Engineers at MIT have studied the simple act of shaving closely, observing how a razor blade can be damaged when it cuts human hair – a material that is 50 times softer than the blade itself. Credit: Mary Elizabeth Wagner
Human hair is 50 times softer than steel, yet it can spew a rage away, a new study shows.
Razors, scalps, and knives are often made of stainless steel, cut to a razor-sharp edge and covered with even harder materials such as diamond-like carbon. Knives, however, require regular grinding work, while razors are regularly replaced after cutting materials much softer than the knives themselves.
Now engineers at MIT have studied the simple act of shaving close, observing how a razor blade can be damaged when it cuts human hair – a material that is 50 times softer than the blade itself. They found that haircuts deform a knife in a way that is more complex than just wearing the edge over time. In fact, a single strand of hair can chip the edges of a sheet under specific conditions. Once a first crack occurs, the blade is vulnerable to further chipping. As more cracks accumulate around the initial chip, the edge of the razor can quickly fade.
The microscopic structure of the blade plays an important role, the team found. The blade is more envious of chips when the microstructure of the steel is not uniform. The approximate angle of the blade to a hairline and the presence of defects in the microscopic structure of the steel also play a role in initiating cracks.
The team’s findings may also provide clues as to how to maintain the sharpness of a blade. When cutting vegetables, for example, a chef may consider cutting directly, instead of at an angle. And when designing longer-lasting, more chip-resistant knives, manufacturers might consider making knives from more homogeneous materials.
“Our main goal was to understand a problem that more or less everyone is aware of: why blades become useless when dealing with much softer material,” says C. Cem Tasan, the Thomas B. King Associate Professor of Metallurgy at MIT. “We have found the key ingredients of failure, allowing us to determine a new processing path to create blades that can last longer.”
Tasan and his colleagues published their results in the journal today Science. Its co-authors are Gianluca Roscioli, lead author and MIT graduate student, and Seyedeh Mohadeseh Taheri Mousavi, MIT postdoc.
An experiment on cutting hair into a scanning electron microscope, using the chipping process. Credit: Gianluca Roscioli
A metallurgy mystery
Tasan’s group in MIT’s Department of Materials Science and Engineering examines the microstructure of metals to design new materials with exceptional damage resistance.
“We are metallurgists and want to learn what regulates the deformation of metals so that we can make better metals,” says Tasan. “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 razors fall when shaving human hair, Roscioli first performed some preliminary experiments, using disposable rockets to shave his own facial hair. After each shave, he took images of the razor with a scanning electron microscope (SEM) to track how the knife dried over time.
Surprisingly, the experiments revealed very little wear, as time ran out of sharp edge. Instead, he noticed chips forming at certain regions of the razor edge.
“This caused another mystery: We saw chipping, but saw no chipping anywhere, only at 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 of the new blade
To answer this question, Roscioli built a small, micromechanical device to perform more controlled shear experiments. The device consists of a movable stage, with two clamps on opposite sides, one for holding a razor blade and the other for anchoring strands of hair. He used blades of commercial rockets, which he placed at various angles and depths of cut to imitate the act of shaving.
The device is designed to fit into a scanning electron microscope, allowing Roscioli to take high-resolution images of both the hair and the knife as he performs multiple cutting experiments. He used his own hair, as hair sampled from several of his lab sizes, and generally represented a wide range of hair diameters.
In-situ cutting experiment performed with one hair to generate the charges generated at the blade edge during shaving. Credit: Gianluca Roscioli
Despite the thickness of a hair, Roscioli observed the same mechanism by which hair damaged a knife. Just as in his first shaving experiments, Roscioli found that hair caused the edge of the knife, but only in certain places.
When he analyzed the SEM images and films made during the cutting experiments, he found that chips did not occur when the hair was cut perpendicular to the blade. However, when the hair was free to bend, chips were more common. These chips are usually formed in places where the blade edge meets the sides of the hair strands.
To see the probable conditions under which these chips were made, the team ran computer simulations in which they modeled a steel blade by cutting a single hair. When simulating each haircut, they changed certain circumstances, such as the cutting angle, the direction of the force applied when cutting, and most importantly, the composition of the steel of the knife.
They found that the simulations failed under three conditions: when the blade approached the hair at an angle, when the steel of the blade was heterogeneous in composition, and when the edge of a strand of hair cut the knife at a weak point met in its heterogeneous structure.
Tasan says that these conditions illustrate a mechanism known as stress intensification, in which the effect of a stress applied to a material is intensified when the structure of the material has microcracks. Once a first micro-jaw is formed, the heterogeneous structure of the material easily converts these cracks into 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 such as hair,” says Tasan.
The researchers have filed a preliminary patent on a process to manipulate steel into a more homogeneous form, to make longer durable, more chip-resistant blades.
“The basic idea is to reduce this heterogeneity while maintaining the high hardness,” Roscioli says. “We learned how to make better knives, and now we want to do it.”
