Stiffeners composite

Composite trailing edge wedges and stiffened panels are quite similar to their metal-bond counterparts and are used in many of the same applications (Fig. 40). Composite stiffened panels have an advantage over metallic bonded panels in that multiple core bays can easily be incorporated in one assembly (Fig. 41). 

Figure 7-25 Shapes of Metal versus Composite Stiffeners...

Further contrast between metal and composite stiffeners is revealed when we examine the objectives and characteristics of stiffener design. For a metal stiffener of uniform or even nonuniform thickness, we attempt to maximize the moment of inertia of the stiffener in order to maximize the bending stiffness of the stiffener. Those two factors are proportional to one another when we realize that the bending stiffness of metal stiffeners about the middle surface of the plate or shell to which they are attached is... 

In contrast, because of the spatially variable (inhomogeneous) nature of material in a composite stiffener, the bending stiffness cannot be separated into a material factor times a geometric term as in Equation (7.6). Instead, the composite stiffener bending stiffness is... 

Figure 7-26 Metal Versus Composite Stiffener Characteristics 7.4.2.2 Types of Stiffeners...

For composite stiffeners, all shapes are builtup from individual layers of material. Of course, some stiffener shapes can be produced by roll forming or pultrusion, for example, and then fastened to panels. Or, the stiffened panel could be made in a single operation involving the placement, usually by hand, of individual laminae of various dimensions in positions such that a builtup structure results. Stiffeners can be fastened to panels by bonding, stitching, or mechanical fastening. 

Standard shapes for composite stiffeners are not likely to occur for most aerospace applications. There, the value and function of the structure warrant optimizing the stiffener design. In contrast, for more everyday applications such as scaffolding, stairways, and walkways in chemical plants, competitive pressures lead to a situation where compromises in stiffener efficiency are readily accepted (overdesign) in order to achieve lower cost than would be associated with optimum design. 

Most of what has been described so far for stiffener design involves shape and size of the stiffener. Those issues involve selection of the type of stiffener, H-shaped cross section, blade, hat-shaped, etc. as well as the specific dimensions and material makeup of each stiffener element. Other obvious factors in the design of a stiffener include how far apart we space them, at what orientation we place them, and, perhaps most obviously in connection with what we addressed in Section 7.3, out of what material we make the elements. As you saw in some of the previous sketches for stiffeners, we are able with a composite stiffener to use different materials in different places very easily and to essentially optimize our materials usage so that the stiffening comes out to be as good as we can possibly make it. 

Unique and unusual stiffening concepts can make composite structures far more effective and efficient than metal structures. We have introduced some of these stiffening concepts, but more are being developed. We should look fonward to even more innovative unique-to-composites stiffening concepts in the future. 

The stiffeners themselves are designed next. A section of the baffle is assumed as acting with the stiffener and as contributing to the overall stiffness. This combined section is known as the composite stiffener. The composite section is checked for stress and deflection. Both vertical and horizontal stiffeners can be added as required. 

Summary of Results for Stress and Deflection in Composite Stiffeners for Sample Problem... 

Liu W, Butler R, Kim HA. Optimization of composite stiffened panels subject to compression and lateral pressure using a bi-level approach. J Struct Multidiscip Optim... 

Bisagni, C. Lanzi, L. 2002. Post-buckling optimisation of composite stiffened panels using neural networks. Composite Structures 58 237-247. 

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