Bending Behavior of Aluminum Honeycomb Sandwich Panels

 Aluminum sandwich construction has been recognized as a promising concept for structural design of light weight systems such as wings of aircraft. A sandwich construction, which consists of two thin facing layers separated by a thick core, offers various advantages for design of weight critical structure. Depending on the specific mission requirements of the structures, aluminum alloys, high tensile steels, titanium or composites are used as the material of facings skins. Several core shapes and material may be utilized in the construction of sandwich among them, it has been known that the aluminum honeycomb core has excellent properties with regard to weight savings and fabrication costs. This paper is theoretically calculate bending behavior, of sandwich panels and to compare the strength to weight ratios of Normal Aluminium rod(panel) and Aluminium Honey Comb Panel .

 1.INTRODUCTION

 Sandwich panels are used for design and construction of lightweight transportation systems such as satellites, aircraft, missiles, high speed trains. Structural weight saving is the major consideration and the sandwich construction is frequently used instead of increasing material thickness. This type of construction consists of thin two facing layers separated by a core material. Potential materials for sandwich facings are aluminum alloys, high tensile steels, titanium and composites depending on the specific mission requirement. Several types of core shapes and core material have been applied to the construction of sandwich structures. Among them, the honeycomb core that consists of very thin foils in the form of hexagonal cells perpendicular to the facings is the most popular. A sandwich construction provides excellent structural efficiency, i.e., with high ratio of strength to weight. Other advantages offered by sandwich construction are elimination of welding, superior insulating qualities and design versatility. Even if the concept of sandwich construction is not very new, it has primarily been adopted for non-strength part of structures in the last decade. This is because there are a variety of problem areas to be overcome when the sandwich construction is applied to design of dynamically loaded structures. To enhance the attractiveness of sandwich construction, it is thus essential to better understand the local strength characteristic of individual sandwich panel/beam members.

 The conventional single skin structure, which is of single plates reinforced with main frames and stiffeners normally necessitates a fair amount of welding, and has a considerable length of weld seams. Further, the lighter but thinner plates employed tend to increase weld distortions that may in some cases require more fabrication work to rectify. More weld seams also mean a greater number of fatigue initiation locations as well. Honeycomb sandwich construction, with a honeycomb core is sandwiched by two outer facing skins is better able to cope with such difficulties. 

  Sandwich panels also provide added structural weight savings in the structure. It is for these reasons that the sandwich construction has been widely adopted for large weight critical structures. Honeycomb-cored sandwich panels have been used as strength members of satellites or aircraft, thus efficiently reducing their structural weight. In the railroad industry, passenger coaches of high-speed trains such as the TGV have been designed and fabricated using aluminum honeycomb sandwich panels. Recently, attempts to use aluminum sandwich panels as strength members of high-speed vessel hulls have also been made.This paper deals with the design and analysis of aerospace lifting surface with honeycomb core. Lifting surfaces are essentially designed to take up bending loads due to lift. Bending stresses will be maximum at the top and bottom surfaces, low stresses at the middle. Honeycomb panel construction suits this requirement, where top and bottom skin takes the bending load. To understand the bending behavior of honeycomb Sandwich panels, analysis is carried out for the specimen level three point bending test. The honeycomb sandwich construction is one of the most valued structural engineering innovations developed by the composites industry.

 2.MATERIALS SELECTION

 The honeycomb sandwich construction can comprise an unlimited variety of materials and panel configurations. The composite structure provides great versatility as a widerange of core and facing material combinations can be selected. The following criteria should be considered in the routine selection of core, facing, and adhesive.

 2.1 Structural Considerations Strength: Honeycomb cores and some facing materials are directional with regard to mechanical properties and care must be taken to ensure that the materials are orientated in the panel to take the best advantage of this attribute. Stiffness: Sandwich structures are frequently used to maximize stiffness at very low weights. Because of the relatively low shear modulus of most core materials, however, the deflection calculations must allow for shear deflection of the structure in addition to the bending deflections usually considered. Adhesive Performance: The adhesive must rigidly attach the facings to the core material in order for loads to be transmitted from one facing to the other. Suitable adhesives include high modulus, high strength materials available as liquids, pastes or dry films. As a general rule, a low peel strength, or relatively brittle adhesive should never be used with very light sandwich structures which may be subjected to abuse or damage in storage,handling or service. Cell Size: A large cell size is the lower cost option, but in combination with thin skins may result in telegraphing, i.e. a .dimpled. outer surface of the sandwich. A small cell size will give an improved surface appearance, and provides a greater bonding area, but at higher cost. Cell Shape: Normally supplied with hexagonal cell shapes, a few honeycomb types can be supplied with rectangular cell shapes 

 2.2 Skin Materials: Skin considerations include the weight targets, possible abuses and local (denting) loads, corrosion or decorative constraints, and costs. Facing material thickness directly affects both the skin stress and panel deflection. Adhesive Materials: For honeycomb sandwich bonding, the following criteria are important: Fillet Forming: To achieve a good attachment to an open cell core such as honeycomb, the adhesive should flow sufficiently to form a fillet without running away from the skin to core joint. Bond Line Control: Every Endeavour should be made to ensure intimate contact between the parts during bonding, as the adhesive needs to fill any gaps between the bonding surfaces. Adhesives are often supplied supported by a carrier cloth, for the purpose of helping them to remain in place where the parts are squeezed particularly tightly together.

 3.SANDWICH PANEL LOADS

 3.1 Loads: Consider a cantilever beam with a load applied at the free end. The applied load creates a bending moment which is a maximum at the fixed end, and a shear force along the length of the beam. In a sandwich panel these forces create tension in the upper skin and compression in the lower skin. The core spaces the facing skins and transfers shear between them to make the composite panel work as a homogeneous structure. Cantilever beam with hexagonal core is shown in Fig 2.

 3.2 Deflections: The deflection of a sandwich panel is made up from bending and shear components. The bending deflection is dependant on the relative tensile and compressive module of the skin materials. The shear deflection is dependant on the shear modulus of the core. Total Deflection = Bending Deflection + Shear Deflection.