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. 

Simply bonding a stiffener to a panel with adhesive is certainly a very feasible and natural procedure with typical composite structure construction. We have not discussed any procedure for joining parts except co-curing. Alternatively, to use film adhesive for bonding parts together, we simply cut a sheet or film of adhesive to the proper size, place it between the two parts that we wish to bond together, and then go through a cure cycle that causes the adhesive to adhere to both the stiffener and to the panel itself. We can also mechanically fasten any stiffener we like to a panel. 

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We usually must go beyond the simple concept of a monocoque or single-thickness skin for whatever structure we design. That is, we must usually consider the bending stiffness, and, to achieve structural efficiency, we often must stiffen a structure in some manner. We will first address the terminology of stiffening and how it is used. Then, we will consider the types of stiffeners that could be used. Next, an important issue that arises in the design of stiffeners is whether the stiffener has an open- or a closed-cross section. Then, we will address some of the... 

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

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. 

To determine the stresses in the base ring as a result of the tailing load, the designer must find the coefficients Kr and Kt based on angle a as shown and the type of stiffening in the skirt/base ring configuration. 

Another type of stiffener is shown in Figs. 8-7 and 8-8. In this case the basic sheet of the part is converted to a series of connected I or T beams. While this construction is not as efficient as the sandwich panel, it does have the advantage that it can be molded or extruded directly in the required configuration and the relative proportions of the legs and sheet can be designed to meet the flexural requirements. One of the limitations is that it imparts increased stiffness in one direction much more than in the other. 

Valve Problems. The primary solution to valve problems has been implantable replacement valves. The introduction of these devices necessitates open-heart surgery. There are two types of valves available tissue (porcine and bovine) and mechanical. The disadvantage of tissue valves is that these have a limited life of about seven years before they calcify, stiffen, and have to be replaced. The mechanical valves can last a lifetime, but require anticoagulant therapy. In some patients, anticoagulants may not be feasible or may be contraindicated. Of the valves which require replacement, 99% are mitral and aortic valves. The valves on the left side of the heart are under much greater pressure because the left ventricle is pumping blood out to the entire body, instead of only to the lungs. Occasionally, two valves are replaced in the same procedure. 

Determine the bounds on E for a dispersion-stiffened composite material of more than two constituents, i.e., more than one type of particle is dispersed in a matrix material. 

A hard-and-fast rule to be followed by all intending to use plastics is to design for plastics. As an example, for the same-size cross-section the strength of conventional plastics (not the high-performance reinforced types) is considerably less than that of most metals. The designer will thus find it necessary to increase thickness, introduce stiffening webs, and/or possibly use design inserts of various types of threads to secure the proposed product. The process will in some instances also require modification to the shape of the equipment used to produce the product. 

Another type of gel expands and contracts as its structure changes in response to electrical signals and is being investigated for use in artificial limbs that would respond and feel like real ones. One material being studied for use in artificial muscle contains a mixture of polymers, silicone oil (a polymer with a (O—Si—O—Si—) — backbone and hydrocarbon side chains), and salts. When exposed to an electric field, the molecules of the soft gel rearrange themselves so that the material contracts and stiffens. If struck, the stiffened material can break but, on softening, the gel is reformed. The transition between gel and solid state is therefore reversible. 

Another type of supramolecular interaction of DNA is the intercalation of fused aromatic compounds into the stacked base pairs in double-stranded DNA (see Figure 6). Intercalation induces not only dehydration from the polar groups in intercalator but also concomitant unwinding, lengthening, dehydration, and stiffening of the DNA double helix. 

Epoxy-PVC plastisols are a type of PVC plastisol adhesive that is used in large quantities in the automobile industry. It is used to bond sheet steel to inner stiffener panels and to seal around the crimped panel edges. These adhesives are formulated as high-solids, thixotropic pastes and are applied as discrete dots or droplets to the stiffener surface or panel edge before joining or crimping. These adhesives are called Hershey drops in the trade because of the characteristic geometry of the droplets. 

Other studies have reported frequency increases for polymer-coated SAW devices upon exposure to water vapor at elevated temperatures (<80°C), consistent with a predominance of elastic stiffening of the coating [57]. At lower temperatures (25-35 C), the same sensors exhibited frequency decreases upon exposure to water vapor of the same concentration, a response consistent with mass loading. Thus, the dominant mechanism may be determined by a variety of factors, including the type of vapor-coating interactions and the ambient temperature. Note that ambient temperature can have profound effects on flim viscoelastic properties (see Chapter 4), which in turn can influence the results of vapor exposure. 

A mixed monolayer consisting of stearic acid (9.9%), palmitic acid (36.8%), myristic acid (3.8%), oleic acid (33.1%), linoleic acid (12.5%), and palmitoleic acid (3.6%) produces an expanded area/pressure isotherm on which Azone has no apparent effect in terms of either expansion or compressibility (Schuckler and Lee, 1991). Squeeze-out of Azone from such films was not reported, but the surface pressures measured were not high enough for this to occur. The addition of cholesterol (to produce a 50 50 mixture) to this type of fatty acid monolayer results in a reduction of compressibility. However, the addition of ceramide has a much smaller condensing effect on the combined fatty acids (ratio 55 45), and the combination of all three components (free fatty acids/cholesterol/ceramide, 31 31 38) produces a liquid condensed film of moderate compressibility. The condensed nature of this film therefore results primarily from the presence of the membrane-stiffening cholesterol. In the presence of only small quantities of Azone (X = 0.025), the mixed film becomes liquid expanded in nature, and there is also evidence of Azone squeeze-out at approximately 32 mN m. ... 

Hand building finishes that retain their stiffening and fullness effects after repeated launderings are considered to be durable. These products are usually aqueous emulsions of polymers that form water-insoluble films on the fibre surface when dried. The three main types of products are vinyl acetate-containing polymers, acrylic copolymers and thermosetting polymers. 

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