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Dull Blades

September 21, 2020

Most boys are excited when they need to first start shaving their beard. The excitement soon fades when shaving becomes just another monotonous morning task, like tooth brushing. Likewise, scientists are excited when gathering data from their first experiment, only to find that their profession requires much more. There's the responsibility of ensure that your statistics are good, that your results are consistently reproduced, that they're adequately explained by theory, and the results are summarized in a cogent and publishable paper. Science is more than just the Eureka! moments.

Both men and women shave unwanted hair, and the global shaving market is more than $10 billion annually. There are also many cultural memes about shaving. Highways in the United States were once decorated with novel advertising signs for the shaving cream, Burma-Shave. Unlike the usual billboard, or barn, ads, the Burma-Shave ads were a humorous rhyming "poems" on a series of small signs, the last of which held the Burma-Shave logo. The signs were used from 1926 to 1963.

Animated Burma-Shave sign

The expansion of the US Interstate Highway System and increased motor vehicle speeds made the Burma-Shave signs less effective, so their use was discontinued in 1963.

The message on this sign relates to pre-pandemic advice from the US Centers for Disease Control and Prevention.[1]

Beard hair can interfere with face piece respirators, such as N95 masks, by preventing an airtight seal, but men can wear cloth masks without trimming their beards.

Widespread mask use slows the spread of COVID-19.

(Modified Wikimedia Commons image, animated using GIMP, the GNU Image Manipulation Program. The base image is a US Library of Congress photograph by Carol M. Highsmith (b. 1946).)


Any article in a science blog about shaving should mention the barber paradox of Bertrand Russell (1872-1970). Russell was a mathematician who co-authored with Alfred North Whitehead (1861-1947) an important book on classical logic, Principia Mathematica. He also won a Nobel Prize. There is no Nobel Prize in Mathematics, but he was awarded the 1950 Nobel Prize in Literature "in recognition of his varied and significant writings in which he champions humanitarian ideals and freedom of thought." The barber paradox is simply stated,
There's a barber who shaves all those, and those only, who do not shave themselves. Does the barber shave himself?
The paradox is that if he does shave himself, he can't be that barber; and, if he doesn't shave himself, he still isn't that barber.

As a testament to how long product development takes, my father had an entire drawer full of electric shavers that he had tried and discarded. Fifty years later, my son has had more luck, and he's a dedicated electric shaving fan, although he's had a problem with the purchase of some counterfeit replacement blades. These are a good target for counterfeiters, since the selling prices are high, and it's simple to make facsimiles. I've always used shaving cream for a "wet" shave, using a double-edged safety razor for most of my life. Now, I've found that my wife's brand of disposable razors work just fine for me. The triple-bladed, laser-processed, men's razors seem like a high-priced advertising gimmick to me.

Safety razor blade

Adouble-edged safety razor blade.

I once used such a blade as the fulcrum in a low friction lever in an experiment.

(Modified Wikimedia Commons photograph by András Bögöly.)


Razor blades, like the one shown in the above image, have been used by telescope makers to test the shape of their reflector mirror blanks using the Foucault knife-edge test. This test, devised in 1858 by French physicist, Léon Foucault (1819-1868), measures mirror surface deviation from a parabolic shape using just a point source of light (historically, a light bulb shining through a pin hole) and the knife edge of a razor blade. It's a stretch to include Ockham's razor in this article, but I'll do it anyways. The English Franciscan friar, William of Ockham (c. 1287-1347), proposed the rule, used by scientists, that the simplest explanation is most likely the right one.

Although wives sometimes complain about their husbands' scratchy beards, hair is still far softer than the steel of a razor blade, and new razor blades are capable of cleanly shaving hair. Since razor blades are produced with alloys and coatings that prevent their corrosion, why do razor blades dull after days, rather than years? That was the problem tackled by a team of materials scientists and mechanical engineers from MIT, and their findings are published in a recent issue of Science.[2-5]

This problem doesn't apply just to razor blades. Even chef's knives are dulled after cutting such soft foods as tomatoes and potatoes.[4] Most cutting steels have a high carbon content that leads to a Martensitic microstructure, and these are often coated with materials intended to enhance hardness and resist wear.[2] The reason why sharp edges become dull after cutting materials significantly softer is poorly understood.[4] Says Cem Tasan, an associate professor in the MIT Department of Materials Science and Engineering and an author of the study,
"We are metallurgists and want to learn what governs the deformation of metals, so that we can make better metals... 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... We found the main ingredients of failure, which enabled us to determine a new processing path to make blades that can last longer."[3]

The MIT research team found that contrary to the expectation that edge rounding and brittle fracture of a blade's hard coating were responsible for the blade dulling, a detailed microstructural investigation showed a different mechanism.[2,4] The dulling is caused by a combination of out-of-plane bending, microstructural heterogeneity, and microscopic chipping along the smooth edge. Fractures at the cutting edge created chipping that dulled a blade faster than other processes.[2]

While the chef who wields a sharp knife would say that "the proof of the pudding is in the eating," scientific proof comes from experiment. Gianluca Roscioli, the lead author of the paper, purchased several brands of disposable razors, saw by microscopic analysis with a scanning electron microscope that their edges were similarly structured, and examined them again over the course of days after shaving his own facial hair.[3,5] He saw that wear was minimal, but chipping was extensive.[3,5] The size of the chips was about a tenth the diameter of a human hair, and they increased in number the more times the blades were used for shaving.[5] These findings signaled the need for a more controlled experiments.

Cutting a human hair in a scanning electron microscope (MIT)

Splitting hairs. This is an in situ video of a human hair being cut by a razor blade in a scanning electron microscope. The left is a real-time graph of the force encountered by the blade during the cutting. As expected, the force increases until the maximum cutting width (the hair diameter) is reached, and it decreases thereafter. From the force curve, it appears that the hair fractures at the very end. My theory is that the acoustic shock wave from this fracture might explain some of the results. (MIT image by Gianluca Roscioli.)


Controlled experiments were conducted using a small, micromechanical apparatus placed inside a scanning electron microscope to mimick the shaving process (see image above).[3,5] commercial razor blades were set at various angles to cut human hair sampled from laboratory members.[3] As in the initial shaving experiments, hair caused the blade’s edge to chip, but only under certain cutting conditions.[3] When the hair was cut perpendicular to the blade, there was no chipping; but, when the hair was free to bend, there was chipping.[3] The razor blades dulled after subsequent cracks accumulated around initial cracks.[3]

computer simulations were done that gave results consistent with the experiments. Blade damage was more likely to occur when the hair was cut at an angle to the blade; when the steel had an heterogeneous composition; and, when the edge of a hair impacted the blade at a weak point in its heterogeneous structure.[3] This type of failure arises from the well-known effect of stress concentration at the tip a crack.[3] Says study author, Tasan, "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."[3]

One way to make blades last longer (although this is likely something a manufacturer might not want to do, since it would limit sales!) is to create them with a more uniform mirosctructure.[3-5] The MIT group has filed a provisional patent application on just such a process.[3] Says team member, Roscioli, "The basic idea is to reduce this heterogeneity, while we keep the high hardness... We’ve learned how to make better blades, and now we want to do it."[3]

References:

  1. Infographic - Facial Hairstyles and Filtering Facepiece Respirators, US Centers for Disease Control and Prevention (PDF File).
  2. Gianluca Roscioli, Seyedeh Mohadeseh Taheri-Mousavi, and Cemal Cem Tasan, "How hair deforms steel," Science, vol. 369, no. 6504 (August 7, 2020), pp. 689-694, DOI: 10.1126/science.aba9490.
  3. Jennifer Chu, "Why shaving dulls even the sharpest of razors," MIT Press Release, August 6, 2020.
  4. How soft hair deforms the sharpest steel blades, American Association for the Advancement of Science Press Release, August 6, 2020.
  5. Nell Greenfieldboyce, "Cutting-Edge Research Shows How Hair Dulls Razor Blades," NPR-All Things Considered, August 6, 2020.

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