Stiffness-to-weight ratio

Fig. 25.7. The combination of properties which maximise the stiffness-to-weight ratio and the strength-to-weight ratio, for various loading geometries.

Compare the flexural stiffness to weight ratios for the following three plastic beams, (a) a solid beam of depth 12 nun, (b) a beam of foamed material 12 mm thick and (c) a composite beam consisting of an 8 mm thick foamed core sandwiched between two solid skin layers 2 mm thick. The ratio of densities of the solid and foamed material is 1.5. (hint consider unit width and unit length of beam). 

Composite materials are ideal for structural applications where high strength-to-weight and stiffness-to-weight ratios are required. Aircraft and spacecraft are typical weight-sensitive structures in which composite materials are cost-effective. When the full advantages of composite materials are utilized, both aircraft and spacecraft will be designed in a manner much different from the present. 

Foamed PDMS elastomers generally play an important role in a number of specific stress absorbing applications. Reports on Polystyrene and Polyurethane foams have tended to dominate the literature in recent years. Such systems generally exhibit high stiffness to weight ratios and some unique mechanical properties that are dependent upon the microstructure of the foam and the properties of the polymer making up the cell walls 10). In general, the desirable material property requirements of foams may be summarised as shown below ... 

ABS structural foam has excellent buoyancy, a very low (desirable) stiffness-to-weight ratio, good screw and staple pull-out strengths, and creep resistance superior to that of high-impact polystyrene (HIPS) and polyethylene foams. This is particularly important in load-bearing applications, such as pellets, tote boxes, fumitine, and parts buried under earth loads (6). 

There is a largely untapped opportunity for wood-plastic composites [10] in packaging and material handling. This is evident from the fact there are nearly 600 million pallets made each year in North America and only about 50 million are plastic. One Canadian manufacturer, Dura-Skid, produces pallets by assembling lineals made from wood-filled HDPE. The main advantage of wood-plastic composites to the pallet manufacturer is the excellent stiffness to weight ratio afforded by wood filler. The greatest opportunity may be in structural foam and twin-sheet thermoformed pallets. 

The Large Millimeter Telescope in the State of Puebla, Mexico designed by Simpson, Gumpertz Heger of Arlington MA, is a 50 m diameter, fully steerable radio telescope, which uses cfrp tube members to minimize deformations due to gravity and thermal loads. It also has low coefficient of thermal expansion and a stiffness to weight ratio that is 40% better than steel or Al. 

Structural foam parts have the combination of performance properties and process features that dehver increased stiffness to weight ratio. Hence they are suitable for metal replacement in some applications. 

Another significant market based on paper is that of honeycomb sandwich structures for aircraft interior floors and panels. The MPDI provides thermal stability during processing and, in the form of honeycombs, gives an exceptional stiffness to weight ratio. 

Fiber-reinforced composites have high stiffness-to-weight ratios, but this ratio may be increased if, instead of a monolithic structure, two thin composite facesheets of the same weight (for example, plastics reinforced with phenolic glass or carbon fibers), separated by a cellular core, are used. A phenolic resin-impregnated aramid paper honeycomb (Nomex honeycomb) is commonly used in the core structure. These structures are appropriate for applications in which weight reduction and fire safety are critical, such as in ground and air transportation. 

Because the advantages of advanced composite materials (FRPs) include light weight, high strength or stiffness-to-weight ratio, corrosion resistance and. 

The optics are made of beryllium because of its attractive physical properties specifically, good dimensional stability with time and after thermal cycling, and a high stiffness-to-weight ratio. The present capability for producing Be blanks limits the size of a finished primary mirror to about 1.6 m, which is the current upper limit for telescopes of this type. 

High strength- or stiffness-to-weight ratio) Tailorable properties can tailor strength or stiffness to be in the load direction Redundant load paths (fiber to fiber)... 

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