In the failure is different for both the outer

In order to obtain the shear strength for the
different bolts that will be used in this study, nine single-bolted double-lap
shear connections are prepared and tested under axial tensile load. The bolts
types include stainless steel bolts, BFRP bolts and hybrid steel-FRP (HSFRP)
bolts with 12 mm diameter. The BFRP bolts were fabricated by using pultruded
BFRP bars which were cut to small pieces and then the bolts shank and the
threaded part were sculpture. For the fabrication of the HSFRP bolts, a small
diameter stainless steel bar was used as a core during the pultrusion process
of the BFRP bars (i.e., inner core made of stainless steel and surrounded by
BFRP). For that purpose, a 6 mm diameter stainless steel bars was used and
surrounded by BFRP till reaching a 12 mm outer diameter. Fig. 2-11 shows the
three types of the bolts.

The specimens were prepared by drilling a single
12.6mm hole in the plate as shown in Fig. 2-12a. The specimens were tested
under a displacement-controlled loading technique with a loading rate of 0.5
mm/min. The
main of this section is to describe the behaviour of the concerned bolts (i.e.,
SS, BFRP, and HSFRP) in single-bolted double-lap shear connections under the
effect of axial tensile loads. The failure modes and the load-displacement
curve is summarized as follow:

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Failure Modes

Fig. 2-13 depicts the failure mode of the three
bolts. As shown in Fig. 2-13a, stainless steel bolts exhibited the familiar
failure for bolts under shear loads. Where the failure occurred in two failure
planes due to the use of double-lap connections. Unlike the stainless-steel
bolts, the failure of the BFRP bolts occurred at one plane see Fig. 2-13b. In
contrast to the SS and BFRP bolts, the HSFRP bolts exhibit different failure
mode. From Fig. 2-13c it can be seen that the failure is different for both the
outer BFRP and the inner SS where the BFRP part failed and separated into two
parts. However, the SS part shows no separation where a bending of the inner
bar was occurred. Additionally, with the increase displacement, slipping
between the two parts was noticed which may indicate the existence of some bond
problems in the fabrication of the HSFRP bolts.  Load-Displacement

Fig. 2-14 shows the recorded load and
the corresponding displacement for the nine specimens. For the specimens with
the SS bolts, the load-displacement curve consists of two parts. The first was
linear with high initial stiffness till reaching approximately 65 kN followed
by nonlinear behavior till reaching the failure point. After the failure point,
the SS bolts lose all of its shear strength due to the two-planes failure. Fig.
2-14b shows that the BFRP bolts exhbited approximately linear behavior till
reaching its ultimate load. It can be notice, that after the failure in
specimen BFRP-1, the specimen did not completely lose its shear capacity and
this behavior can be attributed to the fact that the failure occurred on only
one plane. On the other hand, the HSFRP bolts showed the similar trend like the
BFRP bolts till reaching the ultimate load. However, the remarkable difference
can be noticed beyond this point where unlike the pure SS and BFRP bolts, the
HSFRP bolts did not completely loss their shear strength. As shown in Fig.
2-14c, after reaching the failure point, a sudden drop in the load occurred
which corresponds to the failure of the outer part ( the BFRP part). Then, a
stability plateau in the load-displacement curve can be noticed. The plateau
corresponds to the bending behavior of the inner part ( the stainless steel
core). Based on the recorded load
measurements during the tests, the shear strength of the bolts was calculated
using the following equations:





 is the failure load for the bolt,

 is the average failure load for the three
specimens, A is the cross section area of the bolts,

 is the single shear strength for one bolt. And
the value 2 is used to convert from douple shear strength to single shear
stregth of the bolts.

Table 2-3 summerizes the results of the shear
strength tests. The average shear strength for the SS bolts is 407.5 MPa which
is 3.5 times that of the BFRP bolts (i.e., 116.2 MPa). Using inner stainless
steel core for the HSFRP bolts results in increasing the shear strength by
almost 40.7% compared with that of the pure BFRP.