Conceptual design

CHAPTER 7: FRP COMPOSITES FOR CRUSHABLE CUSHIONS

Figure 75. Details of the leaf spring and its T-groove on he inner facing

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Table 20. Mechanical properties of several foam materials (Marshall 1982).

Foam material

Cellulois acetate rigid

Epoxy closed cell rigid

Phenolics foam-in-place

HD polypropylene foam

Polyurethane rigid c. cell

Polyvinyl chloride rigid

* Acrylonitrile-butadiene-styrene

SI conversion factors: 1 lb = 0.454 kg, 1 psi = 6.89 kPa, 1C = 0.56(F32)

tion would be made. Since the facing material will

(2,000 lb) and an impact speed (v) of 48.3 km/hr

have to wrap around the luminaries supports or

(30 mi/hr) or 13.42 m/s (44 ft/s), we have

the trees, they must be deformable for fabrica-

U (kinetic energy) = (mv2)/(2g) =

Normally, the primary function of the core ma-

81.528 kJ (60,124 ft-lbf).

terial is to stabilize the facings and transfer the

shear loads, but at crush impact, the core mate-

Therefore, 81.528 kJ (60,124 ft-lbf) of energy

rial has to respond to most of the crushing load.

would have to be dissipated in the impact by

Thus, the selection and design of the core mate-

crush deformation of the plastic crash cushion.

rial must be based on energy management dur-

The core must possess the sacrificial and energy

ing vehicular collision with the cushion and the

dissipating mechanisms for this purpose.

fixed object. For a vehicle weight (m) of 908 kg

The use of foam as a structural, as well as a

controlled energy dissipating material, has been

and is now extensive. Because of the single-step

injection molding process, foam is lower in cost

too. It also has the potential to form the CPC with

the two facing plates in a single operation by care-

ful adjustment of the curing reaction and the heat-

sink effect of the mold. This process forms a part

in which the facings are simply the higher den-

sity form of the foam and are common in many

applications in the automotive industries. The

mechanical properties of several candidate foam

materials for the CPC are listed in Table 20.

Conceptual design

A cutout view of the conceptual CPC is illus-

trated in Figure 72. The CPC would be mounted

on a pole or a tree about 203.2 mm (8 in.) above

the ground with the functional height extending

to about 812.8 mm (32 in.) above the ground. This

would give a crush length of about 610 mm (24

in.). Including the cap cushion, the overall length

of the CPC would be approximately 1.02 m (40

in.), as shown in Figure 73. It has been designed

to produce a potentially lightweight, compact, and

Figure 72. Cutout view of the conceptual crush-

radially symmetrical configuration that can be

able plastic cushion.

easily constructed. The body will consist of two

Western Governors University

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