Mechanical properties and experimental researches of new CSIPs sandwich panels

Abstract. The advantages of glass fiber reinforced composites (FRP) and SIPs (structural insulated panels) are combined, and a new type of sandwich panel called composite structural insulated panels (CSIPs) is proposed. Through the adhesive bonding, CSIPs are made of FRP as face sheets and expanded polyethylene foam (EPS) as a core. To master the mechanical characteristics of CSIPs, firstly, adopting the large deflection theory of Reissener in this paper derived the calculation formula of displacement and the stability critical load of CSIPs. Then, ANSYS software was used to carry on the analysis of finite element simulation. Finally, a testing piece of CSIP with length 1000mm and breath 1000mm was made and a test was done. The results show that the theoretical analysis results, finite element simulation results and test results are basically coincide. So the calculating formula of deformation and bearing capacity of CSIPs are correct. And CSIPs have the outstanding advantages of light weight and high strength.

(1) For the thickness of the surface layer is very small relatively comparing to the whole sandwich panel, a thin film was used to simulate face sheets. That is assuming that the surface layer is in the state of film stress; (2) Due to the core is soft, it can be ignored that force components of sandwich panel in parallel the XY plane, this is assuming that σ x =σ y =σ xy =0; (3) It is assumed that the strain in the core and the surface layer along the thickness direction is zero (ε z =0); (4) The stress σ z along the thickness direction is very small; (5) The transverse shear effect was considered and the straight line segment along the direction of thickness keeps a straight line after the deformation even though it is not perpendicular to the middle plane.

The displacement and stress components of the sandwich panel
The displacements of any point in the sandwich panel are as following: The upper face sheet The strain components of the core Where H xi , H yi , H zi , H xyi , H yzi and H xzi (i=1,2,3) are the strain components of the upper, the lower face sheet and the core.
E f and v f are elastic moduli and poisson's ratios. G and G core are the shearing modulus of the face sheet core.
Where V x0 , V y0 and W xy0 are the strains of intermediate surface in sandwich panel.

General bending moment and shear in sandwich panel
Because the transverse equilibrium equations are the same as those of the single layer panel, the bending moment, the torque and the shear force in sandwich panel can be obtained: The boundary conditions Then according to Hu Haichang's methods which cope with the small deflection equations sandwich panel, Mx, My can be express using the new function Z, We can get Z, f can be regarded as two separate parameters, so that we can obtain: The corresponding homogeneous equations are The complete solution is

The single triangle progression method
The exact solution for the bending of the thin rectangular panel under a uniformly distributed load with four edges simply supported is obtained by the single triangle progression method. The vertical displacement can be expressed: Where Y m is an arbitrary function of y, and m is any positive integer.
Whatever x takes on an arbitrary value, the formula (23) can be satisfied, we have The first term on the right-hand side of Eq.(29) is the deflection due to bending, and the second term is the deflection due to shear. k b and k s are the bending and shear deflection coefficients, respectively. The values of both k b and k s depend on the loading and boundary conditions. The bending and shear deflection coefficients under different loading and boundary conditions can be obtained. k b and k s are 5/384 and 1/8, respectively.
According to the theory of combination beam, formula for translation of axis and ASTM C-393, the formula of moment of inertia per unit width sandwich panel can be obtained[13-17]:

Finite element simulation
The universal finite element program-ANSYS12.0 was employed to establish model and analysis using 8-node threedimensional entity unit element (SILOD45). Due to bond together, glass fiber enhanced composite board and polystyrene foam has no relative movement model in sandwich panel. In this paper, the panel is simplified as ideal elastic material. u x , u y , u z for all nodes in the edge of the sandwich panel carry on the restraint. In the static analysis, the uniform load of 1kN/m 2 on the sandwich panel is applied in the model. The finite element analysis model is shown in Fig.2.  The results of the finite element static analysis are shown in Fig.3. The maximum displacement value is located in the middle of the span, which is 3.1024mm. The results are in good agreement with those of the theoretical analysis. Eigenvalue buckling analysis was carried out on the model to obtain the buckling mode and the characteristic value of the coefficient, as shown in Fig.4. The first eigenvalue coefficient of Sandwich panel is 21.117, which is slightly smaller. It is caused mainly by the low shear strength of the core panel.

Model tests
The three same specimens were designed and made whose material are made up the glass fiber reinforced composites(FRP), expanded polyethylene (EPS) foam and a hot-melt thermoplastic spray adhesive. The glass fiber reinforced composite panel and polyethylene foam panel are cut into the length 1000mm and breath 1000mm panel using cutting tools. Applying evenly thermoplastic spray adhesive to the FRP, then placing it on the polyethylene foam board and bringing pressure to bear on it. The specimen after curing was shown in Fig.5. In loading process of experiments, firstly, the wires of the pressure sensor on the self-balancing anti force frame are connected with the equipment and the computer, and opening the test software. Then placing the specimen on the support and simulating the whole process from loading to failure. The calculation results of displacement and the critical load of the specimen are automatically recorded by the sensor. Experimental loading conditions are consistent with the boundary condition of the finite element model. With the increase of load, specimens made a slight sound. We cannot find significant change with the naked eye observation. The displacement of the mid-span increased with the increase of load; in loading process of experiments, specimens made "crack" sound and bended down into arc after observation; at a later stage of loading, obvious tearing sound can be heard and panels on the surface warped. Now it was found that core board and the edges of the upper layer appeared slight debonding phenomenon; with the further increase of load, the core and the lower layer bonding edge also appeared slight debonding phenomenon, the failure area extended constantly to the center and bearing capacity suddenly dropped, specimen failure is shown in Fig.6.

Conclusion
(1) A new type of sandwich panel, namely CSIPs (Composition Structural Insulated Panels), was proposed in this paper. CSIPs are made of low-cost orthotropic thermoplastic glass/polypropylene (glass-PP) laminate as face sheets and expanded polyethylene (EPS) foam as a core and are bonded together through a hot-melt thermoplastic spray adhesive.
(2) CSIPs have the advantages of light weight, high strength, better corrosion-resistant, high durability, energy conservation and environmental protection, low maintenance cost. It is also provided with features of convenient transportation and construction, especially the production methods of industrialization and assembled on site can greatly shorten the building cycle.
(3) Due to the relatively low elastic modulus of the FRP in the composite sandwich panel, the stiffness of the structure is small. Therefore, it is necessary to use the large deflection theory to analyze. (4) Theoretical calculation, finite element simulation and experimental results are consistent. It is showed that the calculating formula of deformation and bearing capacity of CSIPs are correct.