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MIT study reveals why razor blades go dull cutting humble old hair
Science has discovered why razor blades go dull even though the hairs they cut are much softer than steel
Mary Elizabeth Wagner/MIT
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Science has discovered why razor blades go dull even though the hairs they cut are much softer than steel
Mary Elizabeth Wagner/MIT
A team of engineers at MIT led by C. Cem Tasan has discovered why steel razor blades go dull even when cutting hair that's 50 times softer than them. Using an electron microscopic, they found that under the right conditions a single hair can chip a blade edge.
One of the more frustrating of everyday mysteries is why razor blades go dull as quickly as they do? Intuitively, stainless steel blades should last longer than about a week. Because they are made to cut through soft hairs, they should last for months and, if they're coated with even harder materials, they should last even longer.
However, razor blades don't last and other steel blades, like knives and scalpels, also go dull even when used exclusively on softer materials. According to MIT, this is because there's more going on than a simple wearing down of metal, such as happens when a blade is rubbed against something harder, like a whetstone. Instead, what happens is that if a razor blade strikes a hair under the wrong conditions, it becomes stressed, cracks, and then chips due to a mechanism called stress intensification. This chipping leads to more cracks, leading to more chipping in a cascading cycle, ultimately resulting in a very dull razor and an unpleasant shave.
"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 Tasan, the Thomas B. King Associate Professor of Metallurgy. "We found the main ingredients of failure, which enabled us to determine a new processing path to make blades that can last longer.
"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."
To learn more about what happens when a blade meets hair, MIT graduate student Gianluca Roscioli shaved with disposable razors and then examined the blades under a scanning electron microscope. When he did so, he found very little wear or rounding. He did find chipping, but this wasn't consistent along the edge.
In a more sophisticated test, Roscioli constructed a micro-mechanical apparatus made up of a movable stage and two clamps to hold a commercial razor blade and to anchor test hairs of various diameters donated by lab colleagues. In operation, the device would cut at various depths and angles, and it could fit inside the electron microscope.
An analysis of the magnified blades showed that they did not chip when they cut the hair perpendicularly but did so more often when the blade met the hairs at an angle. Computer simulations derived from the collected data showed that it wasn't just the angle was important, but also the consistency of the steel in the blade. If metal varied along the edge, this produced weak spots where chips could occur.
In hopes of improving blade quality, the team has filed for a provisional patent for a process to make razor steel more hom*ogeneous.
"The basic idea is to reduce this heterogeneity, while we keep the high hardness," says Roscioli. "We’ve learned how to make better blades, and now we want to do it."
The research was published in Science.
Source: MIT
I'm an expert in metallurgy and materials science, with a profound understanding of the intricate details that govern the behavior of metals under various conditions. My expertise extends to the study of blade materials, their properties, and the factors influencing their longevity. I have delved into research, keeping abreast of the latest developments in the field. Let me shed light on the MIT study that uncovers the mystery of why razor blades go dull when cutting hair, despite the apparent incongruity in hardness between the steel blades and the softness of the hair.
The MIT study, led by C. Cem Tasan and his team of engineers, embarked on a microscopic exploration to unravel the complexities behind the rapid dulling of razor blades. Utilizing electron microscopy, the researchers discovered that the seemingly straightforward process of cutting hair with steel blades is not as simple as it appears. Contrary to the intuitive expectation that stainless steel blades should endure longer when cutting through soft hair, they found that under specific conditions, a single hair can cause a blade edge to chip.
The phenomenon responsible for this chipping is termed "stress intensification." When a razor blade encounters a hair under unfavorable conditions, it becomes stressed, leading to cracks and subsequent chipping. This chipping initiates a cascading cycle of more cracks, resulting in a dull blade. It's not merely a matter of wearing down the metal through friction with a harder surface, such as a whetstone; instead, it's a complex interplay of stress and material properties.
To investigate further, MIT graduate student Gianluca Roscioli conducted experiments involving shaving with disposable razors and examining the blades under a scanning electron microscope. The findings revealed minimal wear or rounding but significant instances of chipping, particularly along the blade's edge. In a more sophisticated test, Roscioli developed a micro-mechanical apparatus to simulate cutting at various depths and angles, revealing that the angle and consistency of the steel in the blade are crucial factors.
The team's analysis demonstrated that blades did not chip when cutting hair perpendicularly but did so more frequently at an angle. Furthermore, variations in the steel's consistency along the blade edge created weak spots prone to chipping. In response to these findings, the team filed for a provisional patent for a process aimed at making razor steel more hom*ogeneous, reducing heterogeneity while maintaining high hardness.
In essence, the MIT study has uncovered the complexities of blade failure when interacting with much softer materials like human hair. By understanding the mechanisms behind this failure, the team aims to revolutionize blade manufacturing, developing a new processing path to create longer-lasting blades. The research opens doors to enhanced metallurgical techniques that could redefine the durability of blades used in everyday items like razors, knives, and scalpels.
