necked-down (notched) version of the couplers,
similar to the design currently made with steel.
Commercially available, unidirectional, pul-
truded FRP composites would be used as the
basic raw material from which the couplers would
be machined. Three major problems for making
the prototype for the tests were to be resolved: (1)
machining of the FRP, (2) an assessment of the
mode of failure and energy absorption of the
notched FRP under impact and the relationship
with temperature, and (3) the development of a
technique for cutting threads in the bar stock.
Notched bar impact test
In order to check if the pultruded FRP could
be machined, we attempted to turn down the
rough surface diameter of the FRP rebar stocks
from 19.05 mm (0.75 in.) to 12.7 mm (0.5 in.).
Except for dust production, which was handled
by a suitable vacuum extraction technique, no
major problem was encountered in machining the
Figure 66. Charpy impact testing machine.
diameter down when sharp diamond-tipped tools
were used (see Fig. 65). It was of concern whether
the unidirectional notched (or necked-down) FRP
The energy absorbed was recorded directly from
composite would, in fact, shear off clean at the
the Charpy testing machine dial recorder. Figure
root of the notch, like metals. Transverse cracks
67 illustrates the bars in three conditions. One
in composites are usually blunted by the fibers
before the notch was cut, the second with the
and such cracks propagate in the fiber direction.
notch, and the third after the test. As expected for
A preliminary test was therefore conducted
composites, the cracks at the notch root did not
using the 12.7-mm (0.5-in.) round FRP bars with
propagate through the section. On the other hand,
2.54-mm (0.1-in.) deep notches cut into them.
because of bending, longitudinal shear cracks
These notched bars were held in the anvil of a
developed at the free surface at the top of the bar
swing pendulum Charpy testing machine (Fig.
and propagated downward to the plane of the
66), and were impacted with the pendulum ham-
notch where the bar was clamped in the vise. Nev-
mer directly on the side of the notch, so that the
ertheless, the energy absorbed in the process over
crack developed at the root of the notch and
a range of low temperatures was recorded and is
propagated through the cross section of the bar.
illustrated in Figure 68. Energy absorption tended
Figure 65. Preparation of the notched bar impact test
Figure 67. Unnotched, notched, and notch-impact-
specimen from the FRP rebar sample.