Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 21

Figure 1.8. Direct traction tests on Delft University's Hopkinson bar,
plain concrete and very high performance concrete
The simultaneous measurement of the strains on the specimen (Figure 1.8) is
made possible either by extensometers gages glued to the sample [TOU 95a] or by
pre-slotted fiber concrete (where the measurements concerns crack opening), by
gages fixed directly on the sample [TOU 99b]. For the speeds considered, the time
delay between stress and strain signals is about 220 P s, whilst the space difference is
about 1 meter. The “suitable” loading time (from 0 to maximum load) ranges from
100 to 500 P s, and sampling is carried out at 250 kHz. The excellent stress-strain
linearity obtained confirms the validity of the hypotheses. Nevertheless, considering
the time to go through the specimen (about 25 P s, i.e. a difference about 1 MPa), the
rates reached limit the interpretation as far as sample homogenity is concerned.