Dynamic Behavior of Concrete: Experimental Aspects - Dynamic Behavior of Concrete and Seismic Engineering - page 23

1.3.1.2. Hopkinson bar Brazilian test
The test is an expansion of the Brazilian test, whose traditional analysis is based
on the assumption of brittle elastic behavior. We consider an elastic cylinder
compressed perpendicularly to its generators: compression is applied along two
diametric generators. A plane deformation elastic calculation shows that loading
causes practically constant traction maximum stress along the cylinder axle, at right
angles to the compression axle. We assume cylinder failure takes place when the
strain reaches the ultimate value. Carrying out this test in quasi-statics is not
obvious, as it requires strict respect for limit conditions and the ideal elastic model
(stiff supports among others). Nonetheless, this trial is easy to carry out and gives a
consistent order of magnitude for simple traction failure stress.
Extension to the dynamic situation is easy. Compression is applied using a
Hopkinson bar. If we want to analyze the results in the standard way, we suppose
that the situation is not too far from the quasi-static case. To do this, we have to
assume that inertial effects can be neglected. They can be neglected before failure
but, as is the case for simple compression, they cause an apparent increase in the
maximum load after failure, so consequently it is important to detect failure by
direct observation (using high-speed imaging), as it is for dynamic compression tests
where localization of strains with block development does not necessarily lead to
load drop immediately. We should also check that the mechanical fields are not too
far away from the fields we would have in statics at the same applied force value.
Thus, we have to verify that failure occurs at a time when input and output forces
are quasi-equal. Such a situation will only happen when loading is slower than the
homogenizing time (typically the time for the elastic waves to cover the diameter of
the sample several times).
Achieving all these conditions simultaneously is difficult, but as we saw in
section 1.1.2.2, the Hopkinson bar provides us with information about the loads and
displacements applied to the sample all the time. Assuming this data is accurate, we
can then carry out a numeric simulation of the test (assuming brittle elastic
behavior), which gives a more precise assessment of failure strength [TED 93].
However, this hybrid approach (calculation-test association) is that of a structure
trial, and is better suited to model validation than directly determining a behavior
parameter.
1.3.1.3. Scabbing test
The scabbing test is a test with a fundamentally transient analysis. Actually it is
based on analyzing wave propagation inside a bar made of the material itself.
Concrete, a brittle material has a uniaxial compression strength that is clearly
superior to its traction strength.