Design sandwich structure

The solution that has been adopted by makers of composite soundboards is to fabricate a sandwich structure where a layer of high-quality cardboard is glued between two identical layers of CFRP (Fig. 28.22). The philosophy of this design modification is to replace some CFRP by a much lighter material in those regions that contribute least to the overall stiffness of the section. 

The stiffness ratios (i.e. stiffness of the foam sandwich beam relative to the original solid beam) are also given in Fig. 2.21. In both cases the values given are independent of the original solid material or its dimensions, so this provides a good design chart. The design of solid/foam sandwich structures is also considered in Chapter 3 in the laminate analysis. 

There are different techniques that have been used for over a century to increase the modulus of elasticity of plastics. Orientation or the use of fillers and/or reinforcements such as RPs can modify the plastic. There is also the popular and extensively used approach of using geometrical design shapes that makes the best use of materials to improve stiffness even though it has a low modulus. Structural shapes that are applicable to all materials include shells, sandwich structures, and folded plate structures (Fig. 3-8). These widely used shapes employed include other shapes such as dimple sheet surfaces. They improve the flexural stiffness in one or more directions. 

Solid plastic wall thicknesses for most materials should be targeted to be below 0.2 in. and preferably around 0.125 in. in the interest of avoiding the above pitfalls. In most cases ribbing will provide a satisfactory solution in other cases sandwich structures or reinforced materials may have to be considered. As reviewed elsewhere when presenting the ideal target to meet the best design such as the thinner wall just reviewed, does not mean that a thicker wall can not be processed, etc. The thicker wall can be processed requiring closer process controls (Chapter 8). 

Allen, H. G. 1969. Analysis and design of structural sandwich panels. Pergamon Press, Tarrytown, NY. 

All these considerations are purely theoretical and cannot be used for designing, but they give an idea of the broad possibilities of sandwich structures that are also illustrated by Figure 6.22, showing some examples of sandwich flexural moduli. 

Flat-plate PV collectors contain an array of individual cells connected in a series/parallel circuit and encapsulated within a sandwich structure, the front of which is glass or plastic. Unlike thermal collectors, the back of these collectors is not insulated, because for best performance, they need to be cooled by the atmosphere. If this energy could be used and thereby this loss could be eliminated in new designs, the conversion efficiency could be much improved. 

When thicker wall thickness can be used low density foam core products are produced such as in pipe and paneling. This multilayer sandwich structure provides reduced material costs without sacrificing performance, in fact properties can be improved by using the sandwich design advantages.1... 

Operating experience with the sandwich structure in trickle-bed mode is as yet limited, but its behavior is sufficiently similar to that when employed as a catalytic distillation packing, in terms of liquid holdup/residence time, mass transfer, and pressure drop, to allow design of pilot plant for these parameters to be made with confidence. 

A sandwich structure is composed of two skins and a core material. The same or different materials are combined in the form of sandwich construction (Figure 7.46). They can be used in products with an irregular distribution of the different materials, and in the form of large structures or sub-structures. Overall load-carrying capabilities depend on average local sandwich properties, but materials failure criteria depend on local detailed stress and strain distributions. Design analysis procedures for sandwich materials composed of linear elastic constituents are well developed. In principle, sandwich materials can be analyzed as RP structures, but incorporation of viscoelastic properties will be subject to the limitations discussed throughout this book. 

A special module design that is a hybrid between a plate and a spiral-wound module has been developed by the research institute GKSS in Germany. Here, two membrane sheets are welded together (by heat or ultrasonic welding) to a sandwich structure with a permeate spacer between the two membranes. A multitude of these sandwiches, each with a central hole, is arranged on a central perforated tube that removes the permeate. Each membrane sandwich is sealed from the feed to the permeate side at the central perforated permeate... 

P(2) In sandwich structures the properties of the core surrounding the insert have a major effect on the total load-bearing potential of the bonded insert Joint. Therefore the designer shall verify that the core material is capable of carrying the loads transferred from the insert. 

Bonded inserts are also commonly used in sandwich structures, most typically in panels. The insert is bonded into the core and to one or both skins of the sandwich. Loads are then directed to the joint through the mechanical fasteners (i.e. the insert) in their axial direction, causing out-of-plane loading of the sandwich component. Sandwich structures typically have thin skins and therefore the loads from the insert are mainly transferred to the core. If the insert is loaded in the in-plane direction of the sandwich member, the connection is categorised as an embedded insert and it should be designed as a mechanical connection. 

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