Download High Strain Mechanics and Its Importance in Crushing and Grinding

Proposed Henry Krumb Lecture Abstract:
High Strain Mechanics and Its Importance in Crushing and Grinding Technology
Abstract:
The legendary Greek Parthenon was
constructed between 447 and 438 BC. In 1687
a mortar bomb fired from a Venetian battery
besieging the Acropolis of Athens struck
gunpowder stored inside the Parthenon.
Damage to the Parthenon was extensive
reducing the structure to what we see today.
A 2012 study by Caltech, “Explosion at the
Parthenon: can we pick up the pieces?” utilized
high strain rate testing of the Parthenon’s
marble to understand how the marble
stonework fragmented due to the massive
explosion. Brittle materials behave differently when subjected to high strain rates as opposed to quasistatic strain rates. Quasi-static loading occurs when the stress pulse is slow enough for the system to
remain in internal equilibrium. That is, material fracture develops as the stress pulse moves through the
material. In high strain rate loading, however, the stress pulse moves through the material before
fracturing has time to develop. This generally results in higher strength, stiffness and smaller fragment
sizes. High strain testing, using a split Hopkinson pressure bar, was conducted on a series of carbonates
from three limestone quarries and four dolomite quarries. Quasi-static testing showed that the dolomites
have a slightly higher compressive strength than the limestones. High strain rate testing, however, showed
significant differences between the dolomite and limestone with the limestone having significantly higher
compressive strength and stiffness. It is hypothesized that the intercrystalline interface strength of
limestone, which forms as a primary precipitate, is significantly stronger than dolomite. Dolomites in turn
would have lower interface strength due to enlargement of the dolomite crystal during magnesium
replacement. Further, it is hypothesized that the post depositional changes creating dolomites would have
healed existing macro-defects causing slightly higher compressive strength than limestones during quasistatic loading. This suggests that quasi-static loading is controlled in part by the macro-defects since
fracturing would start first with macro-fractures as the load moves through the rock. High strain rate
testing, though, would be dominated by the rock’s intercrystalline interface strength and micro-defects.
This research has implications for utilizing high strain rate testing for developing a better understanding
of the energy requirements and fragmentation behavior in crushing and grinding operations as well as for
rock fragmentation using explosives, which can all be viewed as dynamic fracturing processes.
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