Engineers at MIT have studied the simple act of shaving up close, observing how a razor blade can be damaged as it cuts human hair — a material that is 50 times softer than the blade itself.
By Jennifer Chu, MIT News Office
August 6, 2020 -- Razors,
scalpels, and knives are commonly made from stainless steel, honed to a
razor-sharp edge and coated with even harder materials such as diamond-like
carbon. However, knives require regular sharpening, while razors are routinely
replaced after cutting materials far softer than the blades themselves.
Now engineers at MIT have studied the
simple act of shaving up close, observing how a razor blade can be damaged as
it cuts human hair — a material that is 50 times softer than the blade itself.
They found that hair shaving deforms a blade in a way that is more complex than
simply wearing down the edge over time. In fact, a single strand of hair can
cause the edge of a blade to chip under specific conditions. Once an initial
crack forms, the blade is vulnerable to further chipping. As more cracks
accumulate around the initial chip, the razor’s edge can quickly dull.
The blade’s microscopic structure plays
a key role, the team found. The blade is more prone to chipping if the
microstructure of the steel is not uniform. The blade’s approaching angle to a
strand of hair and the presence of defects in the steel’s microscopic structure
also play a role in initiating cracks.
The team’s findings may also offer clues
on how to preserve a blade’s sharpness. For instance, in slicing vegetables, a
chef might consider cutting straight down, rather than at an angle. And in
designing longer-lasting, more chip-resistant blades, manufacturers might
consider making knives from more homogenous materials.
“Our main goal was to understand a
problem that more or less everyone is aware of: why blades become useless when
they interact with much softer material,” says C. Cem Tasan, the Thomas B. King
Associate Professor of Metallurgy at MIT. “We found the main ingredients of
failure, which enabled us to determine a new processing path to make blades
that can last longer.”
Tasan and his colleagues have published
their results today in the journal Science. His co-authors are
Gianluca Roscioli, lead author and MIT graduate student, and Seyedeh Mohadeseh
Taheri Mousavi, MIT postdoc.
A metallurgy mystery
Tasan’s group in MIT’s Department of
Materials Science and Engineering explores the microstructure of metals in
order to design new materials with exceptional damage-resistance.
“We are metallurgists and want to learn
what governs the deformation of metals, so that we can make better metals,”
Tasan says. “In this case, it was intriguing that, if you cut something very
soft, like human hair, with something very hard, like steel, the hard material
would fail.”
To identify the mechanisms by which
razor blades fail when shaving human hair, Roscioli first carried out some
preliminary experiments, using disposable razors to shave his own facial hair.
After every shave, he took images of the razor’s edge with a scanning electron
microscope (SEM) to track how the blade wore down over time.
Surprisingly, the experiments revealed
very little wear, or rounding out of the sharp edge over time. Instead, he
noticed chips forming along certain regions of the razor’s edge.
“This created another mystery: We saw
chipping, but didn’t see chipping everywhere, only in certain locations,” Tasan
says. “And we wanted to understand, under what conditions does this chipping
take place, and what are the ingredients of failure?”
A chip off the new blade
To answer this question, Roscioli built
a small, micromechanical apparatus to carry out more controlled shaving
experiments. The apparatus consists of a movable stage, with two clamps on
either side, one to hold a razor blade and the other to anchor strands of hair.
He used blades from commercial razors, which he set at various angles and
cutting depths to mimic the act of shaving.
The apparatus is designed to fit inside
a scanning electron microscope, where Roscioli was able to take high-resolution
images of both the hair and the blade as he carried out multiple cutting
experiments. He used his own hair, as well as hair sampled from several of his
labmates, overall representing a wide range of hair diameters.
Regardless of a hair’s thickness,
Roscioli observed the same mechanism by which hair damaged a blade. Just as in
his initial shaving experiments, Roscioli found that hair caused the blade’s
edge to chip, but only in certain spots.
When he analyzed the SEM images and
movies taken during the cutting experiments, he found that chips did not occur
when the hair was cut perpendicular to the blade. When the hair was free to
bend, however, chips were more likely to occur. These chips most commonly
formed in places where the blade edge met the sides of the hair strands.
To see what conditions were likely
causing these chips to form, the team ran computational simulations in which
they modeled a steel blade cutting through a single hair. As they simulated
each hair shave, they altered certain conditions, such as the cutting angle,
the direction of the force applied in cutting, and most importantly, the
composition of the blade’s steel.
They found that the simulations
predicted failure under three conditions: when the blade approached the hair at
an angle, when the blade’s steel was heterogenous in composition, and when the
edge of a hair strand met the blade at a weak point in its heterogenous
structure.
Tasan says these conditions illustrate a
mechanism known as stress intensification, in which the effect of a stress
applied to a material is intensified if the material’s structure has
microcracks. Once an initial microcrack forms, the material’s heterogeneous
structure enabled these cracks to easily grow to chips.
“Our simulations explain how
heterogeneity in a material can increase the stress on that material, so that a
crack can grow, even though the stress is imposed by a soft material like
hair,” Tasan says.
The researchers have filed a provisional
patent on a process to manipulate steel into a more homogenous form, in order
to make longer-lasting, more chip-resistant blades.
“The basic idea is to reduce this
heterogeneity, while we keep the high hardness,” Roscioli says. “We’ve learned
how to make better blades, and now we want to do it.”
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