: In this work the influence of a uniaxial carbon fabric layer on the mechanical performances of a glass mat/epoxy composite used for marine applications has been studied. All the structures have been made, at room temperature, by vacuum bagging technique. Tension and flexural tests have been carried out in order to evaluate the specific mechanical properties of the composite and to compare these with those of the marine aluminium alloy 616-T4. The glass composites have higher specific strength but lower specific modulus than aluminium alloy. To increase the specific modulus of the composites, each layer of glass mat has been replaced with a layer of uniaxial carbon fabric. In addition, a simplified numerical model has been proposed to understand better the relevant dependence of the specific mechanical properties from the position and the orientation of the fibers. The comparison of the predicted numerical results with experiments has shown the accuracy of this model. KEY WORDS: hybrid composite, vacuum bagging, fiber orientation, specific property, FEA. INTRODUCTION GLASS FIBER REINFORCED composite (GFRP) materials are widely used in several key applications such as ships, aircraft, buildings, bridges, automobiles and other transportation vehicles because of their high mechanical strength and stiffness, high impact resistance, low weight, corrosion resistance and very low maintenance cost. In the nautical field, the composite materials have been used since about the 195s to replace traditional *Author to whom correspondence should be addressed. fiore@dicpm.unipa.it Figures 6 8 appear in color online: Journal of COMPOSITE MATERIALS, ol. 44, No. 11/ /1/ $1./ DOI: / ß The Author(s), 21. Reprints and permissions:

2 1352 A. ALENZA ET AL. Table 1. Specific properties of aluminium alloy and reference laminate. Material Aluminium alloy 616 Mat glass/epoxy Density (g/cm 3 ) Flexural specific strength (MPa/g/cm 3 ) Flexural specific modulus (GPa/g/cm 3 ) Tensile specific strength (MPa/g/cm 3 ) Tensile specific modulus (GPa/g/cm 3 ) metal like steel [1]. Other materials used in the previous years in marine applications are the aluminum alloys due to their high mechanical resistance/weight ratio and good corrosion resistance. In the ship design the topside structures show a critical stiffness and the hull structures show a critical strength. For this reason, the stability and the performance of the ships are enhanced by using a high stiffness material for the topside structures and a high strength material for the hull structures [2]. Even if the specific strength of glass composites is higher than aluminium alloys, their specific stiffness is lower (Table 1). So the glass composites are often suited to build the ship hulls but not for the topside structures. The aim of this work is to increase the specific stiffness of the composite materials replacing a layer of glass mat with a uniaxial carbon fabric, in order to use these materials for the topside structures also. Fiber reinforced plastic composite (FRP) structures having a combination of two different reinforcement fibers, usually defined hybrid composites, which have been realized and then tested. The most common hybrid composites are: carbon-aramid reinforced epoxy which combines strength and impact resistance [3,4] and glass-carbon reinforced epoxy which gives a strong material with a reasonable price. Hybrid composites are usually used when a combination of properties of different types of fibers wants to be achieved, or when longitudinal performances as well as lateral mechanical ones are required. In the literature there are many works regarding this kind of composites. Tan and Nieu [5] have investigated the chemical resistance, the thermo-oxidative stability and the mechanical properties (i.e. tensile and flexural strength, interlaminar shear strength (ILSS) and impact strength) of unidirectional vinyl ester composites with varying E glass/carbon fibers ratio. They have verified that increasing the amount of carbon fiber in the hybrid composite chemical resistance and some mechanical properties, especially tensile and flexural resistance, increased, while impact strength decreased. Tsai et al. [6] have studied the influence of the absorption and diffusion of water on thermal and mechanical properties of carbon/glass fiber hybrid composites. Particularly, water-sorption experiments, mechanical tests and dynamic mechanical analysis (DMA) have been performed after immersion in water at different temperatures for up to 32 weeks. The shear properties and the glass transition temperature (T g ) decreased with increasing water. As long as the water concentration in the sample was less than the saturation level, the SBS strength and T g showed 77% and 98% retention, respectively, after drying the samples in a 1 C oven for 2 days to stabilize the weight.

3 Mechanical Properties of Glass Mat Composites for Marine Applications 1353 Cho et al. [7] have investigated the changes in the electrical resistance of composites subjected to tension and bending tests to understand the smart (i.e. good memory) characteristics of hybrid composites with carbon (CFs) and glass (GFs) fibers and epoxy resin as a matrix. They have shown that the fractional electrical resistance of composites (fraction of the change in the electrical resistance relative to the electrical resistance of the unloaded sample) under tension has been affected by the composition of the CF/GF, as well as the applied strain. The slow increase in the fractional electrical resistance has been shown in a relatively low strain region; on further loading, a stepwise increase in the resistance has been shown due to the fracture of CF layers. The strain sensitivity of the samples (the fractional electrical resistance per unit strain) is increased with increasing CF weight percentage. The hybrid composites with a CF content more than 4 wt% had a strain sensitivity higher than The changes of the fractional electrical resistance under bending have not been so dominant when compared to the results under tension. To overcome the limitations of low specific stiffness of the woven glass composites compared with the marine steel, Shivakumar et al. [2] have studied the feasibility using the stitch-bonded carbon fiber, an equibiaxial ( 9 ) fabric bonded with a vinyl ester sizing and knitted by a polyester thread. Compared to the woven glass composites, these CFRP structures have shown more than two times the tensile modulus, strength and the compression modulus, about 15% higher compression strength, almost similar in-plane shear strength, about 95% higher of in-plane shear modulus and the inter laminar shear strength. The objective of the present study is to increase the specific stiffness of the FRP materials, to reach the same or better values than the reference aluminium alloy AA 616, making these structures applicable to both the topside and the hull structures of the ship. For this aim, hybrid composites have been produced starting from a glass mat reference laminate replacing a layer of glass with one layer of uniaxial carbon fabric. The specific properties of these composites have been measured through standard tensile and three point bending tests, and then compared with the reference marine aluminium alloy. Finally, a simplified numerical model has been proposed to evaluate the influence of carbon layer on the mechanical properties of hybrid structures, by employing a commercial code (i.e. Ansys). The experimental data have been compared with the finite element analysis, confirming the good predictive capability of the numerical model. As a result of using this input data of physical and mechanical properties of the composite constituents, it is possible to foresee the behavior of these hybrid laminates to optimize the carbon layer position for design of complex composite structures. Materials and Manufacturing EXPERIMENTAL SETUP All the composite structures have been realized with a single lamination using the vacuum bagging technique. This method involves an initial hand lay-up phase and then the polymerization of the matrix in a flexible bag in which negative pressure is reached by a vacuum pump. acuum bag technology brings some advantages to the final characteristics of composite laminate compared to hand lay up technology. All the laminates have been cured at room temperature for 24 h and then post-cured at 6 C for 8 h.

4 1354 A. ALENZA ET AL. Figure 1. Uniaxial carbon fabric. The stacking sequence of the reference laminate (called Mat ) is constituted of six layers of E-glass mat (randomly oriented fibers, with areal weight of 45 g/m 2 ) in a matrix of epoxy resin (Sicomin SR 85). The total average thickness of the laminates is 2.6 mm. The hybrid composites have been produced replacing a layer of glass mat with one of uniaxial UHM carbon nonwoven fabric with areal weight of 32 g/m 2 (Figure 1). The total average thickness of the hybrid laminates is about 2.9 mm. The reference aluminium alloy is the AA616-T4 received in 1 m 1 m panel of 2.5 mm thickness. The specimens are derived from the panel and then subjected to the tensile and flexural preliminary tests. The specific mechanical properties of the aluminum alloy and composite are reported in Table 1. Mechanical Testing FLEXURAL CHARACTERIZATION To evaluate the influence of carbon on the flexural specific stiffness, each layer of glass mat of the reference laminate has been replaced with one layer of uniaxial carbon fabric. Moreover for each position of the carbon layer, three structures have been realized; i.e. using respectively a different carbon fiber orientation: parallel ( ), perpendicular (9 ) and oriented at 45 to the x direction (Figure 2). A total of 18 hybrid structures have been produced. In Table 2 both the position and the orientation of uniaxial carbon fabric in the hybrid laminates for the flexural characterization are reported. Three point bending tests have been carried out according to ASTM D 79, by using a Universal Testing Machine (UTM) mod by Instron, equipped with a load cell of 5 KN.

5 Mechanical Properties of Glass Mat Composites for Marine Applications 1355 x Figure 2. Laminate geometric configuration. y z Table 2. Position and orientation of uniaxial carbon in the hybrid laminates produced for the flexural characterization. Sample C1 C2 C3 C4 C5 C6 Position Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Orientation Flexural tests have been performed on each of the 18 realized structures, using five prismatic samples with dimensions 2 mm 96 mm. For all tests, the span length is equal to 8 mm and cross-head speed to 4.26 mm/min. TENSILE CHARACTERIZATION As in the previous case, 18 hybrid structures have been produced and tested. Each layer of glass mat of the reference laminate has been replaced with one layer of uniaxial carbon fabric oriented respectively at,45 and 9 to the loading direction, which is the x direction (Figure 2). Tensile tests have been carried out according to ASTM D 339, by employing a UTM by Zwick Roell, equipped with a load cell of 6 kn, with a cross head speed of 2 mm/min. For each structure realized, five prismatic samples with dimensions 25 mm 25 mm have been tested. Flexural Characterization RESULTS AND DISCUSSION In Figure 3 the specific flexural modulus of the hybrid composites and aluminium reference alloy (dotted line) are reported. It is possible to observe that: For the cross angle of the uniaxial carbon layer equal to 45 or 9, the specific stiffness does not show an improvement compared to the reference laminate and, consequently, it remains lower than the reference aluminium alloy. For the perpendicular (9 ) orientation of the carbon layer, the specific flexural modulus varies between GPa/(g/cm 3 ) and GPa/(g/cm 3 ) for the C6 and C2 structures, respectively. Similarly, using uniaxial carbon layer oriented at 45 to the x direction, this specific property varies between GPa/(g/cm 3 ) and GPa/(g/cm 3 ) for the C1 and C2 structures, respectively. For the parallel orientation ( ) of the uniaxial carbon to the x direction, high increases of specific stiffness have been obtained particularly by replacing the external layers of

6 1356 A. ALENZA ET AL E/r (Gpa/ (g/cm 3 )) AI Mat C1 C2 C3 C4 C5 C6 Figure 3. Specific flexural modulus of the laminates. glass mat. Clarifying, the specific flexural modulus of the C1 and C6 structures show about 222% and 258% higher than the reference laminate, respectively. For the C2 and C5 structures we have obtained the fewest improvements (59% and 64%, respectively). By replacing the internal layers of mat glass with uniaxial carbon, the specific stiffness of the hybrid structures does not increase compared to the reference laminates. Particularly, the C3 and C4 structures show low improvements of the specific modulus, equal to 25% and 15%, respectively. It is important to evidence that the average values of specific modulus for the cross angle of the carbon layer equal to, (2.7 and 23. GPa/(g/cm 3 ), respectively) of the C1 and C6 structures are comparable to the alluminium alloy. These results show that, by replacing an external layer of glass mat with one of uniaxial carbon fabric parallel to the x direction, a material with a high stiffness is obtained and, consequently, these hybrid laminates can be used for the topside structures of a ship instead of the reference alloy. Regarding the specific flexural strength (Table 3) it is possible to notice that: In the cases of carbon layer orientation at 45 and 9 to the x direction, the hybrid composites do not show high variations of this specific mechanical property compared to the reference laminate. For the perpendicular (9 ) orientation of the carbon layer, the specific flexural strength varies between MPa/(g/cm 3 ) and MPa/(g/cm 3 ) for the C6 and C5 structures while by orienting the carbon layer at 45 to the x direction the minimum and maximum values of the specific strength are equal to MPa/(g/cm 3 ) ( C6 ) and MPa/(g/cm 3 ) ( C5 ), respectively. By orienting the carbon layer parallel to the x direction, more improvements than the previous cases have been obtained. Particularly for the C6 structure, the increase of the specific strength reaches 127% compared to the reference laminate. The other structures show increase of this specific property equal to about 3% except for the C3 (13.6%) and C4 ( 1.4%) structures.

7 Mechanical Properties of Glass Mat Composites for Marine Applications 1357 Table 3. Specific strenght of the hybrid laminates. Flexural Samples C1 C2 C3 C4 C5 C Tensile It is very important to notice that all the realized hybrid structures keep specific strength higher than the reference alluminium alloy. These results can be explained considering the bending stress r xx trend in a beam loaded in a three point bending mode. In each section of the beam, this stress increases moving through the thickness from the central zone to the external sides of the beam. A neutral axis exists in the middle zone of the beam while the layers in top and bottom side are subjected to compression and tensile stress, respectively. For this reason, the improvements of the specific properties are obtained by orienting the carbon layer parallel to the direction of the bending stress r xx (at to the x direction). For the cross angles of the uniaxial carbon layer equal to 45 or 9, the specific properties show the fewest improvements because the carbon layer works only partially (if oriented at 45 ) or not at all (9 ) along the direction of the bending stress. Moreover by orienting the uniaxial carbon, parallel to the x direction, the increase of the specific properties has been obtained by replacing the external layers of glass mat since the maximum values of the bending stress just occur in these layers of the composite structures. Moving from the external side to the middle zone of the beam, the bending stress decreases and the replacement of the glass mat layer with uniaxial carbon has less influence on the specific performances of the structure. About the specific flexural strength, the hybrid structures show less improvements compared to the specific modulus because of the kind of carbon fiber used (UHM). The ultra high modulus carbon fiber has been chosen in order to increase the specific stiffness of the composite laminates keeping high values of the specific strength so as to use these structures in both the topside and the hull structures substituting the aluminium reference alloy. Tensile Characterization As theoretically foreseen for this load configuration, the specific properties (both modulus and strength) of the hybrid structures are not influenced by the position of the uniaxial carbon layer but only by the orientation: For the parallel orientation ( ) of the carbon layer to the loading direction, all the hybrid composites show values of the specific modulus and strength equal about to 25 GPa/(g/cm 3 ) and 12 MPa/(g/cm 3 ), respectively For the cross angle of the carbon layer equal to 45 and 9 to the loading direction, the specific modulus results equal about to 13.5 GPa/(g/cm 3 ) and 13.8 GPa/(g/cm 3 )

8 1358 A. ALENZA ET AL E/r (Gpa/(g/cm 3 )) AI Mat 9 45 Figure 4. Specific tensile modulus of the laminates. while the specific strength reach values are equal about to 115 MPa/(g/cm 3 ) and 18 MPa/(g/cm 3 ), respectively. In Figure 4 the mean values of the specific tensile modulus for each orientation of the carbon layer in the hybrid structures are shown. The results clearly show that this specific performance is considerably improved by replacing one layer of glass mat with uniaxial carbon oriented at to the loading direction. Particularly, the specific modulus of this hybrid structures is about 1% higher (about two times) than the reference laminate and it is about the same as the reference aluminium alloy (see dotted line in the Figure 4). For the cross angle of uniaxial carbon layer equal to 45 or 9, the specific stiffness shows fewer improvements than the reference laminate and, consequently, it remains lower than the reference aluminium alloy. For the perpendicular (9 ) orientation of the carbon layer, the specific flexural modulus reaches only 8.5% and while using uniaxial carbon layer oriented at 45 to the loading direction, the reach is 6.5% compared to the reference laminate. In Table 3 the specific tensile strengths of the hybrid structures are reported. From these results it is evident that the replacement of a glass mat layer with uniaxial carbon layer does not bring improvements. Particularly, the hybrid structures show lower specific strength than the reference laminate, even if these values remain higher than the aluminium alloy AA 616. As in the previous case, the tensile characterization shows that by replacing one layer of glass mat with one of uniaxial carbon fabric parallel ( ) to the loading direction, hybrid structures are obtained with better or equal values of specific modulus and strength better than the reference aluminium alloy. Consequently, it is possible to use these structures in both the topside and the hull structures of a ship reducing the total weight. Setup FINITE ELEMENT ANALYSIS A 3-D numerical analysis has been conducted in order to simulate both the experimental tests, using a commercial finite element software (i.e. Ansys).

9 Mechanical Properties of Glass Mat Composites for Marine Applications 1359 Table 4. Finite element modeling parameters. Lamina Glass Carbon Young Modulus E x (MPa) 18, ,815 Young Modulus E y (MPa) 5763 Young Modulus E z (MPa) 5763 Poisson Coefficient m xy Poisson Coefficient m yz.43 Poisson Coefficient m xz.32 Shear Modulus G xy (MPa) 2611 Shear Modulus G yz (MPa) 1989 Shear Modulus G xz (MPa) 2611 The sample that has been used to simulate the laminate is constituted by a rectangle having the dimensions defined in the experimental setup sections. A shell element type (i.e. Shell99) has been used to build the model. The shells are a viable alternative to conventional solid elements for the modeling and analysis of laminate structures. These allow to simulate the behavior not only of the plane structures but also of the complex curved profiles in several fields; i.e. aeronautical (fuselages, wings), marine (hulls) and automotive (chassis) ones. In particular Shell99 is an 8-node, 3-D shell, layered element with six degrees of freedom at each node: translation in the nodal x, y and z directions and rotations about the nodal x, y and z axes. It is designed to model thin to thick moderately plate and shell structures with a side-to-thickness ratio of roughly 1 or greater. The Shell99 element allows a total of 25 uniform-thickness layers. In the real constants box of this element the following parameters have been added: number of layers (i.e. six), material (i.e. glass, carbon), thickness (i.e..43 mm for the glass layer,.75 mm for the carbon one) and orientation (i.e.,45,9 ). The mechanical and physical properties of the laminates constituents have been obtained through the experimental tests and analytical studies (i.e. Hahn theory) [8,9]. The finite element modeling parameters are reported in Table 4. The sample has been constrained as in the experimental tests. Moreover, the displacement, corresponding to the start of the inelastic trend, has been applied. In fact the composite structure behavior has been simulated by a numerical procedure performed in elastic regime and the post-elastic behavior has been intentionally neglected; the aim is to obtain a simple and versatile numerical simulation, conditions required for an effective design methodology and, particularly, to characterize the composite structure in the elastic regime, where it exploits its work [1]. Results Comparing experimental and numerical results for all the tests it is possible to observe that the model well matches the linear elastic trend of the stress/strain curves, showing a good prediction of the laminates stiffness. Figure 5 reports the comparison experimental/ numerical for the sample with the external carbon lamina at in the tensile test. Figure 6 shows the stress trends for the tensile test. In Figure 6(a), related to the laminate with only glass fabrics, the stress is uniformly distributed for all the layers. Figure 6(b) is related to the laminate with the external carbon lamina at (i.e. more

10 136 A. ALENZA ET AL Experimental FEA 12 1 Stress (MPa) Strain Figure 5. Comparison Experimental/FEA. (a) (b) C (c) (d) C C z y Figure 6. Maps for the tensile test: (a) Glass; (b) Carbon ; (c) Carbon 9; (d) Carbon 45. thick one identified in the Figure with C ). The deformed sample is characterized by a concavity on the right side, where there are the carbon fibers, due to their higher resistance than the glass ones. It is possible to observe that the carbon fibers are subjected to tensile and compressive stresses, whereas the glass fibers are subjected to a slight traction. The failure occurs at the glass/carbon interface, where there is the highest stress gap. Figures 6(c) and (d) are related to the laminate with the external carbon lamina at 9

11 Mechanical Properties of Glass Mat Composites for Marine Applications 1361 (a) (b) top bottom top bottom 257 C (c) (d) C C Figure 7. Maps for the flexural test: (a) Glass; (b) Carbon ; (c) Carbon 9; (d) Carbon 45. and 45, respectively. The stress is not uniformly distributed, but here the layers are subjected only to a tensile stress. With the lamina at 45 it is possible to observe that at the glass/carbon interface the stress changing is not so sudden. In these cases the failure occurs at the glass fibers placed near the carbon fabric. Figure 7 shows the stress trends for the flexural test. To compare the results for different samples, same displacement (i.e. 5 mm) is considered. In Figure 7(a), related to the laminate with only glass fabrics, the stress distribution is symmetrical; i.e. compression stress on top side and tensile on the bottom. The neutral axis is the x symmetrical axis. Figure 7(b) is related to the laminate with external carbon lamina at (i.e. more thick one identified in the Figure with C ). It is possible to observe that the carbon fibers are subjected to tensile and compressive stresses, whereas the glass fibers are subjected only to compression. So, in this case the neutral axis is in the carbon layer and it is identified with a dotted line. At the glass/ carbon interface there is the highest stress gap. Figure 7(c) is related to the laminate with the external carbon lamina at 9. Both the carbon layer and the glass layer near to this are subjected to tensile stress. The upper glass fabrics are subjected to compressive stress. The neutral axis (dotted line) is at the interface between the glass layers 4 and 5. With the lamina at 45 (Figure 7(d)) the stress distribution is influenced by the fibers orientation. It is interesting to observe the map in the sample bottom, where it is different from the other maps and it is possible to identify the symmetrical axes. For this trend it is more difficult to define the neutral axis. In the figure the latter is identified in glass layer 5 near the interface with glass layer 4.

12 1362 A. ALENZA ET AL. C6 C C C C C Figure 8. Maps for the flexural test with varying the carbon lamina position. Figure 8 shows the stress maps with varying carbon lamina position. In the first case (i.e. C6) the carbon layer is subjected to tensile and compressive stress. In the case C5 a similar behavior is observed but with the fewest values of stress. Also in the case C4 the layer is subjected to tensile and compressive stress, but the compressed region is very small. In the other positions the lamina is always subjected only to compression. In the position C3, C4 and C5 the stress level reached is similar. It is interesting to observe how the neutral axis (i.e. dotted line) moves in the sample with varying the carbon lamina position. In particular this happens in the carbon layer for the configurations C6, C5 and C4. In the configurations C3 and C2 it is at the interface between the layers 3 and 4. In the configuration C1 it is in the glass layer near to the carbon one.

13 Mechanical Properties of Glass Mat Composites for Marine Applications 1363 CONCLUSIONS In the marine ships the strength is a critical parameter for the hull structures while the stiffness is critical for the topside structures. These influence the ship stability. Even though the specific strength of glass composites is higher than the aluminium alloy, its specific stiffness is lower than aluminium alloy and, for this reason, the glass composites are well suited for constructing ship hulls while they are not suited for the topside structures of a ship. The objective of this work is to increase the specific stiffness of composite structures replacing one layer of glass mat with one of uniaxial carbon fabric in order to use these hybrid materials even for the topside structures. The influence of carbon on the tensile specific stiffness has been analyzed by realizing and testing three hybrid structures. The external layer of glass mat laminate has been replaced with one layer of uniaxial carbon fabric oriented respectively at,45 and 9 to the x direction showed in Figure 2. To evaluate the influence of carbon on the flexural specific stiffness 18 hybrid structures have been produced by replacing each layer of glass mat of the reference laminate with one layer of uniaxial carbon fabric oriented at,45 and 9 to the x direction. Consequently, both the flexural and the tensile characterizations have shown that, by replacing an external layer of glass mat with one of uniaxial carbon fabric parallel to the x direction, structures with better or equal values of specific modulus and strength than the reference aluminium alloy have been obtained. For this reason, these hybrid laminates can be used for the topside structures of a ship instead of the reference alloy. Finally Finite Element Analysis allows to build a model that well matches the experimental behavior. The stress maps show how the distribution changes by adding the carbon layers and by varying the fibers orientation. REFERENCES 1. Gerr, D. (1999). Elements of Boat Strength: For Builders, Designers and Owner, International Marine/Ragged Mountain Press, Camden. 2. Shivakumar, K.N., Swaminathan, G. and Sharpe, M. (26). Carbon/inyl Ester Composites for Enhanced Performance in Marine Applications, Journal of Reinforced Plastics and Composites, 25(1): Gustin, J., Joneson, A., Mahinfalah, M. and Stone, J. (25). Low elocity Impact of Combination Kevlar/Carbon Fiber Sandwich Composites, Composite Structures, 69(4): Dorey, G., Sidey, G.R. and Hutchings, J. (1978). Impact Properties of Carbon Fiber/Kevlar 49 Fiber Hybrid Composites, Composites, 9(1): Tan, T.T.M. and Nieu, N.H. (1996). Hybrid Carbon-glass Fiber inyl Ester Resin Composites, Angewandte Makromolekulare Chemie, 234: Tsai, Y.I., Bosze, E.J., Barjasteh, E. and Nutt, S.R. (29). Influence of Hygrothermal Environment on Thermal and Mechanical Properties of Carbon Fiber/Fiberglass Hybrid Composites, Composites Science and Technology, 69(3 4): Cho, J.W., Choi, J.S. and Yoon, Y.S. (22). Electromechanical Behaviour of Hybrid Carbon/ Glass Fiber Composites with Tension and Bending, Journal of Applied Polymer Science, 83(11): alenza, A., Borsellino, C. and Calabrese, L. (24). Experimental and Numerical Evaluation of Sandwich Composite Structures, Composites Science and Technology, 64(1 11):

14 1364 A. ALENZA ET AL. 9. Marino`, A. (1995). L uso Dei Materiali Plastici Rinforzati Nelle Costruzioni Navali: Un analisi Razionale. PhD Thesis, Universita` degli Studi di Trieste. 1. alenza, A., Borsellino, C., Calabrese, L. and Di Bella, G. (26). Geometry and Stacking Sequence Effect on Composite Spinnaker Pole s Stiffness: Experimental and Numerical Analysis, Applied Composite Materials, 13(4):