This is a neat bit of engineering
research. We get a scratch testing
protocol that determines crack resistance in the field using simple tools
usually used for testing hardness and strength.
It may get so an experienced eye is able to determine crack resistance
on a face of ore. That can be mighty
useful as that determines the amount of blast energy needed.
A little bit like tasting the
material.
This is a surprising new tool
that engineers will be happy to have.
Researchers butter up
the old scratch test to make it tough
by Denise Brehm
It might not seem like scraping the top of a cold stick of butter with
a knife
could be a scientific test, but engineers at MIT say the process is very
similar to the "scratch test," which is perhaps the oldest known way
to assess a material's hardness and strength - or, in scientific language, its
resistance to deformation.
Using the scraping of butter as a starting point, the engineers
launched a study to see if the age-old scratch test could be used to determine
a material's toughness, or how well it resists fracturing after a small crack
has already formed. The answer: The scratch test is indeed measuring crack
resistance rather than strength and is valid on material samples of any size.
This means that engineers now have a simple "new" test for
assessing a material's fracture properties.
"Fracture mechanics has not reached the same level of
pervasiveness in most engineering practice as strength theories, and this is
due to the fact that it is difficult to determine fracture properties of
materials, from soft clay to hard concrete," says Franz-Josef Ulm, the
George Macomber Professor of Civil and Environmental Engineering (CEE) at MIT.
"The test which we propose here is just this: a straightforward test for the
engineering practice."
In a paper in Physical Review Letters that appeared online May 20,
co-authors Ulm; Pedro Reis, the Esther and Harold E. Edgerton Assistant
Professor of Civil and Environmental Engineering and Mechanical Engineering;
and CEE graduate student Ange-Therese Akono - who is first author on the paper
- describe their research and findings.
They performed laboratory scratch tests on paraffin wax, which is
similar to butter but more stable at room temperature, Reis says, and used
theory and mathematics to pare the process down to its essential components.
They then created a mathematical model of the entire physical
"scratch" process, which shows that the area of contact between
the scratching implement and the test material is of primary importance in
determining whether the scratch test is assessing strength or toughness.
They knew that when measuring a material's strength, the force required
to make a scratch would always increase at the same rate as the contact area
(width times depth) of the scratching tool.
But when measuring a material's toughness, the mechanics are
complicated by the energy released when chemical bonds break as the new
surfaces are created and a fracture grows. Because of this, the force does not
increase at the same rate as the area of contact.
Instead, the force exhibits a distinct scaling reminiscent of a
fracture process - that is, a wider cut requires more force than a deeper one.
(Specifically, the force increases at the same rate as the width times the
square root of the depth.)
Back in the lab, the engineers changed the dimensions of the test to
see if a wider scratching implement would require more force than a narrow one.
It did. And that seemingly minor change in one dimension gave them their
answer: The scratch test is assessing a material's fracture toughness, not
its hardness nor strength properties.
It assesses the hardness and strength only in cases where the area
of contact between the scratching implement and the material is so small that a
true indentation is made rather than a scratch. Now, knowing the width and
depth of the scratch and the horizontal force, researchers can now determine
the fracture toughness of a material.
"The advantage of a scratch test is that it works on both soft and
hard materials and on very small samples," Akono says. "This method
enables us to isolate brittle-crack propagation and neglect plastic
deformation."
They confirmed their findings with additional tests on cement paste,
limestone and steel.
"You might think that fracture, or how things break, is an old
field of study," Reis says. "But it's relatively new compared to the
tests of a material's hardness. Now, using the very old method of the scratch
test, we have a relatively simple new means for measuring a material's
toughness."
"The scalability of scratching for different probes and depths
will open new venues for the miniaturization of the technique, which will help
us understand fracture properties of materials at very small scales," Ulm says. "We also
know - finally - that it takes less effort to make a narrow, deep cut in cold
butter than a wide one. And that is science we can use at the dinner table."
1 comment:
wow very nice blog you have shared.
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