Showing papers on "Sandwich-structured composite published in 1997"

Process for making carbon foam

Abstract: The process obviates the need for conventional oxidative stabilization. The process employs mesophase or isotropic pitch and a simplified process using a single mold. The foam has a relatively uniform distribution of pore sizes and a highly aligned graphic structure in the struts. The foam material can be made into a composite which is useful in high temperature sandwich panels for both thermal and structural applications.

Full scale testing of precast concrete sandwich panels

Abstract: This paper describes newly developed precast concrete sandwich panels. The panels use a new system of fiber reinforced plastic (FRP) connectors, developed at the University of Nebraska's Center for Infrastructure Research, for transferring shear forces from one concrete wythe to the other The use of FRP as the connector material increases the thermal efficiency of the panels compared to panels that contain steel or concrete connectors. The geometry and material properties of the new connector provide sufficient strength and stiffness for a significant transfer of shear between the two concrete wythes. The results of structural tests are analyzed and design recommendations are presented. Description of an accompanying thermal performance testing of these panels is also included. For full-scale specimens were tested in a vertical position. Two of the specimens contain the new FRP connectors, and two contain steel truss connectors. Measurements of load versus panel deflection and load versus connector stress are provided. The ultimate strength of the panels containing the new connector were found comparable to the strength expected of fully composite panels. The design of panels containing the proposed FRP connecting system can be undertaken ina manner similar to that composite panels. Ultimate strength of the panels can be computed assuming full composite action between the concrete wythes, it sufficient shear connectors are provided and the panels are not over-reinforced.

Stability of Functionally Graded Shape Memory Alloy Sandwich Panels

Abstract: The stability of sandwich panels subjected to the simultaneous action of a uniform temperature and a uniaxial compression is considered. At elevated temperatures, the buckling load can be increased by using shape memory alloy (SMA) fibers in resin sleeves embedded within the core, at the midplane of the sandwich panel. The best results are achieved when the spacing of SMA fibers across the panel is nonuniform, i.e. the spacing, which is minimum at the centerline, gradually increases with the approach to the edges (sinusoidal distribution). The effectiveness of SMA fibers increases with temperature due to larger tensile recovery stresses. The example of stability of sandwich panels considered in the paper illustrates that functionally graded SMA composites may present significant advantages in engineering design.

Resin compositions for fiber-reinforced composite materials and processes for producing the same, prepregs, fiber-reinforced composite materials, and honeycomb structures

Abstract: Resin compositions for fiber-reinforced composite materials characterized by comprising (A) epoxy resin, (B) hardener, and (C) additive as the essential ingredients and having a storage rigidity G' of 5 to 200 Pa throughout the whole temperature range of 70-100 °C when heated from 50 °C at a temperature rise rate of 1.5 °C/min by using plane parallel plates having an oscillation frequency of 0.5 Hz; and prepregs made by using the same. The prepregs are excellent in self-adhesion to honeycomb cores, give skin panels having a reduced porosity, an excellent surface smoothness, and appropriate tackiness and draping properties, thus being usable in aircrafts, automobiles, and industrial applications, especially as structural materials for aircrafts.

Composite honeycomb sandwich structure

Abstract: We eliminate resin flow into the cells of honeycomb in sandwich structure by using an unsupported film adhesive (108), a barrier layer (110), and a scrim supported adhesive layer (112) between the composite laminate (102) and the core (106). We produce superior panels with lighter weights, improved mechanical properties, and more predictable structural performance by keeping resin in the laminate rather than losing it to the core cells. We reduce core crush and ply wrinkling in composite honeycomb sandwich structure by preventing slipping of tiedown plies relative to the mandrel and to one another during autoclave curing. We produce superior panels with lighter weights, improved mechanical properties, and more predictable structural performance. The method involves applying a film adhesive to the tiedown plies in the margin of the part outside the net trim line. During heating of the autoclave and prior to the application of high pressure to the composite structure, the film adhesive cures to form a strong bond between the plies and to the mandrel. When pressure is applied, the tiedown plies are locked together and to the mandrel to prevent slippage between any layers in the panel.

Composite detail having Z-pin stubble

Abstract: I improve the impact shock resistance of bonds between composite elements by including Z-pin reinforcement. I prepare stubbled composite structure by using peel plys over the appropriate surface of the composite during pin insertion using conventional processes. I then use the stubbled composite structure with padups, as necessary, to produce the Z-pin reinforced joint or bond between composite elements using any of adhesive bonding, cocuring, or thermoplastic welding.

Cellular Solids: The mechanics of honeycombs

Abstract: Introduction and synopsis The honeycomb of the bee, with its regular array of prismatic hexagonal cells, epitomizes a cellular solid in two dimensions. Here we use the word ‘honeycomb’ in a broader sense to describe any array of identical prismatic cells which nest together to fill a plane. The cells are usually hexagonal in section, as they are in the bee's honeycomb, but they can also be triangular, or square, or rhombic. Examples were shown in Fig. 2.3. Man-made polymer, metal and ceramic honeycombs are now available as standard products. They are used in a variety of applications: polymer and metal ones for the cores of sandwich panels in everything from cheap doors to advanced aerospace components; metal ones for energy-absorbing applications (the feet of the Apollo 11 landing module used crushable aluminium honeycombs as shock absorbers); and ceramic ones for high-temperature processing (as catalyst carriers and heat exchangers, for example). And many natural materials – wood is one – can be idealized and analysed as honeycombs (as we do in Chapter 10). If such materials are to be used in load-bearing structures an understanding of their mechanics is important. There is a second good reason for studying honeycombs: it is that the results shed light on the mechanics of the much more complex three-dimensional foams. Analysing foams is a difficult business: the cell walls form an intricate three-dimensional network which distorts during deformation in ways which are hard to identify.

Efficient structural components using porous metals

Abstract: Porous metals can now be produced by a variety of techniques After reviewing the available processes we describe the potential for their use in efficient structural components, including honeycomb beams, sandwich panels and cylindrical shells with porous cores

Low-weight and water-resistant honeycomb sandwich panels made by resin transfer molding process

Abstract: The instant invention pertains to a low-weight and water-resistant composite panel made from an open cell core, such as honeycomb, using the resin transfer molding (RTM) process. The open cell core is sealed to a degree of at least 95 %, and even up to 100 %, by use of a resin/moisture barrier before the liquid resin is injected into the fibrous reinforcement This prevents resin fill into the open cell core during the RTM process and isolates the core from water migration after the structure is put into service.

A Comparative Study on the Impact Resistance of Composite Laminates and Sandwich Panels

Abstract: An experimental study was conducted to compare the damage resistance and energy absorbing capabilities of graphite/bismaleimide composite laminates and foam-core sandwich panels. Two types of tests were conducted: static indentation and drop weight impact. The load-displacement response and energy absorbed were recorded for static and impact tests. The amount of core compression in sandwich panels was also measured. Damage in the laminates and sandwich panels were assessed by using ultrasonic C-scan, X-radiography, and photo-micrography. The load-displacement responses and damage area obtained from static and impact testing matched very well for the laminates. The static and impact responses for the sandwich panels have the same overall trend, but differ due to the viscoelastic properties of the foam core. Test results show that the sandwich panels can absorb more energy than the laminates and undergo less deflection to absorb the same maximum energy as the laminates. Impact energy above a statically determined threshold level has shown to cause extensive foam core damage.

Method of manufacturing composite articles

Abstract: A composite article is prepared by positioning composite material precursor between a layup tool and a sculpted block of an open-celled foam material. The block has a first face sculpted to define a first face of the composite article. A pressure is applied between the layup tool and the block, and thence to the composite material precursor. Simultaneously with the pressure application, the assembly is heated to compact and cure the composite material. The open-celled foam block acts both as a breather material to allow gas to escape from the compacting composite material and a bleeder material to receive resin that is forced out of the composite material under pressure.

On the design and analysis of continuous sandwich panels

Abstract: Interaction between the bending moment and support reaction introduces new failure modes at intermediate supports of continuous sandwich panels. Two different loading cases have to be separated in the design. Positive support reaction causes a compressive contact pressure between the supporting structure and sandwich panel, and negative support reaction tensile forces in the fasteners of the panels. In the paper, the loading case named the positive support reaction is studied by means of analytical models, numerical analyses and experimental results. The analyses result in recommendations for the design at the serviceability state.

Curved Sandwich Panels Subjected to Temperature Gradient and Mechanical Loads

Abstract: The results of a detailed study of the nonlinear response of curved sandwich panels with composite face sheets, subjected to a temperature gradient through the thickness combined with mechanical loadings, are presented. The analysis is based on a first-order shear-deformation Sanders-Budiansky-type theory, with the effects of large displacements, moderate rotations, transverse shear deformation, and laminated anisotropic material behavior. A mixed formulation is used with the fundamental unknowns consisting of the generalized displacements and the stress resultants of the panel. The nonlinear displacements, strain energy, principal strains, transverse shear stresses, transverse shear strain energy density, and their hierarchical sensitivity coefficients are evaluated. The hierarchical sensitivity coefficients measure the sensitivity of the nonlinear response to variations in the panel parameters, the effective properties of the face sheet layers and the core, and the micromechanical parameters. Numerical results are presented for cylindrical panels subjected to combined pressure loading, edge shortening or extension, edge shear, and a temperature gradient through the thickness. The results show the effects of variations in the loading and the panel aspect ratio, on the nonlinear response, and its sensitivity to changes in the various panel, effective layer, and micromechanical parameters.

Z-pin reinforced, bonded composite structure

Abstract: I improve the impact shock resistance of bonds between composite elements by including Z-pin reinforcement. I prepare stubbled composite structure by using peel plys over the appropriate surface of the composite during pin insertion using conventional processes. I then use the stubbled composite structure with padups, as necessary, to produce the Z-pin reinforced joint or bond between composite elements using any of adhesive bonding, cocuring, or thermoplastic welding.

Moisture Ingression in Honeycomb Core Sandwich Panels: Directional Aspects

Abstract: Moisture ingression was studied in several composite sandwich panels.No observable ingression was found in a panel with HRP core of density 0.13 g/cc (8.0 lb/ft'). Significant moisture ingression occurred in a panel with Korex 3.0-lb core with density 0,048 g/cc, (3.0 lb/cu ft). It was as faster in the Y-axis (core ribbon) direction, per unit distance than in other directions.

Optimum design via panda2 of composite sandwich panels with honeycomb or foam cores

Abstract: PANDA2 has been extended to handle panels with sandwich wall construction by inclusion of the following failure modes in addition to those previously accounted for: (1) face wrinkling, (2) face dimpling, (3) core shear crimping, (4) core transverse shear stress failure, (5) core crushing and tension failure, and (6) facesheet pull-off. Transverse shear deformation effects are included both for overall panel buckling and for local face sheet dimpling and face sheet wrinkling. The new PANDA2 code will optimize stiffened sandwich panels in which the stiffener segments as well as the panel skin may have sandwich wall constructions. The effects of panel buckling modal initial imperfections as well as initial face sheet waviness are accounted for during optimization cycles. The updated PANDA2 code will also handle optimization of a panel supported by an elastic Winkler foundation. Examples are presented for a uniformly axially compressed perfect and imperfect unstiffened panel without and with a uniform temperature gradient through the panel wall thickness. Initial face sheet waviness and initial overall buckling modal imperfections both have major influence on optimum designs of sandwich panels with honeycomb cores.