Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 25

1.3.2. Tests implementing compression behavior
1.3.2.1. Hopkinson bar trial
As explained in section 1.1.2.2, the Hopkinson bar allows an accurate
measurement of the forces and displacements applied on a both faces of a sample,
especially in compression. Particular precautions pointed out give access to the weak
strain area in the case of concrete. Figure 1.11 shows an example of the forces
measured on each face, as well as the rates applied to each face of the sample
(Figure 1.12) and the associated displacements (Figure 1.13).
For this test, the specimen is initially 40 mm in both length and diameter. Its
relative density is 2.25 kg/m 3 , with a largest aggregate diameter of 8 mm. It is
loaded via an aluminum Hopkinson bar, 40 mm in diameter. The 1.3 m long
impactor is projected with a speed of 14.5 m/s. When observing the speeds to be
measured, we notice that the specimen absorbs little of the available energy, since
the loading bar speed is roughly equal to the initial speed of the impactor at the end
of the test, i.e. when the sample has failed. The induced displacements are very low,
as the displacement associated with the force peaks is below 1 mm. The post-peak
phase observed on the loads says a lot about the existence of inertial confinement.
Figure 1.11. Hopkinson bar compression test input and output loads