Protective steel doors are widely used in buildings due to their high resistance against the impact loads. However, its heavy weight has been always considered as a major drawback for these doors. In this paper, a new optimized stiffened impact-protective steel door incorporating Sandwich panel with aluminum foam core (OSSA) is examined. This door consists of two face sheets, main and secondary stiffeners, and aluminum foam as the inner core. In order to optimize the door, at first the rigidity and weight functions of the stiffened steel door were extracted. Then an optimal door weighing 42% less than the primary door was obtained. Due to the high energy absorption capacity of the combined foam core and stiffened steel door structure, the use of aluminum foam core in the optimized steel door was proposed. By doing numerical analysis, and depending on the thickness of the face sheet of OSSA, 20 to 32% reduction in the maximum displacement was observed. The results also showed that, with 67% increase in the peak overpressure, OSSA has kept almost the same maximum displacement as that of the steel door without an aluminum foam. In other words, by using aluminum foam core in the optimized stiffened door, the door will resist 67% more impact load.

Metal faced Polyurethane/Polyisocyanurate Sandwich panels are used in construction sites and temporary accommodation especially after destructive events such as flooding and earthquake, that a clear example was the temporary settlement of earthquake victims after earthquake occurrence in the Kermanshah province at November 2017. However, flammability of Polyurethane foam core of these panels and the higher risk of fire in these types of buildings, highlight the importance of assessing fire performance of these panels. In this study, fire performance of several types of metal faced Sandwich panels with PUR/PIR foam core produced in the country, was evaluated by reaction to fire and fire resistance tests. The reaction to fire behavior of foams was also evaluated separately. The results showed that the Polyurethane foam was not fire retarded and met reaction to fire class F,but the poly-isocyanurate foam depicted a better fire behavior and met fire class E. Fire resistance tests were performed on common types of Sandwich panels in the temporary buildings with two different execution details including a steel sheet fixed to the joint position in the panels and the other, fireproof paint and their fire performance was compared to unprotected panel. According to the results, deformation of the joint in Sandwich panel is the main disadvantage and it is very critical in real fire due to flame spread through the joints which is critical in a real fire incident, when evacuating the occupants and acting fire brigades. Hence, protection of the joints by insertion of a protective sheet, increases fire resistance and improves the integrity by increasing the time by 40 minutes compared to the unprotected panel. Finally, fire safety recommendations were provided for the safe use of these panels in temporary buildings.

Flexible Polyurethane foam (PU) and monolithic Polyurethane Sandwich panel samples reinforced with different weight percent of TiO2 nanoparticles (0.25, 0.5, 0.75, 1, 1.5 and 2) have been successfully prepared. The effects of TiO2 nanoparticles on the physical and thermal properties of mentioned samples were examined. In order to observe morphological structure of Polyurethane (PU) samples, scanning electron microscopy (SEM) were used. Research results showed that by increasing TiO2 nanoparticles to 2 wt.%, the density of foam and Sandwich panel increased to 32.2% and 62.6%, respectively in comparison with pure sample and amount of water absorption decreased to 45% and 58.8%, respectively. The results of UV analysis showed that the maximum amount of UV rays absorption in PU foam reinforced with 2 wt.% of TiO2 nanoparticles equals to 3.21 at wavelength of 450 nm. Investigation results of TGA analysis showed that the presence of TiO2 nanoparticles caused improvement thermal stability of PU Nano composites. By increasing TiO2 nanoparticles to 1 and 2 wt.%, the degradation temperature at weight loss equals to 10 wt.% of PU increased 14.5oC in comparison with pure sample. Also the degradation temperature of weight loss equals to 50 wt.% of samples reinforced with 1 and 2 wt.% of TiO2 nanoparticles increased 1 and 3oC, respectively.

Sandwich structures with cores made of porous materials such as various types of metal and polymer foams are effective in absorbing the energy of explosive loads and due to their lightness can be used well in various industries. In this paper, deformation and energy absorption of Sandwich panels with aluminum and steel face sheet and cores made of Polyurethane foam, which is filled with two types of mineral pumice of different sizes (so-called chickpea size and almond size), under experimental free explosion loading. And numerical simulation has been studied with the help of Abaqus finite element software. In this study, the mechanical behavior of the core was first studied by making samples of Polyurethane foam and mineral pumice under compression testing and the results were used in software behavioral models. After comparing the numerical results with the results of experimental test, and validating the results of numerical methods, numerical parametric studies were performed and the effect of pumice type, core thickness, face sheet material and thickness on the rate of deformation and energy absorption of Sandwich panel components have been studied. The results show that the panel with a thicker back face sheet has a better performance in absorbing explosion energy. Also, as the strength or thickness of the face sheets increases, the role of the core in energy absorption decreases.

A novel geometrically nonlinear high order Sandwich panel theory considering finite strains of Sandwich components is presented in this paper. The equations are derived based on high order Sandwich panel theory in which the Green strain and the second Piola-Kirchhoff stress tensor are used. The model uses Timoshenko beam theory assumptions for behavior of the composite face sheets. The core is modeled as a two dimensional linear elastic continuum that possessing shear and vertical normal and also in-plane rigidities. Nonlinear equations for a simply supported Sandwich beam are derived using Ritz method in conjunction with minimum potential energy principle. After obtaining nonlinear results based on this enhanced model, simplification was applied to derive the linear model in which kinematic relations for face sheets and core reduced based on small displacement theory assumptions. A parametric study is done to illustrate the effect of geometrical parameters on difference between results of linear and nonlinear models. Also, to verify the analytical predictions some three point bending tests were carried out on Sandwich beams with glass/epoxy face sheets and Nomex cores. In all cases good agreement is achieved between the nonlinear analytical predictions and experimental results.

Reaction to fire of fire-retarded rigid PUR foams and two types of metal faced rigid Polyurethane foam core Sandwich panel was evaluated by using cone calorimeter test method. The tests were carried out in various radiative heat fluxes from 15 to 75 kW/m2. The radiation rate effect on reaction to fire parameters, including time to ignition (TTI), peak of heat release rate (PRHR), total heat release (THR), average heat release rate (Av.RHR) and average heat of combustion (Av.EHC) was investigated. The phenomenon of char forming, when the foam is exposed to heat, leads to the formation of a protective layer on the surface of foam and hence no direct relation exists between AV.RHR and average specific mass loss rate (Av.Spec.MLR) of foam with increased radiation rate. In addition, the increased PRHR with foam density was also very smooth. The relation between TTI and heat flux was investigated for the foam and its corresponding correlation has been achieved with a specified density. Fire hazard assessment of foams and sandwitch panels was carried out by adopting Petrella and Richardson fire risk classification methods. The assessment results showed that rigid PUR foam and PUR Sandwich panels may have a high contribution to bring the room to critical flashover condition, but the risk is intermediate from the viewpoint of fire endurance. The reasons of these risk levels are attributed to a very short TTI, relative high PRHR and an intermediate amount of THR. Decrease in foam density reduces heat release but it shows no significant effect on reducing flashover hazard.

Auxetic cellular solids in the forms of honeycombs and foams have great potential in a diverse range of applications, including as core material in curved Sandwich panel composite components, radome applications, directional pass band filters, adaptive and deployable structures, filters and sieves, seat cushion material, energy absorption components, viscoelastic damping materials and fastening devices etc. In this paper, the characteristic of wave propagation in Sandwich panel with auxetic core is analyzed. A three-layer Sandwich panel is considered which is discretized in the thickness direction by using semi-analytical finite element method. Wave propagation equations are obtained through some algebraic manipulation and applying standard finite element assembling procedures. The mechanical properties of auxetic core can be described by the geometric parameters of the unit cell and mechanical properties of the virgin core material. The characteristics of wave propagation in Sandwich panel with conventional hexagonal honeycomb core and re-entrant auxetic core are discussed, and effects of panel thickness, geometric properties of unit cell on dispersive curves are also discussed. Variations of Poisson’s ratio and core density with inclined angle are presented.

A new improved high-order theory is presented to investigate the dynamic behavior of Sandwich panels with flexible core. Shear deformation theory is used for the face sheets while the three-dimensional elasticity theory is used for the core. Displacements in the core are assumed as polynomial with unknown coefficients. Inertia forces, moments of inertia and shear deformations in the core and the face sheets are taken into consideration. Unlike the previous improved theory, the in-plane normal and shear stresses in the core are considered. The governing equations and the boundary conditions are derived by Hamilton's principle. Closed form solution is achieved using the Navier method and solving the eigenvalues. The numerical results of present analysis are compared with the available numerical or theoretical results in the literatures. It indicates that the present new modified theory is more accurate than the other developed theories for Sandwich panels. The variations of the non-dimensional natural frequency with respect to the various geometrical and material parameters are investigated.

In this article, vibration and supersonic flutter analyses are studied for trapezoidal Sandwich panels. Functionally graded trapezoidal panel as well as reinforced Sandwich panel by graphene nano platelets are considered. It is assumed that the graphene platelet (GPL) nanofillers are distributed in the matrix either uniformly or non-uniformly in the direction of thickness. UD, FG-X, FG-V, FG-O and FG-A are the distribution patterns of GPLs. Based on the Kant higher-order theories, the dynamic equations of Sandwich panels reinforced with graphene nanoplates are obtained using extended Hamilton’ s principle. Dynamic pressure is estimated according to the quasi-stable theory of supersonic piston. Then, using a transformation of coordinates, the governing equations and boundary conditions are converted from the original coordinates into new computational ones. Finally, the differential squares method (DQM) to obtain the natural frequencies, the shape of the modes, and the critical aerodynamic pressure is used. The effect of different porosity distribution, porosity coefficients, distribution of graphene nanoplates, weight fraction, geometry of graphene nanofillers and geometric dimensions on natural frequencies and system instability behavior are studied.