Table 17. Comparison of FRP breakaway coupler
them to yield at about 111.25 MPa (25,000 lbf),
material and steel (MMFG Co. 1994).
and the failure would have occurred with a frac-
ture through the entire cross section of the necked-
Carbon steel
Coupler material
down portion of the bolt. However, the FRP com-
Properties
(M1020)
(MMFG Co. 1994)
posite breakaway couplerbolt failure mode, as
Tensile strength (psi)
35,000
30,000
suspected, was completely different from the steel
Fiber direction
failure. In the case of the FRP female-threaded
90 to fiber direction
35,000
7,000
coupler, the failure was initiated by interfacial
Tensile modulus (psi 106)
30
2.6
shear bond failure, so that as the tensile load con-
Fiber direction
90 to fiber direction
tinued to be applied, a plug of material, roughly
30
1.0
Izod impact (ft-lb/in.)
equal to the cross sectional area of the turned-
Fiber direction
N/A
25
down section of the stud, was pulled out (see Fig.
90 to fiber direction
4
71 [left]). For the male-threaded coupler, the fail-
SI conversion factors: 1 ft = 0.305 m, 1 in. = 25.4 mm, 1 lbf = 4.45
ure occurred by thread shearing at the thread
N, 1 psi = 6.89 kPa.
roots (Fig. 71 [right]), when an approximately
31.7-mm (1.25-in.) length of threads at each end
was engaged to the loading column of the testing
machine. The tensile loads at which these cou-
plers failed are given in Table 18.
Discussion
The above experimental effort clearly shows
that the one-to-one replacement of steel break-
away couplers by FRP material cannot succeed.
Fiber-reinforced composites properties are
strongly direction oriented, and the failure mecha-
nism is controlled by the fiber architecture. In
order to have a clean transverse shear break in a
necked-down breakaway coupler made with
FRP composites, the fiber architecture has to be
designed and developed such that the failure can
be induced by interlaminar or interfacial bond
failure in the transverse direction to the bolt. At
the same time, sufficient longitudinal strength has
to be available to withstand the tensile load occur-
ring from bending under wind and ice loads.
Figure 71. Tension testing of the male- (left) and female-
Theoretically, this could be accomplished by a
threaded FRP breakaway coupler.
microstructural analysis of load sharing between
fibers and matrix, determining the optimum fiber
Tension tests of the
volume fraction and the optimum orientations of
breakaway couplers
the fibers. The composite breakaway coupler in
One each of male- and female-threaded break-
this case would possibly have to be precast rather
away couplers were tested in a testing machine
than machined from the commercially available
by applying a load to them. If these FRP couplers
bar stocks. Because of the limited scope of this
had been made of steel, we would have expected
study, no such attempts were made.
Table 18. FRP composite breakaway coupler test results.
FRP breakaway
Rate of loading
Tensile breaking load
coupler type
(mm/min, in./min)
(kN, lbf)
Mode of failure
Male-threaded type
2.54 (0.1)
24.373 (1800)
Thread shear
Female-threaded type
2.54 (0.1)
6.675 (1500)
Fiber shear bond failure
50
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