Why shaving dulls even the sharpest of razors

August 12, 2020
Shaving

RAZORS, scalpels, and knives are commonly made from stainless steel, honed to a razor-sharp edge and coated with even harder materials such as diamond-like carbon. However, knives require regular sharpening, while razors are routinely replaced after cutting materials far softer than the blades themselves.

Now engineers at MIT have studied the simple act of shaving up close, observing how a razor blade can be damaged as it cuts human hair -- a material that is 50 times softer than the blade itself.

They found that hair shaving deforms a blade in a way that is more complex than simply wearing down the edge over time. In fact, 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 razor's edge can quickly dull.

The blade's microscopic structure plays a key role, the team found. The blade is more prone to chipping if the microstructure of the steel is not uniform. The blade's approaching angle to a strand of hair and the presence of defects in the steel's microscopic structure also play a role in initiating cracks.

The team's findings may also offer clues on how to preserve a blade's sharpness. 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.

Researchers explored the microstructure of metals in order to design new materials with exceptional damage-resistance.

To identify the mechanisms by which razor blades fail when shaving human hair, they studied disposable razors first. Then they took images of the razor's edge with a scanning electron microscope (SEM) to track how the blade wore down over time.

Surprisingly, the experiments revealed very little wear, or rounding out of the sharp edge over time. Instead, he noticed chips forming along certain regions of the razor's edge.

They built a small, micromechanical apparatus to carry out more controlled shaving experiments. The apparatus consists of a movable stage, with two clamps on either side, one to hold a razor blade and the other to anchor strands of hair. He used blades from commercial razors, which he set at various angles and cutting depths to mimic the act of shaving.

Regardless of a hair's thickness, MIT researchers observed the same mechanism by which hair damaged a blade. They found that hair caused the blade's edge to chip, but only in certain spots.

When he analyzed the SEM images and movies taken during the cutting experiments, he found that chips did not occur when the hair was cut perpendicular to the blade. When the hair was free to bend, however, chips were more likely to occur. These chips most commonly formed in places where the blade edge met the sides of the hair strands.

To see what conditions were likely causing these chips to form, the team ran computational simulations in which they modeled a steel blade cutting through a single hair. As they simulated each hair shave, they altered certain conditions, such as the cutting angle, the direction of the force applied in cutting, and most importantly, the composition of the blade's steel.

They found that 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.

They found that once an initial microcrack forms, the material's heterogeneous structure enabled these cracks to easily grow to chips. (Massachusetts Institute of Technology)