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  • Why Shaving Dulls Even the Sharpest Razor

  • 2023-06-24 18:12:29
  • Human hair is 50 times softer than steel, yet it can shave a razor blade, a new study suggests.
  • Human hair is 50 times softer than steel, yet it can shave a razor blade, a new study suggests.

    Razors, scalpels, and cutting tools are typically made of stainless steel, with sharpened edges and coated with materials such as diamond-like carbon to make them more durable. However, these blades require regular sharpening, and for razors, the cutting material is often replaced more frequently than the blades themselves, as it is softer.

    Engineers at MIT have now studied the simple act of shaving, observing how the blade cuts as it encounters hair, a material 50 times softer than the blade. They found that shaving hair deforms the blade, not just simply dulling the edge. In fact, a single hair can cause tiny chips in the blade edge under certain conditions. Once an initial crack forms, the blade is prone to further chipping. With more and more chips accumulating at the initial site, the crack can quickly propagate along the razor’s edge.

    The microstructure of the blade plays an important role, the research team found. Blades are more prone to chipping if the steel’s microstructure is not uniform. The angle of approach of a hair and the presence of microstructural defects in the steel also play a role in initiating cracks.

    The team’s findings could also provide clues on how to protect the sharpness of blades. For example, when cutting vegetables, chefs could consider reducing the downward angle rather than cutting at an angle. Manufacturers could also consider making blades from more homogeneous materials to create more durable and less brittle blades.

    “Our main goal was to understand a problem that more or less everyone is aware of: why blades become useless when interacting with soft materials,” said C. Cem Tasan, associate professor of materials science and engineering at MIT, who led the research with Thomas B. Kingery Professor of Metallurgy at MIT. “We identified the main ingredients of failure, which allowed us to come up with a new processing approach to make blades last longer.”

    Tasan and his colleagues published their results today in the journal Science. His co-authors are Jiang Luka Roscioli, lead author and MIT graduate student, and Seyedeh Mohadeseh Tamassobi, an MIT postdoc.

    A Metallurgical Mystery

    Tasan’s group explores the microstructure of metals at MIT’s Department of Materials Science and Engineering to design new materials with outstanding stability.

    “We are gold diggers, trying to understand how to control the deformation of metals, so we can make better metals,” Tasan said. “In this case, it’s interesting because if you have something soft, like human hair, and something hard, like steel, the hard material will fail.”

    To identify the mechanism of blade failure when shaving human hair, Roscioli first conducted some preliminary experiments, using a disposable razor to shave his own facial hair. After each shave, he used a scanning electron microscope (SEM) to track how the blade edge was wearing.

    In situ hair-cutting experiments with a scanning electron microscope showed the chipping process. Credit: Jiang Luka Roscioli

    Surprisingly, the experiments showed little wear or dulling of the sharp edge. Instead, he noticed that chips formed in certain areas along the razor’s edge.

    “This created another mystery: we saw chipping, but we didn’t see chipping everywhere, only in certain locations,” Tasan said. “We wanted to figure out what conditions led to chipping, what the ingredients of failure were.”

    To answer this question, Roscioli built a small, micromechanical device for controlled shaving experiments. The device consisted of a movable stage with two clamps on either side, a blade, and an anchor to hold a hair. Based on commercial razor blades, the blade was set at different angles and cutting depths to simulate shaving.

    The device was designed to fit inside a scanning electron microscope, where Roscioli conducted multiple cutting experiments with high-resolution images of hair and blades. He used his own hair, hair sampled from several labmates, representing a range of hair diameters.

    Regardless of hair thickness, Roscioli observed the same mechanism of hair damaging the blade. As in his initial shaving experiments, Roscioli found that hair caused chips in the blade edge, but only at certain moments.

    When he analyzed SEM images and movies taken during cutting experiments, he found that chips did not occur when hair was cut perpendicular to the blade. When hair bent randomly, however, chips were more likely to occur. These chips typically formed where the blade edge encountered a strand of hair.

    To see what conditions might lead to these chips forming, the team modeled the steel blade cutting a hair. As they simulated each hair being shaved, they changed certain conditions, such as the cutting angle, the direction of force applied to the cut, and, most importantly, the steel composition of the blade.

    They found that the simulations predicted failure under three conditions: when the blade approached a hair at an angle, when the blade’s steel was composed of heterogeneous components, and when the edge of a hair encountered a weak point in the blade’s microstructure.

    Tasan said these conditions suggest a mechanism called stress intensification,where the stress on the blade is amplified at certain points, leading to crack initiation and propagation. The team also found that when the blade was made from more homogeneous materials, it was less prone to chipping.

    The team’s findings have important implications for the design and manufacture of cutting tools, including razors and scalpels. Manufacturers could consider using more homogeneous materials for blades to increase their durability and reduce the likelihood of chipping. Chefs and other professionals who use cutting tools could also adjust their cutting angles to reduce stress on the blade.

    “This work suggests that by controlling the microstructure of the blade, we can make it more resistant to failure,” Tasan said. “We hope our findings will inspire new approaches to the design and manufacture of cutting tools, leading to longer-lasting and more effective blades.”

  • Data Source: famshop.com
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