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Thick or thin: Scientists make hair-raising discovery about strength

Scientists found thin strands were able to withstand greater tension before they broke.

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Scientists looked at the strength of hair (Nick Ansell/PA)

Scientists looked at the strength of hair (Nick Ansell/PA)

Scientists looked at the strength of hair (Nick Ansell/PA)

Lustrous, thick hair may be desirable but it is not necessarily the strongest.

Scientists said thin hair tends to be stronger than thicker locks, after looking at the way they break.

The University of California took samples of hair from humans, bears, boars, horses, capybaras, javelinas, giraffes and elephants, and tied individual strands to a machine that gradually pulled them until they tore apart.

They found that thin hair was able to withstand greater tension before it broke compared to thick hair.

The notion of thick being weaker than thin is not unusual, and we have found that happening when studying brittle materials like metal wiresRobert Ritchie, University of California

On analysing the strands with a scanning electron microscope after the procedure, researchers noticed the samples broke in different ways even though most hairs have a similar structure.

Mammals with hair strands among the widest in diameter – such as boars, giraffes and elephants – generally made a clean break, similar to what you would expect if a banana breaks in the middle.

But those with a smaller diameter, including humans, horses and bears, succumb with an uneven break, more like a tree branch snapping in a storm – known as a shear mode.

“We were very surprised by the result,” said Wen Yang, a nanoengineering researcher at the University of California, San Diego, and first author of the paper.

“Because, intuitively, we would think thick hair is stronger.

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A jagged broken hair of a human, left, versus the clean break of a boar hair, right (Wen Yang/PA)

A jagged broken hair of a human, left, versus the clean break of a boar hair, right (Wen Yang/PA)

A jagged broken hair of a human, left, versus the clean break of a boar hair, right (Wen Yang/PA)

“Shearing is when small zig-zag cracks are formed within the material as a result of stress.

“These cracks then propagate, and for some biological materials, the sample isn’t completely broken until the small cracks meet.

“If a material shears, it means it can withstand greater tension and thus is tougher than a material that experiences a normal fracture.”

Co-author Robert Ritchie added: “The notion of thick being weaker than thin is not unusual, and we have found that happening when studying brittle materials like metal wires.

“This is actually a statistical thing, which is a bigger piece will have a greater possibility of having a defect.

“It’s a bit surprising to see this in hair as hair is not a brittle material, but we think it’s because of the same reason.”

The findings are presented in the Matter journal.

PA