Weight savings

Explosion-welded constmction has equivalent or better properties than the more compHcated riveted systems. Peripheral benefits include weight savings and perfect electrical grounding. In addition to lower initial installation costs, the welded system requires tittle or no maintenance and, therefore minimizes life-cycle costs. Applications of stmctural transition joints include aluminum superstmctures that are welded to decks of naval vessels and commercial ships as illustrated in Figure 11. 

Cell Construction. Nickel—2iac batteries are housed ia molded plastic cell jars of styrene, SAN, or ABS material for maximum weight savings. Nickel electrodes can be of the siatered or pocket type, however, these types are not cost effective and several different types of plastic-bonded nickel electrodes (78—80) have been developed. 

The biggest potential weight saving, however, is in the body panels, which make up 60% of the weight of the vehicle. Here the choice is more difficult. Candidate materials are given in Table 27.3. 

Fig. 27.7. A 1994 Lotus Elan, with a GFRP body (but still mounted on a steel chassis - which does not give anything like the weight saving expected with an all-GFRP monocoque structure). (Reproduced with the kind permission of Group Lotus Ltd.)...

Retains all existing technology Weight saving only appreciable in designing against plastic flow... 

Despite the high cost of composites, the weight-saving they permit is so great that their use in trains, trucks and even cars is now extensive. But, as this chapter illustrates, the engineer needs to understand the material and the way it will be loaded in order to use composites effectively. 

Large-scale adhesive bonding of steel is of great interest to the automotive and appliance industries because of the opportunities it provides for design flexibility, weight savings, and manufacturing economy. Because of its economic importance. 

Example 2.8 A polypropylene sandwich moulding is 12 mm thick and consists of a foamed core sandwiched between solid skin layers 2 mm thick. A beam 12 mm wide is cut from the moulding and is subjected to a point load, IV, at mid-span when it is simply supported over a length of 200 mm. Estimate the depth of a solid beam of the same width which would have the same stiffness when loaded in the same way. Calculate also the weight saving by using the foam moulding. The density of the solid polypropylene is 909 kg/m and the density of the foamed core is 6(X) kg/m. ... 

A polypropylene pipe with an outside diameter of 80 mm is required to withstand a constant pressure of 0.5 MN/m for at least 3 years. If the density of the material is 909 kg/m and the maximum allowable strain is 1.5% estimate a suitable value for the wall thickness of the pipe. If a lower density grade of polypropylene (p = 905 kg/m ) was used under the same design conditions, would there be any weight saving per unit length of pipe ... 

Weight savings can also mean the difference between whether the structure we design can perform its mission or not. The current Space Shuttle payload is limited to 60,000 lb (27,200 kg). If we have an object that we wish to carry up into space that weighs 65,000 lb (29,500 kg), then we are out of luck. That object does not satisfy the Shuttle s weight limit. We must wait for a new-generation Space Shuttle, or sufficient weight in the object to be carried must be saved to fit within the current Space Shuttle limitations. 

The potential weight savings in a variety of structures are displayed in Figure 1-27, There, the savings range from a modest 25/lb ( 55/kg), barely justifying the use of some composite materials, to the enormous 15,000/lb ( 33,000/kg) in the Space Shuttle. In the case of the Space Shuttle, use of composite materials fairly shouts for attention. In between those two extremes, composite materials have very strong justification for use. 

Everyone is familiar, to some degree, with space activities. However, few are conversant with the role that various composite materials play in these activities. Weight savings are a crucial arena for space structures because of the enormous cost of boosting every structure from earth into space. Thus, composite materials are playing a compelling role in virtually all space structures, but not as much as they will in the future as more applications are developed. 

Next, we can, of course, make a material substitution. We can often substitute one specific graphite-epoxy for another member of the graphite-epoxy family. We can obviously substitute one metal for another metal. We can also substitute a composite material for a metal. All those approaches are taken in the interest of weight savings or cost savings, although the substitutions could also be made solely to achieve the required function of the structure. 

Steam and hot-water coils should be constructed of seamless steel tube and preferably be without joints within the tank. These coils can be either plain or finned tube. However, due to their greater surface area, finned tubes generally have a higher rate of heat transfer than plain tubes. On the basis of cost per unit surface area, finned tubes are less expensive than conventional tubes. There is also an advantage of weight saving, and complete coverage of the base area is not necessarily required to achieve specified temperatures. 

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