The Static Behavior of a Ship Deck Panel Made of Composite Materials

A study on the static analysis of a naval panel made of composite sandwich materials is presented. By using FEM, the modeling of a naval floor with a length of 5 m and a width of 2.5 m is performed. Two distinct cases, have been performed: the first model consists of the plate and stiffeners made of steel and the second model concerns a panel made of composite material sandwich type steel / SANFoam103 / steel, and the stiffeners made of steel. A parametric study has been performed. The thickness of the steel faces have 6 mm, and for the core of SANFoam have been selected the thicknesses 5 mm, 10 mm, 20 mm, 40 mm.


Introduction
Composites represent the most important material used to build performing structures in all industrial domains. The basic feature of composite materials is the high strength/weight ratio, which makes them more efficient than some of the best steel categories. A sandwich panel made of composite materials is performant if it has thin and rigid faces with high strength, and the core has low density.
Sandwich panels with metal faces and foam cores are very often used as lightweight structures in shipbuilding applications [1][2][3][4][5], especially because of their performance on light weight and high rigidity. In the shipbuilding industry, the application of polymeric sandwich-type composites is constantly evolving, introducing into their composition new basic materials such as: fibers, resins, adhesives, cure accelerators, additives, etc.
As an alternative to panels with a classic stiffened metal structure [6][7][8], sandwich structures justify their use in the shipbuilding, making these materials less complex, eliminating the need for secondary stiffening (stiffeners). At the same time, other advantages of sandwich-type composite materials are related to corrosion resistance, increased tensile strength, for easier durability, and thermal insulation [9,10].
The faces of sandwich panels can be made of metal materials (aluminum alloys or stainless steel), non-metallic (polymeric laminates with glass fibers or carbon fibers), various composite materials (reinforced polymeric) or hybrid. The core consists of a light, porous or non-porous material, granulated or special types, consisting of cells (honeycomb type) - [19] or profiles (U, I, T, etc.). As a rule, the core is a composite material that, at a long time load, slowly deforms.
As an alternative solution in shipbuilding, the concept of SPS (sandwich plate system) material has developed. This material allows the replacement of the classic panels with a stiffened steel structure with a polyurethane elastomer [5,[11][12][13][14].
In the case of naval decks, in the classical system, longitudinal structures are used as stiffening elements, whose welded joints are the sensible to corrosion and fatigue. In the case of SPS, the space between two steel plates is filled with an elastomer and thus the need for longitudinal welds on the coating is eliminated. In the case of decks repair, the process is much simplified, the main stiffeners (currents, longitudinal or transverse frames) are welded, and the remaining spaces are stiffened with elastomer, with an important financial and time advantage versus the classical methods.
In order to obtain answers such as stresses and strains for various cases of loading, of the natural vibration modes or of behavior of the structural elements to the buckling in the structural analyses that are applied in the ship structures, analytical calculations or the finite elements method are often used [15][16][17][18]. In general, in order to correctly define a structural analysis, various stages are taken, starting from the setting of objectives, type and size of the analysis, modeling of the structure and boundary conditions, modeling itself and evaluation of the results. In certain cases, linear analyses of ship structures are often sufficient.
In the work a study on the static analysis of a ship deck structure made of sandwich-type composite materials, by using the FEM is performed.

Materials
The ship deck structure has length of 5 m and a width of 2.5 m. Two cases are considered: a first case in which the floor and stiffeners are made entirely of steel, and the second where the deck panel is made of steel / SANFoam103 / steel sandwich composite material and the stiffeners are of steel. The thickness of the steel faces is 6 mm, and the core of SANFoam has thicknesses of 5 mm, 10 mm, 20 mm, 40 mm respectively.
The dimensions are chosen so that the weights of the resulting structures are approximately the same with the initial one. In the case of analysis of the static behaviour of a ship deck panel using FEM, when defining the geometry of the model, for the steel panel and stiffeners, steel is considered as materials used in modeling, steel having the characteristics of Figure 1. The core of the sandwich is SAN Foam whose characteristics are presented in Table 1.  The structure of the ship deck panel (Figure 2) was modeled using COSMOS/M, static analysis: extensive library of 1D, 2D and 3D elements supports isotropic, orthotropic, anisotropic, multi-layer composite materials, and temperature-dependent material properties. Capabilities include linear gap/contacts, stress stiffening, sub-structuring, multi-point constraints, constraint equations and much more.    The ship deck pane model is simple supported on all sides. The loading is produced by the tires pressure of the vehicles having the masses 1.5 t, 2 t, 2.5 t, 3 t and 3.5 t, as are presented in the Table 3.

Results
According to the FEM analysis of the ship deck panel, the results obtained for the all five models M1÷M5 and five analysed cases (m1÷m5) are illustrated in the Figures 9÷13.

Discussions
In the case of the deck panel made of steel (M1) it can be seen that with the increase of the mass of the vehicle, the value of the maximum displacement increases, but also increase the values of the equivalent stresses, as are illustrated in Figure 14.    masses (m1, m2, m3, m4, m5), are applied to the deck panel, but the thickness of the sandwich core varies, it can be seen that the maximum displacement decreases with the increase of this thickness. The same situation can be seen in the case of the equivalent stress ( Figures 19÷23).

Conclusions
In the case of the deck panel made of sandwich composite material with a core thickness that varies between 5 mm and 40 mm and the steel faces of 6mm, it can be seen that with the increasing of forces produced by the vehicle masses, the value of the maximum displacement increases (e.g. from 0.095 mm (model M2, mass m1) to 0.22 mm, (model M2, mass m5)) ( Figure 24).
Similarly, in the case of the deck panel made of sandwich composite material with a core thickness that varies between 5 mm and 40 mm and the steel faces of 6mm, it can be seen that with the increasing of forces produced by the vehicle masses, the value of the equivalent stress (e.g. from 2.65 MPa (model M5, mass m1) to 6.13 MPa, (model M5, mass m5)) ( Figure 25). If a single mass type is applied to the ship deck panel, but the thickness of the core of the sandwich composite varies, it can be seen that the maximum displacement decreases with the increase of core thickness, the lowest displacement value, 0.034 mm, being recorded in the case of the model M5 -deck material: face 1 -steel with a thickness of 6mm, core -SAN Foam 103 kg/m 3 with a thickness of 40 mm, face 2 -steel with a thickness of 6mm, at a load produced by the m1 mass of the vehicle (1.5 t) (Figure 26).
Similarly, if a single mass type is applied to the ship deck panel, but the thickness of the core of the composite sandwich varies, it can be seen that the equivalent stress decreases with the increase of core thickness, recording a minimum value of 2.65 MPa (model M5, mass m1) (Figure 27).  Even in these upper conditions, both in the case of displacements and equivalent stresses obtained by FEM modeling, it is observed that the highest values of displacements and stresses are recorded on the ship deck panel made of steel, with a thickness of 12 mm, these varying upwards with the increase in the values of the loading, which demonstrates that the composite panels have a better behavior compared to the metal ones, even if their weight is approximately equal. This observation is provided by the metal stiffeners, which exist in all the analyzed constructive models.