Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 6

The forces on the input and output faces are respectively:

ª º

F () t SE t H () H () t

¬ ¼

e i r

b

[1.4]

F () t SEt H ()

s t

b

Measures on the two opposite faces of the sample allow estimation of strain field
homogenity by comparing the forces on each face (section 1.3.2, Figure 1.12). We
note that for this test, the assumption of homogenity in mechanical fields is
hazardous. As a consequence, the notion of average strain velocity is also hazardous.
In section 1.3.2 we will see the best way to use the available measurements. Thus,
we should stress that the Hopkinson Bar leads to overall values of loads and
displacements on both sides of the sample. All mechanical quantities are obtained by
making additional assumptions completely separate from the test facilities. These
have been widely reported in the literature [NIC 80].

1.1.2.2.2. Limitations of the conventional system

Accurate analysis of wave transport
To carry out a precise virtual wave transport between the measured points and
the sample (forward transport for the incident wave and backward transport for the
others), the three-dimensional feature of the bars need to be considered, and the
dispersal correction must be introduced using a signal treatment technique. This
parameter corresponds to signal modification during transport. An accurate time
calibration (to within a micro-second) is also necessary [ZHA 96]; it is especially
important for measurement of small strains, and thus for brittle materials such as
concrete.

Multi-axial characteristics of the test
The uniaxial characteristic of the test is also an approximation. Let us examine
this aspect in the case of compression. Whenever the material presents a Poisson
effect, the longitudinal strain comes with a lateral strain (as is the case in statics if
the support conditions are well controlled), which is opposed by radial inertial
effects. This causes an induced confinement. The confinement explains the obvious
sensitivity of concrete to strain velocity that is universally observed in dynamic
compression (see section 1.3.2).

Measurement duration
The proportionality between the mechanical values associated with a wave inside
a bar, on which the Hopkinson bar technique is based [1.3]-[1.4], only applies to a