Back Face Sheet Research Articles

Dynamic Responses of Sandwich Beams with Gradient-Density Aluminum Foam Cores

The dynamic responses of end-clamped sandwich beams comprised of aluminum face sheets and close-celled aluminum foam cores are numerically simulated by loading the beams at mid-span. Three types of aluminum foam core having identical areal density are considered: homogeneous core, low-high gradient-density core, and high-low gradient-density core. Two-dimensional (2D) finite element (FE) models are created from tomographic images of the foam, which represent the cell shape and geometric distribution of real foams fabricated using the powder metallurgy foaming technique with blowing agents. To determine the mechanical properties of cell wall material, the numerically predicted uniaxial stress versus strain curve for homogeneous aluminum foam is fitted to that measured experimentally. The shock impulse is simulated using a uniformly distributed pressure versus time history. Depending upon the initial impulse applied, it is demonstrated that the deformation and deflection of the sandwich beam can be either bend or stretch dominated. Within the bend dominated domain, the foam-cored sandwich beams outperform monolithic solid beams of equal mass. However, the benefits diminish as the response of the sandwich becomes stretch dominated. Whilst the sandwich beam with homogeneous foam core shows the smallest mid-span deflection of its back facesheet, that with low-high gradient-density core achieves the largest average compressive strain and absorbs the most internal energy.

Dynamic response of metallic lattice sandwich structures to impulsive loading

The dynamic response of metallic lattice sandwich plates under impulsive loading is studied by experimental investigation. The sandwich structures composed of two identical face sheets and tetrahedral lattice cores, were designed and fabricated through perforated metal sheet forming and welding technology. The air blast experiment of lattice sandwich structures was performed by use of a four-cable ballistic pendulum system. The deformation/failure mechanisms were investigated through experimental observation and analysis. The impulsive resistance of the tetrahedral lattice sandwich structures is quantified by the maximum permanent transverse deflection of the back face sheet as a function of transmitted impulse. The maximum transverse deflections of tetrahedral lattice sandwich plates are compared with that of hexagonal honeycomb ones with identical parent materials and core relative density. The comparison implies that the tetrahedral lattice sandwich structures possess a better impulsive resistance.

Blast Mitigation in a Sandwich Composite Using Graded Core and Polyurea Interlayer

The dynamic behavior of two types of sandwich composites made of E-Glass Vinyl-Ester (EVE) facesheets and Corecell™ A-series foam with a polyurea interlayer was studied using a shock tube apparatus. The materials, as well as the core layer arrangements, were identical, with the only difference arising in the location of the polyurea interlayer. The foam core itself was layered with monotonically increasing wave impedance of the core layers, with the lowest wave impedance facing the shock loading. For configuration 1, the polyurea interlayer was placed behind the front facesheet, in front of the foam core, while in configuration 2 it was placed behind the foam core, in front of the back facesheet. A high-speed side-view camera, along with a high-speed back-view 3-D Digital Image Correlation (DIC) system, was utilized to capture the real time deformation process as well as mechanisms of failure. Post mortem analysis was also carried out to evaluate the overall blast performance of these two configurations. The results indicated that applying polyurea behind the foam core and in front of the back facesheet will reduce the back face deflection, particle velocity, and in-plane strain, thus improving the overall blast performance and maintaining structural integrity.

Response of Curved Sandwich Panels Subjected to Blast Loading

Curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally and numerically. Specimens with two values of radius of curvature and different core/face sheet configurations with the same projected area were tested for three blast intensities. All four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. The impulse acting on the front face of the assembly, the deflection history at the center of the back face sheet, and the strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analyzed systematically. The commercial software LS-DYNA was employed to simulate those physical processes. The finite-element (FE) model was validated by the data from experiments. Detailed deformation and energy dissipation mechanisms were further revealed by the FE models. The valuable experimental data and results from FE models show that the initial curvature of a curved sandwich panel changes the deformation/collapse mode with an extended range for bending-dominated deformation mode, which suggests that the performance of the sandwich shell structures slightly exceeds that of both their equivalent solid counterpart and a flat sandwich plate in certain blast intensity ranges.

Low-velocity Impact Behaviour of 3-D Glass Fibre Hollow Integrated Core Sandwich Composites

In the current work, 3-D glass fibre woven fabric-reinforced composites were fabricated by impregnating epoxy resin in a hand lay-up procedure for the hollow integrated core sandwich structure. Four types of 3-D structures were investigated in terms of the impact properties as a function of the z-direction fibre length (5, 6 and 7 mm) and covered skins (covered with aluminium) in core structures. Impact properties including peak load, time to peak load, and absorbed energy were evaluated. It was found that the 3-D hollow integrated core sandwich composites covered with aluminium skin considerably enhanced the impact resistance. Photographs of the impact damage taken after impact tests were analyzed and discussed in terms of a hollowing on the front face sheet against the load applied, and whitening spread in the impact point on the back face sheet of the composites was also studied.

Experiments on curved sandwich panels under blast loading

In this paper curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally. Specimens with two values of radius of curvature and different core/face sheet configurations were tested for three blast intensities. All the four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. Impulse acting on the front face of the assembly, deflection history at the centre of back face sheet, and strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analysed systematically. The experimental data show that the initial curvature of a curved sandwich panel may change the deformation/collapse mode with an extended range for bending dominated deformation, which suggests that the performance of the sandwich shell structures may exceed that of both their equivalent solid counterpart and a flat sandwich plate.

Experimental analysis of perforation of glass/polyester structures subjected to high-velocity impact

In this paper the ballistic behaviour of several glass/polyester laminate structures was studied, evaluating the residual velocity of the projectile and the damage area. Three monolithic laminates of different thicknesses and two multiplate laminate structures were analyzed: one of a sandwich type with face-sheets of glass/polyester and a foam core, and another made with the same face-sheet, which were separated by a distance equal to the thickness of the core. It was found that laminates of greater thickness show a larger damage area and a greater ballistic limit. The influence of the core on the ballistic limit of multiplate laminate structures is negligible but, nevertheless, the extension of the damage area in the back face-sheet is increased.

Dynamic models for low-velocity impact damage of composite sandwich panels – Part B: Damage initiation

Equivalent single and multi degree-of-freedom systems are used to predict low-velocity impact damage of composite sandwich panels by rigid projectiles. The composite sandwich panels are symmetric and consist of orthotropic laminate facesheets and a core with constant crushing resistance. The transient deformation response of the sandwich panels subjected to impact were predicted in a previous paper, and analytical solutions for the impact force and velocity at damage initiation in sandwich panels are presented in this second paper. Several damage initiation modes are considered, including tensile and shear fracture of the top facesheet, core shear failure, and tensile failure of back facesheet. The impact failure modes are similar to static indentation failure modes, but inertial resistance and high strain rate material properties of the facesheets and core influence impact damage loads. Predicted damage initiation loads and impact velocities compare well with experimental results.

Optical tests of an intelligently deformable mirror for space telescope technology

The concept of an actively deformable composite reflector is demonstrated and test verified. The objective of developing such a concept is to correct the low-order long-wavelength distortions in a space reflector. This is achieved by bonding a limited number of piezoelectric ceramic strips to the back face sheet of the reflector. A cryointerferometric test of a 0.5-m reflector with six groups of actuators was performed to verify the concept experimentally. Analysis of and test results for the development of an active deformable composite reflector panel are summarized.