Showing papers on "Sandwich panel published in 2007"

Hierarchical Corrugated Core Sandwich Panel Concepts

Abstract: The transverse compression and shear collapse mechanisms of a second order, hierarchical corrugated truss structure have been analyzed. The two competing collapse modes of a first order corrugated truss are elastic buckling or plastic yielding of the truss members. In second order trusses, elastic buckling and yielding of the larger and smaller struts, shear buckling of the larger struts, and wrinkling of the face sheets of the larger struts have been identified as the six competing modes of failure. Analytical expressions for the compressive and shear collapse strengths in each of these modes are derived and used to construct collapse mechanism maps for second order trusses. The maps are useful for selecting the geometries of second order trusses that maximize the collapse strength for a given mass. The optimization reveals that second order trusses made from structural alloys have significantly higher compressive and shear collapse strengths than their equivalent mass first order counterparts for relative densities less than about 5%. A simple sheet metal folding and dip brazing method of fabrication has been used to manufacture a prototype second order truss with a relative density of about 2%. The experimental investigation confirmed the analytical strength predictions of the second order truss, and demonstrate that its strength is about ten times greater than that of a first order truss of the same relative density. DOI: 10.1115/1.2198243

Impact and post impact behavior of composite sandwich panels

Abstract: Assessing the residual mechanical properties of a sandwich structure is an important part of any impact study and determines how the structure can withstand post impact loading. The damage tolerance of a composite sandwich structure composed of woven carbon/epoxy facesheets and a PVC foam core was investigated. Sandwich panels were impacted with a falling mass from increasing heights until damage was induced. Impact damage consisted of delamination and permanent indentation in the impacted facesheets. The Compression After Impact (CAI) strength of sandwich columns sectioned from these panels was then compared with the strength of an undamaged column. Although not visually apparent, the facesheet delamination damage was found to be quite detrimental to the load bearing capacity of the sandwich panel, underscoring the need for reliable damage detection techniques for composite sandwich structures.

Deformation and fracture modes of sandwich structures subjected to underwater impulsive loads

Abstract: Sandwich panel structures with thin front faces and low relative density cores offer significant impulse mitigation possibilities provided panel fracture is avoided. Here steel square honeycomb and pyramidal truss core sandwich panels with core relative densities of 4% were made from a ductile stainless steel and tested under impulsive loads simulating underwater blasts. Fluid-structure interaction experiments were performed to (i) demonstrate the benefits of sandwich structures with respect to solid plates of equal weight per unit area, (ii) identify failure modes of such structures, and (iii) assess the accuracy of finite element models for simulating the dynamic structural response. Both sandwich structures showed a 30% reduction in the maximum panel deflection compared with a monolithic plate of identical mass per unit area. The failure modes consisted of core crushing, core node imprinting/punch through/tearing and stretching of the front face sheet for the pyramidal truss core panels. Finite element analyses, based on an orthotropic homogenized constitutive model, predict the overall structural response and in particular the maximum panel displacement.

Micromechanical Analysis of Composite Corrugated-Core Sandwich Panels for Integral Thermal Protection Systems

Abstract: termsA44 andA55 werecalculatedusinganenergyapproach.Usingtheshear-deformableplatetheory,aclosed-form solution of the plate response was derived. The variation of plate stiffness and maximum plate deflection due to changing the web angle are discussed. The calculated results, which require significantly less computational effort and time, agree well with the three-dimensional finite element analysis. This study indicates that panels with rectangular webs resulted in a weak extensional, bending, and A55 stiffness and that the center plate deflection was minimum for a triangular corrugated core. The micromechanical analysis procedures developed in this study were used to determine the stresses in each component of the sandwich panel (face and web) due to a uniform pressure load.

Investigation of Parameters Governing the Damage and Energy Absorption Characteristics of Honeycomb Sandwich Panels

Abstract: Honeycomb sandwich panels of various skin thicknesses and core densities have been investigated under quasi-static loading in bending and indentation with both hemispherical (HS) and flat-ended (FE) indenters. Core crushing, top skin delamination, and top skin fracture are identified as major damage mechanisms. Their characteristics and energy-absorbing capabilities are established using load—displacement and load—strain curves and inspections of cross-sectioned specimens. The effects of varying skin thickness, core density and type, indenter nose shape, and boundary conditions on the damage and energy-absorbing characteristics are examined. The variation of the indenter nose shape is shown to induce a change in the damage mechanisms and have the most significant effect on energy absorption, especially for panels with relatively thicker skins. Increasing the skin thickness significantly increases not only the initial threshold and ultimate loads but also the absorbed energy (AE) of the panels. Increasing ...

Manufacture of lattice truss structures from monolithic materials

Abstract: Methods and systems to manufacture lattice-based sandwich structures from monolithic material. Such methods and systems eliminate the bonding process which is conventionally used to join lattice based truss cores to facesheets to form sandwich structures. This bonded interface is a key mode of failure for sandwich structures which are subjected to shear or bending loads because the nodes transfer forces from the face sheets to the core members while the topology for a given core relative density dictates the load carrying capacity (assuming adequate node-bond strength exists). An aspect comprises a core and related structures that provide very low density, good crush resistance and high in-plane shear resistance. An aspect of the truss structures may include sandwich panel cores and lattice truss topology that may be designed to efficiently support panel bending loads while maintaining an open topology that facilitates multifunctional applications.

Incremental forming of sandwich panels

Abstract: This paper presents a first investigation of the applicability of incremental sheet forming (ISF) to sandwich panels. Two initial tests on various sandwich panel designs established that sandwich panels which are ductile and incompressible are the most suitable for the process. Further tests on a sandwich panel with mild steel face plates and a continuous polypropylene core demonstrated that patterns of deformation and tool forces followed similar trends to a sheet metal. It is concluded that, where mechanically feasible, ISF can be applied to sandwich panels using existing knowledge of sheet metals with the expectation of achieving similar economic benefits. Potentially this will increase the range of applications for which sandwich panels are viable.

Effective elastic characteristics of honeycomb sandwich composite shells made of generally orthotropic materials

Abstract: This paper works on the analytical development of the method of two-scale asymptotic homogenization. The technique is used to determine the effective elastic stiffnesses of hexagonal honeycomb-cored structural sandwich composite shells made of generally orthotropic materials. Orthotropy of the constituent materials leads to much more complex unit-cell problems and is considered in the present paper for the first time. At first, a 3D-to-2D general shell model based on a set of unit-cell problems is derived. This is followed by the exploitation of the model to the derivation of analytical estimate formulae; used to calculate the force and moment resultants present in the sandwich shell structure. The implication of the general shell model is further indicated by calculating similar design characteristics for a three-layered composite sandwich panel reinforced with hexagonal and triangular shaped cellular core made from the generally orthotropic material.

Structural modeling of sandwich structures with lightweight cellular cores

Abstract: An effective single layered finite element (FE) computational model is proposed to predict the structural behavior of lightweight sandwich panels having two dimensional (2D) prismatic or three dimensional (3D) truss cores. Three different types of cellular core topology are considered: pyramidal truss core (3D), Kagome truss core (3D) and corrugated core (2D), representing three kinds of material anisotropy: orthotropic, monoclinic and general anisotropic. A homogenization technique is developed to obtain the homogenized macroscopic stiffness properties of the cellular core. In comparison with the results obtained by using detailed FE model, the single layered computational model can give acceptable predictions for both the static and dynamic behaviors of orthotropic truss core sandwich panels. However, for non-orthotropic 3D truss cores, the predictions are not so well. For both static and dynamic behaviors of a 2D corrugated core sandwich panel, the predictions derived by the single layered computational model is generally acceptable when the size of the unit cell varies within a certain range, with the predictions for moderately strong or strong corrugated cores more accurate than those for weak cores.

Buckling and nonlinear response of sandwich panels with a compliant core and temperature-dependent mechanical properties

Abstract: This paper deals with the buckling response and nonlinear behavior of sandwich panels with soft cores that have temperature-dependent mechanical properties and are subjected to thermally induced deformations and mechanical loads simultaneously. This study investigates the effects of the degradation of properties of the core as a result of rising temperature on the response of the sandwich panel. Analyses are carried out for cases of pure thermal loading, with either uniform or gradient temperature fields through the depth of the panel, as well as for thermal loading acting simultaneously with external mechanical loads. The formulation is based on variational principles along with the high-order sandwich panel approach. It takes into account the flexibility of the core in the vertical direction as well as the dependency of the mechanical core properties of the temperature distribution through the core depth. The stress and deformation fields of the core have been solved analytically, including the case where the temperature-dependent properties attain a complex pattern. The buckling equations are derived using the perturbation technique, yielding a set of nonlinear algebraic equations for the case of a simply-supported panel and a uniform temperature field. The critical temperatures and modes of wrinkling and global buckling are determined numerically for some foam types of core made by Rohacell and Divinycell. The nonlinear response caused by thermally induced deformations is presented for Divinycell foam core with different temperature distributions through the depth of the core. Finally, the nonlinear response caused by the simultaneous action of external mechanical loading and increased temperatures on the compressive or the tensile side of the panel, with a thermal gradient through the core depth, is presented. The interaction between elevated temperatures and mechanical loads changes the response from a linear into an unstable nonlinear one when the degradation of the mechanical properties due to temperature changes is considered and the panel is unrestrained. Moreover, the unstable nonlinear behavior becomes even more severe when the face, loaded in compression, is subjected to elevated temperatures. This study reveals that a reliable, realistic design of a sandwich panel that is subjected to elevated temperature (within working temperature range) and mechanical loads must take into account the degradation of the properties of the core as a result of the thermal field even at working temperature range, especially when cores made of foam are considered.

Composite Sandwich Panel and Method of Making Same

Abstract: A method of manufacturing a composite panel includes manufacturing a composite panel having a first skin, a second skin, a core, and a plurality of distinct groupings of Z-axis fibers that extend through the core from the first skin to the second skin, wherein the Z-axis fibers include opposite ends respectively terminating at and integrated into the first skin and the second skin; and creating structural stringers in the composite panel by removing the second skin and substantially all of the core and the Z-axis fibers down to or adjacent to the first skin.

Effect of Soft Honeycomb Core on Flexural Vibration of Sandwich Panel using Low Order and High Order Shear Deformation Models

Abstract: Low order and high order shear deformation models are applied to investigate the effect of honeycomb core (transversely shear deformable) on flexural vibration of thick rectangular sandwich panel w...

Crushing energy absorption of GFRP sandwich panels and corresponding monolithic laminates

Abstract: Steady quasi-static compression of GFRP monolithic laminates and sandwich panels made of a randomly oriented continuous filament mat/polyester were undertaken. The effects of facing/laminate thickness, trigger collapse system and aspect ratio on their failure mechanisms, hence their energy absorption capability were examined. A numerical model, using a non-linear finite element explicit code, LS-DYNA, was used for pre-analysis of the effect of aspect ratio. A collapse trigger configuration was also studied numerically. The experimental data showed that high values of energy absorbed per unit mass were a predominant feature of the thickest monolithic laminates and sandwich panels with the thickest facings. The monolithic laminates showed higher specific energy than their sandwich panel counterparts. It seems that this difference was due to instability of the sandwich specimens.

Suppression of interfacial crack for foam core sandwich panel with crack arrester

Abstract: Since delamination often propagates at the interfacial layer between a surface skin and a foam core, a crack arrester is proposed for the suppression of the delamination. The arrester has a semi-cylindrical shape and is arranged in the foam core and is attached to the surface skin. Here, energy release rates and complex stress intensity factors are calculated using finite element analysis. Effects of the arrester size and its elastic moduli on the crack suppressing capability are investigated. Considerable reductions of the energy release rates at the crack tip are achieved as the crack tip approached the leading edge of the crack arrester. Thus, this new concept of a crack arrester may become a promising device to suppress crack initiation and propagation of the foam core sandwich panels.

Modelling of cold-formed steel sandwich purlin-sheeting systems to estimate the rotational restraint

Abstract: Sandwich panels are attached to cold-formed steel purlins in roofs of industrial buildings to provide insulation. As the strength of the attached purlins is considerably increased due to the lateral and rotational restraints provided by the sandwich panels, estimating these restraints is important in the design of purlins. The rotational restraint is generally determined by experiments, as no design rules exist for sandwich purlin-sheeting systems. In this paper, a non-linear finite element model is presented to estimate the rotational restraint provided by the sandwich panels to the attached purlin. The model is validated with experimental tests and is in good agreement. In order to develop a design method for estimating the rotational restraint in sandwich purlin-sheeting systems, the model could be useful for parametric studies to investigate the influencing factors.

High-order nonlinear contact effects in cyclic loading of delaminated sandwich panels

Abstract: The interaction between the delaminated surfaces and its influence on the static and on a special type of cyclic load behavior of delaminated sandwich panels is studied. In addition, the results for a cyclic push–pull load and the analysis for the detection of damage in sandwich panels is presented. The analysis uses the high-order theory of sandwich panels (HSAPT) and considers geometrical nonlinear effects and the nonlinearity associated with the contact characteristics of the delaminated surfaces. The nonlinear contact phenomena, which are associated with variation of the contact conditions during motion, result in differences in the stiffness of the delaminated panel, which are detected under a push or pull type of loads. Quantitative assessment of these differences sheds light on the existence, size, and location of the delaminated region and can be used for a nondestructive evaluation of the damage characteristics of the delaminated panel. The field equations and the corresponding boundary and continuity conditions of the nonlinear analytical model are derived via the variational principle of virtual work. The mathematical formulation uses the ordinary unidirectional panel theory for the face-sheets and a two-dimensional elasticity approach for the core. It yields a set of nonlinear differential equations that are solved using the Multiple Shooting Method combined with a Newton–Raphson iterative scheme. The influence of the nonlinear contact phenomena on the behavior of the panel under two loading schemes is numerically studied. The first loading scheme is a push–pull loading simulation that demonstrates the concept of the proposed damage detection method. The second loading scheme simulates the conditions that evolve during free vibration response. The results reveal the role of the nonlinear contact phenomena in the cyclic bending behavior of the sandwich panel and its influence on the localized and overall nonlinear response. A summary and conclusions close the paper.

Dynamic modeling of honeycomb sandwich panel

Abstract: In this paper a mathematical model based on a multi-scale asymptotic technique for the dynamic description of honeycomb structures is presented. The technique is used to evaluate an equivalent orthotropic model of the honeycomb. The derivation is based on an asymptotic analysis for periodic structures developed by Bensoussan, Lions and Papanicolaou. The method is totally general; in fact, it is applied to Cauchy’s partial differential equations that describes the dynamics of an elastic material. Elastic and density characteristics are determined in terms of the cell geometry and material of the honeycomb. A numerical validation is carried out by using a finite-element modeling (FEM) numerical simulation.

A new design for steel bridge decks using laser fabrication

Abstract: While recognising the value of steel orthotropic decks in providing a lightweight form of construction, essential for weight-critical structures, their cost and poor record of fatigue durability have discouraged their use for mainstream construction. The authors take a look at an innovation proposed to overcome these problems and transform the design of steel decks. Using laser welding to produce an enclosed sandwich panel profile, many of the constraints are overcome. The authors describe how the sandwich profile is constructed, outlining the advantages such as the low cost of production, the durability of the product and its long term potential uses. Figures, tables and illustrations accompany the descriptions. Also addressed are the current state of the art in steel deck designs, the new design concept for steel bridge decks, a comparison of structural performance in table form, the switch panel design for bridge deck applications, and the fatigue durability of laser welded joints including the results of web bending fatigue tests and deck bending fatigue tests. The authors also look at the economic viability of sandwich panel decks and their support and jointing arrangements.