CHAPTER 7: FRP COMPOSITES FOR CRUSHABLE CUSHIONS
interior facing, and the other for the exterior fac-
Background
Every year, side impact collisions with road-
ing, (2) a thick, lightweight, crushable core to sepa-
side fixed objects involve approximately 225,000
rate the two facings and carry crushing loads from
people, of which 1 in 3 is injured and 1 in 100 is
one facing to the other, and (3) an attachment or
killed. This level of injury represents a societal cost
bonding of the facings to the core so that not only
of more than billion (Troxel et al. 1991). Trees,
the direct normal loads, but also the shear loads,
utility poles, and light supports form the majority
are transmitted to the core. Thus the material sys-
of the fixed objects (77%) in these accidents.
tem that would be suitable for this design could
The objective of this study was to explore the
be a structural "sandwich." A sandwich construc-
feasibility of making a crushable plastic cushion
tion would also be lightweight, but stiff, strong,
(CPC) that could be used on wooden utility poles
and durable. A fourth requirement would be an
to reduce the severity of side impact collisions of
arrangement of deformable spring inserts to the
vehicles. It was conceived that the plastic cush-
interior facing, which would essentially clamp the
ions could be designed such that they could
crash cushion to the pole or tree trunk under an
accommodate shrinkage of the wooden utility
active force.
pole. Possible future applications on trees could
The primary functions of the facing material
also accommodate some increase in the diameter
would be to (1) provide the bending and in-plane
of the tree due to growth. Thus, the plastic cush-
shear stiffness, and (2) carry the axial, bending,
ions would not be attached directly to the wooden
and in-plane shear loading. The facing material
utility pole with nails, staples, or screws. Other
must also be impact resistant and tough. It must
desirable features would be that the cushion's cov-
have a mode of fracture that would not let the
ering should be made of a reflective material or
cracks to propagate easily throughout the fac-
glass-beads coating to provide delineation. It must
ing, and finally, it must be weather resistant and
also be vandal-resistant, and its top cover should
durable. A number of materials initially consid-
be such that rainwater or melted snow water could
ered as facing materials were investigated. Their
easily drip down.
mechanical properties are briefly summarized in
Table 19. The list includes a few materials, e.g.,
plywood, which could hardly be considered for
Materials and design approach
A design of the crushable plastic cushion (CPC)
the crushable cushion surface, but they were
would basically have three essential elements: (1)
included for the convenience of comparison with
a pair of stiff plastic composite skins, one for the
the composites and metals from which the selec
Table 19. Mechanical properties of a number of facing materials for the
crushable plastic (Marshall 1982).
Yield strength
Modulus of
Weight of 1-mil
thickness (lb/ft2)
Facing material
(psi)
elasticity (psi)
Comments
Composites
0.92 106
Glass/polyethylene
14,000
0.0070
3.50 106
Glass/epoxy
62,000
0.0088
3.50 106
Glass/phenolic
48,000
0.0094
High temp resist.
3.50 106
Glass/polyester
48,000
0.0100
3.50 106
Glass/polyimide
60,000
0.0095
High temp resist.
10.0 106
Graphite/epoxy
80,000
0.0080
Woven graphite
4.4 106
Kevlar/epoxy
60,000
0.0070
Very tough
Metals
10.0 106
Aluminum 7075-T6
66,000
0.014
High strength
30.0 106
Mild steel
50,000
0.040
Low cost, heavy
Stainless steel 316
60,000
1.80 106
Plywood (Douglas fir)
2,650
0.003
1.80 106
Southern pine
2,650
0.003
SI conversion factors: 1 psi = 6.89 kPa, 1 lb = 0.454 kg, 1 ft = 0.305 m.
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