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Strength of
Materials Primer:
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Modulus is an expression of how “flexible” something is
(The units are in Kips which stands for “thousand pounds per square inch”).
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Strain is “Length as a result of stress” divided by
“Unstressed Length.” (Strain has no units, but is often designated as
inches/inch or feet/feet.)
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Stress is a measurement of the load applied (the units
are in “Kips”).
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The amount of
stress can be calculated by observation/measurements by the following
formula:
Stress = Strain x Modulus
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Compressive
Strength is how much load can be
applied before the rock starts to crumble (also measured in Kips).
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Tensile
Strength is the amount of tension
that can be applied without failure.
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Rocks have
lower tensile strength than compressive strength. Failures due to tensile strength result in
large gaps in the rock strata.
Determining
rock strength parameters requires very sophisticated tests, and even then,
the results are quite elusive. Due to
fracturing, larger samples provide weaker strength data than small samples of
the same rock. Fortunately, we can use
general rock “compressive strength” and “modulus” relationships to solve our
equations. Generally, the stronger the rock, the higher the modulus (the
stronger the rock the less flexible it is).
However, this ratio does vary significantly.
Key Point: The compressive strength of a
rock can range between 0.003 to 0.001 times the same rock’s modulus. In the best case, Stress = 0.003 x
Modulus. In other words, rocks can be
bent a very little bit before they break. The question is: is a syncline like
Sideling Hill possible after the rock solidifies without multiple cracks or
crumbling?
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