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Analysis of a New Composite Material for Watercraft Manufacturing
Alexandre Wahrhaftig1, Henrique Ribeiro2, Ademar Nascimento3, Milton Filho4     
1. Department of Construction and Structures, Polytechnic School, Federal University of Bahia, Rua Aristides Novís, 02, 5°andar, Federação, Salvador-BA, Brazil, CEP: 40210-910;
2. Bahia Federal Institute of Education, Science and Technology, RuaEmidio dos Santos, Barbalho, Salvador-BA, Brazil, CEP: 40301-015, Brazil, CEP: 40210-910;
3. Department of Mechanical Engineering, Polytechnic School, Federal University of Bahia, Rua Aristides Novís, 02, 5° andar, Federação, Salvador-BA, Brazil, CEP: 40210-910;
4. Research and Education Center in Composite Materials (CEPEC), Avenida Laurindo Régis, 360, Cond. Castro Alves, BL.11A, AP 412, Bela Vista de Brotas, Salvador-BA, Brazil, CEP: 40240-550
Abstract: In this paper, we investigate the properties of an alternative material for use in marine engineering, namely a rigid and light sandwich-structured composite made of expanded polystyrene and fiberglass. Not only does this material have an improved section modulus, but it is also inexpensive, light, easy to manipulate, and commercially available in various sizes. Using a computer program based on the finite element method, we calculated the hogging and sagging stresses and strains acting on a prismatic boat model composed of this material, and determined the minimum sizes and maximum permissible stresses to avoid deformation. Finally, we calculated the structural weight of the resulting vessel for comparison with another structure of comparable dimensions constructed from the commonly used core materialDivinycell.
Key words: naval construction     computational analysis     composite material     sandwich-structure     expanded polystyrene     fiberglass     composite structure concepts     finite element method     economic viability.    

1 Introduction

Steel is traditionally used in the hull construction of ships and submarines, although aluminum is often used as an alternative material in applications in which the minimization of structural weight can have a great impact on energy efficiency or provide other advantages. The use of lightweight structures in shipbuilding can, for example, allow an increase in payload, a higher speed, and a reduction in fuel consumption and environmental emissions (Crupi et al., 2013). In addition, such replacement is often necessary when steel would make a surface ship top-heavy, and if aluminum is not a viable option, composite materials offer a reasonable alternative (Chalmers, 1994). The use of composite and sandwich-structured materials in modern engineering applications, such as civil and military aircraft, launch vehicles, wind turbine blades, and assorted marine structures (Cerracchioa et al., 2015), has grown significantly over the past few decades due to their high strength-to-weight and stiffness-to-weight ratios (Yang et al., 2013). The fact that the density of a composite can be fairly easily controlled is also particularly valuable in increasing structural buoyancy (Craugh and Kwon, 2013). However, many other features have also been lauded, including greater material strength, flexibility, environmental resistance, and damage tolerance, as well as reductions in weight, size, and cost (Kimpara, 1991).

Traditionally, the preferred form of fiber reinforcement for marine applications has been woven fabric, which is often combined with layers of chopped strand mat. The selection of such materials currently available for fast vessels such as the surface effect ship is quite advanced, as in these applications’weight savings are critical and structural optimization is essential (Davies et al., 1994). Since the middle of the last century, fiberglass has also proven to be particularly popular in boat construction as it has the advantages of being chemically inert (for both general applications and in marine environments), lightweight, strong, easily moldable, and competitively priced. On the other hand, it also has a low modulus of elasticity and low fatigue strength when compared to steel and aluminum. Of course, these properties are somewhat dependent on the macrostructure of the material, and the two most commonly used for marine applications are single-skin sheets and sandwich panels. Sandwich panels address some of the aforementioned problems in that they improve the modulus and thickness of sections without increasing their weight, but the fact that the material is still relatively flexible means that framing is necessary in most applications. Such modification increases the weight and reduces space inside a boat, the former being a problem in high-speed boats, and the latter causing issues in cargo transport ships. In addition, sandwich panels made of Divinycell and polyurethane are not only expensive, but are also only available in a limited range of thicknesses.

Given the ubiquity of fiberglass, it is no surprise that significant research has been devoted to developing fiberglass composites that retain the advantageous features of fiberglass, while eliminating those that are problematic. Because Kevlar and carbon fiber both have a high tensile strength, modulus of elasticity, and specific resistance in addition to their low density, they are well suited to being coupled with fiberglass to create new, more highly functional materials. The use of fiberglass, meanwhile, helps to offset the relatively high cost of these more advanced materials. Cost concerns can be further offset by introducing an additional core material, which typically consists of a honeycomb structure of balsa wood, Divinycell, polyurethane, or polystyrene. A review of the advances made in composite structures for naval ships and submarines up until 2001 can be found in a paper by Mouritz et al. (2001).

In this paper, we investigate a new composite material in which fiberglass is impregnated with epoxy resin by the lamination of external and internal surfaces that are separated by a core of Expanded Polystyrene (EPS). To evaluate the mechanical properties of this material and to assess its suitability for use in boat construction, we numerically tested the material in a computational simulation of a prismatic ferry structure, and also verified its economic viability in comparison with other composites traditionally used in this type of application.

2 Basic core materials

The honeycomb structure of various woods and foamed plastics makes them among the most commonly used core materials in the composites used in hull construction. In general, these core materials should have good shear strength and rigidity, the ability to bond easily, low weight, good resistance to aqueous deterioration, and sufficient crushing strength resistance to withstand loading (Scott, 1996). Honeycomb structures are available in various sizes and weights, and can be constructed from a range of materials including aluminum, fiberglass laminates, and waterproof paper. These materaials are generally both light and rigid, but as they cannot support concentrated loads, they require special treatment in order to ensure an effective bond between the core and the faces of the laminate. Consequently, the use of these materials is usually restricted to the interiors of boats; i.e., in bulkheads.

Softwoods are the most common choice for a wood-based core material, as when hardwoods like balsa are used, they can crack the laminate when they swell and do not bond well to fibers. While softwoods in particular can drape very well over curved boat surfaces, their use is generally avoided below the waterline because of the possibility of rotting, swelling, and degradation. Currently, PVC is the most commonly used foamed plastic, and is used in large part due to the fact that it softens when heated and can be draped over curved surfaces such as those found on boats. This tendency to soften with temperature, however, also makes it unusable in places on boats that are subjected to high temperatures, such as decks. Cross-linking polyurethane with PVC can overcome this issue, but results in the loss of some of the desirable properties of the original material.

EPS can serve as a reasonable replacement for many of the core materials currently used in marine applications as it is very light, less expensive, resistant to fungi, and impermeable. Unfortunately, it is also a somewhat weaker material, and its low shear and compressive strength makes it susceptible to delamination and damage. However, these limitations can generally be overcome by increasing the core thickness or using shear webs. This material is also incompatible with polyester resin, although this problem can typically be overcome by binding a thin PVC cover to the central polystyrene core to act as a barrier. Epoxy resin can also be mixed with glass strands to produce shear webs. These various core designs are compared in Table 1.

Table 1 Mechanical properties of different core materials
Property PVC (Renicell 240) Balsa LD7 Honeycomb PP30-5 EPS 3 EPS 3 with shear webs
Specific gravity /(kg∙m–3) 240 90 100 14 38
Compression strength / psi 580 783 235 9 26 000
Compression modulus / ksi 19 268 10.5 -- 22 000
Tensile strength / psi 479 1015 175 18 30 000
Shear strength / psi 363 232 75 9 11 000
Shear modulus / ksi 14 14 2 0.48 --
3 Composite structure concepts

The design of stiffened composite panels is a key factor in the effectiveness of composite structures. The assessment of such structures in a design environment consequently requires models that can provide a rapid assessment of the reliability of the final construction. Given that stiffeners play an indispensable role in enhancing the strength and stiffness of these structures (Yu et al., 2015), panels for use in ship fabrication have been previously investigated through experimental testing with preform frames under in-plane uniaxial compressive loads (Mouring, 1999).

Typically, sandwich-structured composites contain a web that separates the outer layers. These webs are usually oriented in a specific direction to provide continuous longitudinal support, with added stands providing transverse support. which makes the overall structure highly orthotropic (Romanoff and Varsta, 2007). Loads applied in such structures are supported by internal traction and compression forces in the outer material (Atkinson, 1997), while shear forces are transferred to the core, allowing it to work as a homogenous structure. This means that the materials used must be able to handle significant tension and compression.

Fig. 1 shows simplified top and side views of a newly proposed structure that overcomes these aforementioned difficulties.

Figure 1 Structural arrangement of a base plate made from the new composite

Here, vertical epoxy shear webs make up for EPS’lack of strength and rigidity, and while a core thickness of 15 mm is used in this instance, the availability of EPS in different thicknesses makes the structure readily scalable. Moreover, as the webs are both longitudinal and transverse there is no need for formal framing, which thereby provides a light and rigid composite material while freeing up internal space. Shear webs can even be used as part of the framing system when thicker cores are present. Of course, if this is the case, then it would also be possible to use thinner faces and webs, such that the faces that cover the core would act like the flanges of an I-beam. As the shear, compressive, and tensile strengths of the faces and webs are far higher than those of the EPS core, the voids between the webs provide a molded surface for the whole assembly and can prevent buckling.

The material can be delivered already bonded to one side, thus allowing it to more effectively drape along the surface of a boat under construction. This is fairly straightforward when using core blocks that form voids between the webs, as shown in Fig. 2.

Figure 2 Basic structural arrangement, as per the dimensions in Fig. 1, wherein laminated shear webs are placed between the cores

An added advantage of using an epoxy resin in this structure is that, unlike polyester resins, it can elongate as much as or even more than the reinforcement. This helps prevent structural failure prior to reaching the limit of the reinforcement and improves the overall strength of the composite. To clarify, Fig. 3 shows an example of a typical fabrication process in the construction of a reduced model of a boat using the material investigated, along with a description of the principal steps.

Figure 3 Typical process for fabricating a vessel using the test material
4 Economic viability 4.1 Cost

Ultimately, the cost of any new composite must be competitive with that of other materials, even if its performance is significantly greater. The basic structural arrangements of EPS and Divinycell units are shown in Fig. 4, while a comparison of the price of these two panel types is given in Table 2. These prices were budgeted in Brazil, but are listed in American dollars (USD).

Figure 4 Structural arrangements of the two possible core materials
Table 2 Price breakdown of panels made using EPS or Divinycell as the core material
Item Panel with EPS core Panel with Divinycell core
Matrix (300 g/m2) 7.56 7.56
Roving Wire 1.41 0.00
Epoxy Resin 84.92 0.00
Polyester Resin 0.00 35.28
EPS (1 m2) 1.00 0.00
Divinycell (1 m2) 0.00 75.59
Total Price 94.89 118.43

From a cost analysis perspective, we note that the costs of producing and maintaining the internal structure of a naval vessel constitute a large percentage of the lifetime costs of a ship. Vessels designed to have minimal maintenance costs over the entirety of their service life will therefore typically consist of larger members and be more expensive to produce. This means that it is important to design the internal structure of naval vessels in way that strikes an effective balance between these two competing cost aspects in order to minimize the total monetary cost to the ship owner (Temple and Collette, 2015).

The naval ship structure community is also currently pursuing improved structural performance while simultaneously reducing the costs of construction and life-cycle maintenance. In addition to reducing structural weight, efforts have been made over the last seventy years to improve many other properties related to boat design, and which composite structures have the very clear potential to help achieve. Of the low-cost options available for fabricating composite ship structures, the most promising are the resin transfer molding and filament winding techniques described by Critchfield et al. (1994).

Historically, the model used for these structures has been based on a plate, as the curvature of ship plating is sufficiently small for individual panels to be approximated as flat plates (Rajendran and Lee, 2009).

4.2 Weight comparison with Divinycell structure

For comparison, we chose the Divinycell with the lowest available density (38 kg/m3). As a boat is prismatic in all directions, its weight can be calculated for each linear meter of the structure. The thicknesses of the internal and external faces, as well as that of the core, were set to the same value as that of the EPS structure. The only noticeable difference in this structure is that the reinforcements are external to the plate. The weight breakdown of the aforementioned Divinycell plate is provided in Table 3, and the values for the EPS plate are provided in Table 4. Note that the two structures are very close in weight, although the EPS plate is somewhat heavier. However, the lower density Divinycell structure is not as useful mechanically and is much more expensive (http://boatbuildercentral.com, 2015).

Table 3 Weight of a Divinycell plate
Item Weight
Divinycell plate 14.906 N (1.52kgf)
External reinforcements in the transverse direction 10.101 N (1.03kgf)
External reinforcements in the longitudinal direction 10.101 N (1.03kgf)
Internal/external fiberglass plate faces 211.824 N (21.6kgf)
Inner lining of reinforcements (Divinycell) 1.177 N (0.12kgf),
Total weight 248.108 N (25.3kgf)
Table 4 Weight of the new composite
Item Weight
Base polystyrene plate 5.492 N (0.56kgf)
Inner fiberglass reinforcements 56.486 N (5.76kgf),
Internal/external fiberglass plate faces 211.824 N (21.6kgf)
Total weight 273.606 N (27.9kgf)
5 Vessel simulation 5.1 Structural arrangement and geometry

The girder of a ship’s hull is a large floating structural system made up of plate panels and stiffeners, and is subjected to both still-water and wave loads. The static forces acting on it in still water are created by the longitudinal distributions of weight and buoyancy (Shu and Moan, 2011), however, the structure may collapse in sea conditions if the structural capacity is less than the work load. Consequently, both the working load and hull girder capacity are vital aspects to the safety of a ship (Pei et al., 2015).

To test the composite in this study, we used a rectangular prismatic barge for use in calm, deep waters as the model, using the following main dimensions: an overall length of 20.00 m, a molded breadth of 4.00 m, a molded draft of 2.13 m, a maximum draft of 0.50 m, a displacement at full load of 364.754 kN (41.0 tf), a light draft of 0.023 m, a light displacement of 16.12 kN (1.812 tf), and a load capacity of 348.634 kN (39.188 tf).

This simulated boat has eight compartments, separated by seven watertight bulkheads. In order to simulate both hogging and sagging conditions, we carried out two distinct simulations, in which four separate compartments were alternately loaded, as shown in Fig. 5.

Figure 5 Load arrangement for the two simulations

The vertical bending moment is of crucial importance for ensuring the survival of vessels in rough seas, and in the case of conventional vessels, it is normally considered that the wave-induced maximum vertical bending moment is experienced in head seas applied to maximum sagging and hogging (Zhu and Moan, 2014). As implied by the load capacity and molded draft in Fig. 5, a load of 8.715kN/m2 (0.9796 tf/m2) is distributed at the bottom of each loaded compartment and hydrostatic pressure is present at the bottom and on the portions in contact with water. In order to maintain static equilibrium, we applied the sum of the gravitational loads distributed over the floor area from bottom to top. To clarify, Fig. 6 shows the locations and directions of the hydrostatic forces that maintain a balance with the applied external forces, and which represent the boundary condition of the model. Thus, these are the only forces acting on the system, which provide the self-equilibrium in each loading condition shown in Fig. 5. It is for these loading conditions that we calculated the distributions of the stresses and strains in the ferry structure.

Figure 6 Hydrostatic load acting on the hull
5.2 Computational modeling

Any design strategy needs to take into account two important aspects: material selection, and the structural arrangement and scantlings most appropriate for the chosen material (Stenius et al., 2011). As such, engineering design consists of several steps including mathematical modeling, the application of physics, and theoretical and computational analysis (Lee et al., 2003). For the purposes of this study, we computationally elaborated a basic plate (Fig. 7) using the Finite Element Method (FEM) with a commercial finite element package (SAP2000, 2015), which has been used in the past to evaluate the design of high-speed marine craft (Townsend et al., 2012). We used a three-dimensional model for the discretization of a plate containing solid elements, and built a model by taking into account the actual dimensions and properties of the materials of each separate component of the plate. These included details of the corner and middle of both models, as shown in Figs. 1 and 4. The basic plate of the new composite material served as the basis of the numerical experiment and constituted the main piece covering the entire barge and bulkheads of the model.

Figure 7 Basic three-dimensional FEM model of the plate

We determined the mathematical relationship between the solid elements using an isoparametric formula that includes nine optional, incompatible bending modes. These incompatible bending modes significantly improve the bending behavior of the element if the element geometry is rectangular. We used a 222 numerical integration scheme to evaluate the stresses in the local coordinates at the integration points, which were then extrapolated to the joints. We estimated the approximate error for the stresses from the difference between the calculated value and other elements attached to the same joint. This, in turn, provided an indication of the accuracy of the calculation that could be used in the selection of a new and more accurate finite element mesh. All six stress and strain components were active for this element (Cook, 1974; Bucalem and Bathe, 2011).

We completed the computational modeling of the vessel described in subsection 5.1 using 35183 eight-node solid modeling elements. The construction process was based on replicating the basic plate as many times as necessary in each of three orthogonal directions (X, Y and Z) until the boat was fully formed. This process included replication of the frontal part, back, bottom, cover, laterals, and internal divisions. Special care was taken to ensure that all nodes of each plate were connected to each other at all points of the model, which included both the internal nodes of the plate and the union between plates. A similar technique using FEA has been employed to design what is primarily a Glass-Reinforced Polymer (GRP) composite with an airfoil-shaped sail and canopy-style configuration known as the Composite Advanced Sail (CAS), which can reduce weight and maintenance costs and improve the load capacity. For modeling, this CAS structure is divided into four separate components to account for variation in material composition (Eamon and Rais-Rohani, 2009).

5.3 Computational results and discussion

We classified the hogging and sagging conditions summarized in Fig. 5 as COMB1 and COMB2, respectively, with both simulations assuming a draft depth of 0.50 m. The overall displacement of the vessel for both loading conditions indicated a maximum vertical absolute displacement of 22 and 30 mm, respectively, while the maximum relative observed sag was 2.6%. We note that epoxy resin impregnated with fiberglass can elongate by up to about 5%. A maximum horizontal displacement of 66 and 30 mm was obtained for the front and sides (Fig. 8) of the deformed structure, respectively. Table 5 summarizes the stress results and Table 6 shows the mechanical properties of interest for fiberglass impregnated with epoxy resin (Torabizadeh, 2013). It is important to note that these results were obtained by considering the initial dimensions of the plate, which can be modified and adjusted at any time.

Figure 8 Deformation from pressure (front cover and side)
Table 5 Maximum stresses (N/mm2)
Condition Max. tensile stress (deck) Max. tensile stress (bottom)
COMB1 55 (T) 343 (C)
COMB2 159 (C) 628 (T)
Table 6 Mechanical properties of fiberglass complexed with epoxy resin
Property Value
Tensile strength 715 N/mm2
Compressive strength 570 N/mm2
Shear strength 70 N/mm2
Poisson's ratio 0.22
6 Conclusions

In this study, simulation and comparison showed that the proposed composite is not only feasible for use in boat hull construction, but also quite competitive with existing materials. Even considering the relatively poor mechanical properties of EPS, its use as a simple core in conjunction with shear webs made of epoxy resin improves the overall strength of the proposed composite to the extent that it is competitive with the much higher-quality sandwich-structured Divinycell. Furthermore, as EPS is at worst roughly the same weight as Divinycell, it is a particularly desirable construction material for speedboats, especially given that it is approximately 20% less expensive. Overall, this new composite appears to provide an excellent alternative for use in light and medium vessels, although further laboratory research has yet to be undertaken to address its other mechanical properties and weight capacity. To address these issues, new computational models will be developed with plate elements that should prove much simpler and quicker to use than solid element models. However, models made with FEM three-dimensional elements will be used for comparison.

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Article Information

Alexandre Wahrhaftig, Henrique Ribeiro, Ademar Nascimento, Milton Filho
Analysis of a New Composite Material for Watercraft Manufacturing
Journal of Marine Science and Application, 2016, 15(3): 336-342
DOI: 10.1007/s11804-016-1364-8

Article History

Received date: 2015-11-02
Accepted date: 2016-02-10