Hair v razor. Why hair wins

Any non-bearded male will tell you that (a) shaving is monotonous and (b) razors don’t last. No matter how much they claim to have improved the technology, and how many blades they add, you have to keep replacing them.

So how come stainless steel, honed to a razor-sharp edge and coated with even harder materials, can’t cut it (pun intended) with something as soft as hair?

Metallurgists from Massachusetts Institute of Technology, US, now think they know.

By observing the action at very close quarters, Gianluca Roscioli and colleagues found that shaving deforms a blade in a way that is more complex than simply wearing down the edge over time.

In fact, they say, 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 edge can quickly dull.

The blade’s microscopic structure plays a key role, the team reports in a paper in the journal Science.

The blade is more prone to chipping if the microstructure of the steel is not uniform. Its approaching angle to a strand of hair and the presence of defects in the steel’s structure also play a part in initiating cracks.

The research started simply: Roscioli shaving himself then taking images of the razor’s edge with a scanning electron microscope (SEM).

When intriguing results began to appear, he and colleagues C Cem Tasan and Seyedeh Mohadeseh Taheri Mousavi designed a special apparatus to fit inside the SEM, providing high-resolution images of the hair and the blade.

Regardless of a hair’s thickness, they observed the same mechanism by which hair damaged a blade. To see what conditions were likely causing chips to form, they ran computational simulations in which they modelled a steel blade cutting through a single hair.

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. There may be hope, however. “We found the main ingredients of failure, which enabled us to determine a new processing path to make blades that can last longer,” he says.

And the findings, the researchers say, may also offer clues on how to preserve the sharpness of other blades.

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.