8.3 Test Results and Discussions
247
height, and the impact force versus time curves can be generally divided into three
stages (i.e. the peak value stage, the plateau stage and the unloading stage). Firstly,
the impact force increases sharply to a peak value when the hammer is in contact
with the specimen, and then the specimen accelerates from a zero velocity to a
speed approaching that of the drop hammer. This impact force could induce the
drastic vibrations of the drop hammer, leading to the fast change of the contact
area between the specimen and the drop hammer, reflected as fluctuations in the
impact force history. The other reason for the fluctuations in the impact force is the
sequential crushing and cracking of core UHPCC. Subsequently, the specimen and
the drop hammer move downwards together and remain in contact, which reflects
the actual structural capacity under the lateral impact. The average value of the
impact force keeps almost constant for a relatively long period, which is regarded
as the “plateau” stage. Finally, when the specimen reaches its maximum mid-span
deflection, the specimen and the drop hammer start to rebound upwards, accompanied
by an unloading on the specimen. The curve goes into the descending stage and
the impact force gradually decreases to zero due to the complete separation of the
specimen and the drop hammer.
8.3.4 Deflection-Time History
The deflection-time histories of specimens under three release heights are shown in
Fig. 8.10. It can be seen that all curves show the following similar trends. The deflection increases rapidly after the impact, and the gradient of the curve decreases continuously with the impact energy is dissipated. The curve attains the maximum value
when the velocity of the specimen declines to zero. Then, the specimen rebounds due
to the release of elastic energy which leads to the deflection undergoes a descending
stage. Finally, the specimens oscillate at a new equilibrium position and approach to
a certain value.
Table 8.5 lists the experimental maximum and residual deflections of specimens,
where E K represents the impact energy and can be defined as:
E K = mgH
(8.2)
where m and H are the mass and release height of drop hammer, respectively; g is
the gravity acceleration and equal to 9.8 kg·m/s
2 . “—” denotes that the data is not
collected due to the accidental damage of LVDTs. As can be seen, both the maximum
and residual deflections increase obviously with the increase of release heights.
247
height, and the impact force versus time curves can be generally divided into three
stages (i.e. the peak value stage, the plateau stage and the unloading stage). Firstly,
the impact force increases sharply to a peak value when the hammer is in contact
with the specimen, and then the specimen accelerates from a zero velocity to a
speed approaching that of the drop hammer. This impact force could induce the
drastic vibrations of the drop hammer, leading to the fast change of the contact
area between the specimen and the drop hammer, reflected as fluctuations in the
impact force history. The other reason for the fluctuations in the impact force is the
sequential crushing and cracking of core UHPCC. Subsequently, the specimen and
the drop hammer move downwards together and remain in contact, which reflects
the actual structural capacity under the lateral impact. The average value of the
impact force keeps almost constant for a relatively long period, which is regarded
as the “plateau” stage. Finally, when the specimen reaches its maximum mid-span
deflection, the specimen and the drop hammer start to rebound upwards, accompanied
by an unloading on the specimen. The curve goes into the descending stage and
the impact force gradually decreases to zero due to the complete separation of the
specimen and the drop hammer.
8.3.4 Deflection-Time History
The deflection-time histories of specimens under three release heights are shown in
Fig. 8.10. It can be seen that all curves show the following similar trends. The deflection increases rapidly after the impact, and the gradient of the curve decreases continuously with the impact energy is dissipated. The curve attains the maximum value
when the velocity of the specimen declines to zero. Then, the specimen rebounds due
to the release of elastic energy which leads to the deflection undergoes a descending
stage. Finally, the specimens oscillate at a new equilibrium position and approach to
a certain value.
Table 8.5 lists the experimental maximum and residual deflections of specimens,
where E K represents the impact energy and can be defined as:
E K = mgH
(8.2)
where m and H are the mass and release height of drop hammer, respectively; g is
the gravity acceleration and equal to 9.8 kg·m/s
2 . “—” denotes that the data is not
collected due to the accidental damage of LVDTs. As can be seen, both the maximum
and residual deflections increase obviously with the increase of release heights.